Source : http://www.pelicanparts.com/bmw/techarticles/Borrowed/mult_engine_rebuild-1.htm
This article is the one in a series that will be released in conjunction with Wayne's upcoming book, 101 Projects for Your BMW 3-Series.
The book will be 256 pages of full color projects detailing everything
from performance mods to timing the camshafts. With more than 350+
full-color glossy photos accompanying extensive step-by-step procedures,
this book should be a staple in any 3-Series owner's collection. See The Official Book Website for more details. The book is due out in October 2005.
When to Rebuild?
Indeed, this is a very common question, and one that is often not
easily answered. Obviously, if there is the end of a rod sticking out of
your engine case, then chances are, it’s time for a rebuild. However,
with more subtle noises, broken pieces, and poor performance, the
rebuild decision may not be crystal clear. In this section, I will
provide you with some questions to ask yourself and some answers to
common myths, in the attempt to correctly determine whether your engine
needs to be rebuilt. As
with any serious medical condition, it’s always a wise idea to get a
second opinion. The same is true with BMW engine rebuilds. Very often, I
have heard of unscrupulous (or even over-meticulous) mechanics who have
recommended, or even insisted on a rebuild, when not all of the signs
pointed in that direction. Keep in mind that no matter how
well-intentioned your mechanic may be, he may have a vested financial
interest in seeing you rebuild your BMW engine. Of course, not knowing
that you’re armed with this book and prepared to do it yourself, he
might recommend a full rebuild. Rebuilding engines is a good business,
and will guarantee about 40 hours of labor for complete job. My
recommendation is that you take your car to a second, independent
mechanic, and pay to have the car evaluated. Have him perform a
leak-down test on the engine (see later in this chapter), and let him
know up-front that you have a master mechanic friend waiting in the
wings to rebuild the engine for you. The goal is to try to get an
independent, unbiased expert view of the condition of your engine. Many
of the problems with BMW engines can be somewhat subtle, and difficult
for a novice to detect and decipher. I’ll give you some hints, tips,
procedures, and clues to help you in the following sections, but getting
at least two expert opinions is always a wise idea.
High Mileage Engines
Each derivative of an BMW engine has it’s own quirks and problems. Some engines are known for their longevity, and some are decidedly not. Just because your engine has a lot of miles on it, doesn’t mean that it’s automatically time for a rebuild. With proper care and maintenance, certain engines can easily last 250,000 miles or more. Of course, some years have had better track records than others, but the basic rules apply: if the engine was well cared for, and not abused, then it should last a long time, and gradually wear out. In general, the rule of thumb is that high-mileage is not a good yardstick measurement of engine condition. The methods by which the car was operated and maintained during its life affect the condition of the engine much more than the mileage total. High mileage engines often show signs of their age in compression and leak-down tests, described later in this section. As the engines age and mileage increase, the small tolerances within the engine slowly become larger. While this usually doesn’t result in a catastrophic breakdown, high-mileage engines will gradually see their performance degrade as the mileage increases. Such an engine may be referred to as 'tired'.
High Mileage Engines
Each derivative of an BMW engine has it’s own quirks and problems. Some engines are known for their longevity, and some are decidedly not. Just because your engine has a lot of miles on it, doesn’t mean that it’s automatically time for a rebuild. With proper care and maintenance, certain engines can easily last 250,000 miles or more. Of course, some years have had better track records than others, but the basic rules apply: if the engine was well cared for, and not abused, then it should last a long time, and gradually wear out. In general, the rule of thumb is that high-mileage is not a good yardstick measurement of engine condition. The methods by which the car was operated and maintained during its life affect the condition of the engine much more than the mileage total. High mileage engines often show signs of their age in compression and leak-down tests, described later in this section. As the engines age and mileage increase, the small tolerances within the engine slowly become larger. While this usually doesn’t result in a catastrophic breakdown, high-mileage engines will gradually see their performance degrade as the mileage increases. Such an engine may be referred to as 'tired'.
Stock
engines almost always last longer than modified engines. Higher
compression ratios, aftermarket turbos, or superchargers will almost
always place added stress on engines and make them wear out or fail
quicker. Engines driven constantly on the track may especially show
signs of wear. Race engines have such a typically short lifespan that
their usage is usually tallied in hours run, rather than miles traveled.
Failed Smog Tests
Out here in sunny California, we have one of the most strictest emissions tests in the world. Cars are held up to high standards that seem to get higher each year. Recently the California Air Resource Board (CARB) instituted a dynometer test where the wheels of the car are placed on a roller, and tested for emissions at a specific speed. In addition, the tests monitor hydrocarbons, carbon monoxide, and nitrogen oxides (NOx). These tests are designed to monitor the emissions for engine conditions that might produce smog. Unfortunately, as the tests get tougher and tougher to pass, more engines tend to fail. In some cases, the tests hold the cars to emissions standards that they were never designed to meet. Just because your car fails the smog test, doesn’t necessarily mean that its engine needs to be rebuilt. In fact, a recently rebuilt engine will most certainly fail the test if it hasn’t been fully run-in yet. The best thing that you can do to get your car to pass a smog test is to make sure that it is running perfectly. Most of the time a non-passing car simply has its timing set incorrectly, or has a fuel injection problem. You must make sure that all of your fuel injection and ignition components are working 100% properly before you can assume that the engine mechanicals may be suspect. A compression or leak-down test should be able to let you know if your failure to pass smog is caused by internal engine wear.
Poor Performance and Poor Gas Mileage When rings and valve guides begin to wear, the result is an increase in burnt oil inside the engine. Also seen is a decrease in compression. Both will have a negative impact on the power generated by your engine. The burnt oil is a contaminant in the combustion chamber and will interfere with the combustion process. The loss of compression will reduce your compression ratio, and limit the power output of the engine. Both will result in poor performance and poor fuel economy. However, there are plenty of other factors that can affect fuel economy and power. Most notably, the fuel injection system needs to be maintained in top shape in order to achieve the most power out of your system. Make sure that you have eliminated both the fuel system and ignition system as a potential source of problems before you decide you need a rebuild. Also try to isolate and fix other obscure problems that you might not think of. Improper suspension alignment can seriously reduce power, as can improper tire inflations. Brake problems (especially with the emergency hand brake) can drag on the wheels and create some pretty significant drag. My upcoming book in this series, “101 Projects for Your BMW 3-Series” will have more information on fixing these problems.
Failed Smog Tests
Out here in sunny California, we have one of the most strictest emissions tests in the world. Cars are held up to high standards that seem to get higher each year. Recently the California Air Resource Board (CARB) instituted a dynometer test where the wheels of the car are placed on a roller, and tested for emissions at a specific speed. In addition, the tests monitor hydrocarbons, carbon monoxide, and nitrogen oxides (NOx). These tests are designed to monitor the emissions for engine conditions that might produce smog. Unfortunately, as the tests get tougher and tougher to pass, more engines tend to fail. In some cases, the tests hold the cars to emissions standards that they were never designed to meet. Just because your car fails the smog test, doesn’t necessarily mean that its engine needs to be rebuilt. In fact, a recently rebuilt engine will most certainly fail the test if it hasn’t been fully run-in yet. The best thing that you can do to get your car to pass a smog test is to make sure that it is running perfectly. Most of the time a non-passing car simply has its timing set incorrectly, or has a fuel injection problem. You must make sure that all of your fuel injection and ignition components are working 100% properly before you can assume that the engine mechanicals may be suspect. A compression or leak-down test should be able to let you know if your failure to pass smog is caused by internal engine wear.
Poor Performance and Poor Gas Mileage When rings and valve guides begin to wear, the result is an increase in burnt oil inside the engine. Also seen is a decrease in compression. Both will have a negative impact on the power generated by your engine. The burnt oil is a contaminant in the combustion chamber and will interfere with the combustion process. The loss of compression will reduce your compression ratio, and limit the power output of the engine. Both will result in poor performance and poor fuel economy. However, there are plenty of other factors that can affect fuel economy and power. Most notably, the fuel injection system needs to be maintained in top shape in order to achieve the most power out of your system. Make sure that you have eliminated both the fuel system and ignition system as a potential source of problems before you decide you need a rebuild. Also try to isolate and fix other obscure problems that you might not think of. Improper suspension alignment can seriously reduce power, as can improper tire inflations. Brake problems (especially with the emergency hand brake) can drag on the wheels and create some pretty significant drag. My upcoming book in this series, “101 Projects for Your BMW 3-Series” will have more information on fixing these problems.
Strange Engine Noises
Water-cooled
engines are designed to expand and contract as they heat and cool. As
such, it is very difficult to diagnose strange engine noises that occur
when the engine is cold. It is not uncommon for the engine to make some
unusual tapping or knocking noises when started stone cold. It’s the
strange noises that are made when the engine is warm and running that
are the ones to watch out for. All engine also tend to get noisier as
they age, and clearances between parts inside the engine become larger. Engine
noises are indeed difficult to hear at times. What may be a loud noise
from one area of the engine, may in fact be inaudible from another
angle. Sometimes sitting inside the car, you will hear more of the
lower-pitched noises, as the higher-pitched ones are filtered out by the
cars’s insulation. Closing your eyes when listening to the engine helps
to eliminate potential distractions, and allows you to concentrate on
isolating the engine noises from one another. An automotive stethoscope
is a useful tool for listening closely to the engine. This tool works
best when placed against a solid piece of the engine. Local sounds from
troubled components can be heard better through the stethoscope because
it helps to isolate outside noise. A long wooden dowel is a good
alternative to the stethoscope, but be careful not to stick it in your
ear, as intermittent engine vibrations can sometimes knock it into the
inside of your ear. A piece of rubber vacuum hose will work as well.
There are four basic types of noises that can come from the typical
engine. Intermittent noises occur at irregular intervals and seem to
have no reasonable pattern to them. An example would be something
rattling around inside one of the valve covers. There are noises that
emanate with the crankshaft speed, and occur once every revolution. Then
there are valve-train noises which come and go once every two
revolutions (on a four-stroke engine, the valve train operates at half
the speed of the crankshaft). Such a noise would include the rockers and
valve noise. This is probably the most common noise heard, and the fix
may be to simply adjust the valves.
A
common noise to hear is a loud squeaky noise from inside the engine
while running. Such a noise can often be attributed to worn alternator
bearings or a bad fan belt. Take the fan belt off, and run the engine
for no more than 10-15 seconds and see if the noise disappears. If it
does, you know the problem is with your alternator or belt system.
Another
common noise is piston slap. This is the sound that the piston makes on
its power stroke when clearances between the piston and the cylinder
are somewhat excessive. It’s a dull-thud clunk that can be heard every
two rotations of the crankshaft. Piston slap is most commonly heard when
the engine is warming up, before the piston to bore clearances have
decreased due to the pistons expanding.
There are a whole host of noises associated with problems such as rod
knock, noisy valves, broken rings, chain tensioner failure, detonation,
and broken or pulled head studs. Unfortunately, I have discovered that
it’s nearly impossible to accurately describe these noises in writing so
that someone can diagnose them. The best suggestion would be to take
your car to your mechanic and have him listen to the engine. An engine
can be loud and noisy, and if you haven’t listened to a whole lot of
them, your imagination can get the best of you. Of course, listening to
other finely tuned engines in cars owned by your friends will help you
with an idea of what a normal engine should be sounding like.
Oil Consumption & Smoking
As your engine ages, it will consume more oil. When the engine is brand
new, all of the clearances inside the engine are easily filled with a
thin film of oil. As the surfaces wear, the clearances enlarge, and oil
begins to slip by them. This oil is then burned in the combustion
chamber, as it seeps past the valve guides and piston rings. The wider
the clearances, the more oil will be burned away. Also, some oils have
different viscosities, and tend to burn at a higher rate than others. In
general, thinner, lighter-weight oils have a tendency to flow more
easily past worn parts in the engine. Use of a heavier weight oil in a
tired engine may help to slightly reduce oil consumption.
In addition, excess clearances mean that the oil films that float the
crankshaft bearings require more oil to work properly. Looser gap
clearances between bearings means that oil flows more easily around the
bearing journals. The result is that more oil is required to do the same
task, and there is a corresponding drop in oil pressure and an increase
in wear. This small drop in oil pressure can sometimes be seen if
careful observance to oil pressure readings are taken over the life of
the engine. In general, an increase in oil consumption, coupled with a
decrease in oil pressure, is a sure-fire sign that the clearances in the
engine have increased, and the engine needs to be rebuilt. In addition,
the presence of oil in the combustion chamber may have an adverse
affect on the combustion process. Oil tends to lower the effective
octane rating of the fuel mixture, thus making the engine a bit more
prone to harmful detonation.
So how much oil should your engine be consuming? One quart per 1000
miles is about the standard amount for most engines, as quoted by BMW.
Newly rebuilt engines with about 5,000 miles on them will usually burn
this amount. If your engine is consuming significantly more oil than
this, you have a problem. Consumption of two quarts per 1000 miles is
certainly cause for concern. Check your owner's manual for information
about how much oil your particular model and year car should use.
There are two places that the oil can be lost, either past the piston
rings, or the valve guides. If the car is excessively smoking, then
there is significant oil being burned in the combustion chamber. Engines
expand significantly when they are run. It’s not uncommon for some
engines to expand more than 1/8 of an inch side-to-side when heated from
stone cold to operating temperature. This means that certain clearances
that are designed to be optimum at operating temperature are sometimes
not ideal when the engine is cold. Oil seepage when the engine is cold
is considered normal. For example, just about every older engine smokes
when it’s started, primarily because some oil has seeped into the
combustion chamber when the engine was cold. This smoking is not
necessarily a sign that the engine needs to be rebuilt. A more accurate
test would be to check for significant smoke when the car is completely
warmed up.
What smoke should you look for? White smoke is typically caused by
condensation in the engine, and is generally harmless when seen on an
air cooled engine. Black smoke means that there is a lot of unburned
fuel in the combustion chamber that may be a sign that the car is
running too rich. In general, blue, sooty smoke is burning oil. If your
engine puts out a big puff of bluish smoke when pulling away from a
stoplight, it’s probably a sign that the rings are significantly worn.
Worn rings also produce what is known as blow-by. Just as oil can enter
into the combustion chamber, exhaust gases can also be blown into the
crankcase when the piston fires. Such blow-by, as it is called, often
comes out of the crankcase through the crankcase breather hose on the
top front of the engine. This hose connects to the oil tank, and the
exhaust gases are recirculated back into the engine through the filler
neck on the oil tank. On some cars, blow-by is typically funneled back
into the air filter through the positive crankcase ventilation valve
(PCV).
Worn valve guides can also contribute to oil loss, although typically
less than worn rings. In the mid-1970s, manufacturers sometimes
experimented with new types of valve guides that did not last long at
all. As a result, many of the engines had to have their guides replaced
at about 60,000 miles. Most of these engines have had this repair done,
however, if you find that your engine has not, then you can expect that
your guides will be well worn. Worn guides not only leak compression,
but also can cause the heads of valves to overheat and break off. This
is because close valve guide clearances are necessary for proper cooling
of the valve. It should be noted that puffs of smoke on deceleration
are usually a sign of worn guides and valve seals.
In addition to the oil burned naturally by the engine, your engine can
also lose a lot of oil due to leaks. Many oil leaks drip onto the
exhaust pipes and are burned off by the high heat. As such, sometimes
it’s very difficult to gauge exactly how much oil is being burned by the
engine, and how much is actually being lost to oil leaks.
Air-cooled engines in particular, are infamous for oil leaks. Whether
it’s a Porsche 911 engine or a Volkswagen engine, air-cooled owners will
fondly describe that burning oil smell that is characteristic of these
cars. To be fair, the air-cooled cars must get their passenger
compartment heat from heat exchangers that wrap around the exhaust
pipes. If there is an oil leak onto these pipes, then the smell of
burning oil will waft up into the passenger compartment. This is the
reason why many air-cooled owners diligently try to chase down and
repair oil leaks in their engines.
Your engine can leak oil from one of many different places.
Fortunately, many of these oil leaks can be repaired without tearing
down and rebuilding the engine. See the upcoming book, “101 Projects for Your BMW 3-Series”
for details on all of the common leaks that can be easily fixed without
engine disassembly. If your main goal of rebuilding the engine is to
fix some of these major oil leaks, I suggest that you read that section
first.
There are a few major leaks that cannot be fixed without major engine
work. Crankcase parting line leaks require disassembly, as do leaks
between the heads and the cam towers or engine cases. Many times a leak
will appear to be coming from one of these places when in fact it is
leaking from a different point that is significantly easier to fix. Wash
the underside of the car and track down all of the easy oil leaks
before you decide that it’s time for a rebuild.
Reading Spark Plugs
The spark plug is really the best way to visually ‘see’ what is going
on inside your combustion chamber. You need to pull out all of the spark
plugs to perform a compression test, so you might as well take a close
look at them while their out.
While today’s modern fuels make plug-reading much more difficult, you
can still glean a lot of information from looking at them. A good, well
balanced engine will produce a plug that is light brown in color, and
dry. If the engine is running too rich, the plug will often be coated
with a lot of extra carbon. Keep in mind that the rest of your
combustion chamber probably looks the same. An engine running too lean
will have a powdery white coating on it, and the outer porcelain ring
may have a burned appearance.
When reading spark plugs, pay close attention to the white porcelain
ring around the plug. This white area will give you an excellent
background to inspect the color of the plug, and to help determine how
your combustion chamber looks inside.
If the plug is wet with oil, then that indicates that there is
significant leakage into the combustion chamber past either the valve
guides or the piston rings. This is generally a bad sign, and an
indicator that your compression test may not yield good results.
Compression Tests
One of the most common tests that can be performed on a engine is the
standard compression test. This particular test measures the amount of
pressure that is built up inside the combustion chamber when the engine
is turned over. The typical compression tester is a pressure gauge that
is attached via a short hose to a plug that is screwed into the spark
plug hole. As the engine turns over, the compression gauge will read the
maximum pressure exerted within the combustion chamber. The overall
value is one method of testing your engine to determine the condition of
the rings or valves.
Your engine needs to be setup before you can start the compression
test. With the car cold, loosen the spark plugs with a spark plug socket
and extension. Then tighten them up very lightly. You want to test the
engine when it’s warm, yet if the spark plugs are very tight in the
heads, you can damage the threads in the heads by removing them when the
engine is hot. Loosening them up a bit when the engine is cold will
minimize any damage you could possibly do to the threads in the heads.
Although you might think that it’s good practice to use anti-seize
compound on the plug threads, one manufacturer, Porsche, specifically
recommends against this. The anti-seize compound seems to interfere with
the proper grounding of the plugs. Also, temporarily remove any heater
hoses that might get in the way of removing the spark plugs.
Warm the car up to operating temperature and then turn it off. Wait
about 5 minutes or so, as head temperatures tend to spike right after
you turn the engine off. At this point, the engine fan has stopped, and
the heat tends to build up with no place to dissipate to. Removing the
spark plugs right after turning off the engine can cause the threads in
the aluminum to gall. After about five minutes, remove the spark plugs
from their holes. If you’re working on an early car, then simply
disconnect the power line (+) from the coil. If you’re testing a car
with the Motronic Engine Management System, then remove the small square
DME relay that powers the system. Doing this will disable the car’s
ignition system, and prevent the spark plug wires from firing. It’s also
a wise idea to remove the fuel pump relay at this time, if your car has
one. You are going to be cranking the engine over several times, and
you don’t want raw fuel to be dumped into the system.
Having a helper around is useful, as you can watch the gauge while he
or she cranks the engine. I recommend that you attach a battery charger
to your battery to avoid running it down. Don’t fire it up at 50 Amp,
but instead leave it on about 10 amps, which should help it recover when
it’s not cranking.
With the engine warm, install the compression tester into the spark
plug hole. A bit of patience and skill are required in order to properly
manipulate and screw in the compression tester so that you don’t cross
thread and damage the threads in the cylinder heads. With the
compression tester installed, crank the engine over 12-16 times. Make
sure that you place your foot all the way down on the throttle. This
will allow maximum air flow into the engine, otherwise your compression
readings will be off. The engine should make six to eight full complete
compression strokes (12-16 turns of the crankshaft). You can tell when
the engine is on a compression stroke because the compression gauge will
jump and show an increase when the cylinder is compressed. Carefully
watch how the compression tester gauge increases, and record the maximum
value when you have completed the last compression stroke. The gauge
will jump at first, and then increase slowly until cranking the engine
over more and more has no additional effect on the reading. Remove the
compression tester and repeat for each of the other cylinders.
So what to do with the results? In general, compression tests are
limited in what they can tell you. It is important to remember that
different compression testers may give different readings as well.
Cranking the engine faster (with a stronger battery or high powered
starter) may also skew readings. The most useful piece of information
that you can glean from them is how each cylinder compares to the
others. All of the cylinders should give readings that are very close to
each other. This would generally indicate an engine in good health. A
good rule of thumb is that each cylinder should read a minimum of 85% of
value of the highest cylinder. So, if the highest reading is 150 psi,
then the minimum acceptable reading would be about 128 psi.
It is important to note that this would be an acceptable figure, but
not necessarily ideal. In all practicality, all of the cylinders should
be very close to each other (within about 5-10 psi). On a newly
assembled and run-in motor, compression numbers are usually within this
range. As the engine ages and certain parts wear faster than others, one
or more cylinders may experience a bit more wear than the others. This
will definitely show up in the compression tests. Needless to say, if
you have all of your cylinders in the 150 psi range, and one cylinder is
down around 120 psi, that should give you cause for concern. The
important thing is to remember is that you want to gather consistent
readings across all of the cylinders, without focusing on the actual
values. If a reading is significantly off, go back and test that
cylinder again to make sure that the measurement was not caused by some
sort of fluke, which is often the case.
So what causes variations in compression tests, and why can’t they be
used as the final word on engine rebuilds? The problem is that there are
several factors that effect the final pressure read by the tester.
Engines running with very aggressive camshafts have a tendency to give
low compression readings. This is because there is significant overlap
between the intake and the exhaust stroke on the cam. During high-rpm
operation of the engine, this overlap works to give the engine more
power. However, when turning the engine at a low RPM, as with a
compression test, the overlap causes some of the pressure in the
combustion chamber to leak out before the valve is closed. An early 911S
engine, for example (with its high-overlap cams ) has a tendency to
give lower compression readings than the 911 CIS engines (1974-83),
despite having a higher compression ratio. This is caused by the
aggressive overlap of the camshaft.
Altitude and temperature also affect the compression readings.
Manufacturer’s specifications are almost always given at a specific
altitude (14.7 psi at sea level), and 59° Fahrenheit. Both temperature
and barometric pressure change as you go up in altitude, so you will
need to correct your measurements if you wish to compare it with a
factory specification. The following chart provides conversion factors
for correctly compensating for changes in altitude:
| Compression Test Altitude Compensation Factors | |
| Altitude | Factor |
| 500 | 0.987 |
| 1500 | 0.960 |
| 2500 | 0.933 |
| 3500 | 0.907 |
| 4500 | 0.880 |
| 5500 | 0.853 |
| 6500 | 0.826 |
| 7500 | 0.800 |
8500
|
0.773
|
A standard compression reading of about 150 psi at sea level in Los Angeles would measure significantly less in the surrounding mountains. For example, at an elevation of 6000 feet, the expected reading would be 150 psi X .8359 = 125 psi. The cylinders would be reading low if compared to sea level measurements, yet perfectly fine at this altitude.
Another factor that can alter compression test readings are incorrectly
adjusted valves. If the valves are not opening or closing at the
correct time, then one cylinder may read vastly different than another.
Make sure that your valves are adjusted properly prior to performing the
test. Along the same lines of thought, premature camshaft wear can also
lead to variances in compression readings.
You can determine if the rings are causing low compression readings by
squirting about a tablespoon of standard 10-30W engine oil into the
cylinder. Crank the engine 2-3 times to spread the oil around inside the
combustion chamber. Then retest the compression. If the readings shoot
up significantly (45 psi or so), then the problem is most likely with
the piston rings seating to the cylinders. Squirting the oil inside the
combustion chamber in this manner allows the rings to temporarily seal
quite a bit more than they would dry. If the compression readings do not
change, then most likely culprit is a leaky valve.
Leak-Down Testing
Without a doubt, the most comprehensive test that you can perform on
your engine is a leak down test. While somewhat similar to the
compression test, it eliminates nearly all of the extraneous variables
that may alter the final compression readings in a typical compression
test. In simple terms, the leak-down test involves pressurizing the
cylinder and measuring the amount of air that is leaked out past either
the rings, the valves, or out a gap between the heads and the cylinder.
The leak-down test equipment uses an external air compressor to
pressurize the cylinder. The engine is held stationary, and the test is
not dependent upon outside variables like the cranking speed, altitude,
temperature, or the camshaft overlap. In fact, the leak-down test can be
performed on just about any engine, whether or not it is inside the car
or not.
Unfortunately, the leak-down test equipment is somewhat specialized,
requires an air compressor, and is not exactly inexpensive. Most local
repair shops have a leak-down tester, but it’s not common to find one in
your neighbor’s garage. The good news is that most shops will be able
to perform a leak-down test on your engine for a nominal fee. Most BMW
engines doesn’t require any special leak-down adapters, so you should be
able to take your BMW to any good foreign repair shop, and they should
be able to do the test for you. Similar to the compression test
leak-down test should give you information on the condition of the rings
and valves, but from a slightly different perspective. The leak-down
test can be performed on an engine that is not installed in the car.
However, if the leak-down test is performed on an engine that isn’t
warmed up, then the test may not give accurate results.
The leak-down test is performed by initially setting the engine to
top-dead-center (TDC) on the compression stroke for the piston that you
are checking. Make sure that it’s exactly at TDC, otherwise the engine
will begin to turn over as soon as you pressurize the cylinder. You want
to make sure that both the intake and exhaust valves are completely
closed (as they should be at TDC) otherwise air will immediately leak
out of the cylinder. To make sure that you are at TDC for cylinder
number 1, remove the distributor cap, and rotate the engine clockwise
until the rotor is lined up with the small notch.
When you are running the test, it is a wise idea to make sure that the
crank doesn’t turn at all. Have an assistant hold the crank steady or
place a flywheel lock on the engine if it’s out of the car. Connect the
leakage tester to the engine in the same manner that you would with the
compression tester. Pump up the cylinder and let the leakage tester
measure the amount of air lost. The gauge on the tester should give
readings in percentage numbers. A newly rebuilt engine should have
leak-down percentages of around 3-5%. An engine in good running
condition should show 10% or less. Numbers around 20% indicate some wear
of the engine, but are still adequate for good engine operation.
Leakage numbers of around 30% indicate that there are problems brewing,
and that a rebuild may be necessary. Needless to say a large leakage
amount like 90% indicates that there is a hole in the combustion
chamber, and the engine is probably not firing on this cylinder at all.
Rotate the engine crankshaft clockwise 180° when you’re done, and check
the next cylinder. Repeat the process for each of the six cylinders.
Another good quality of the leak-down test is the ability to pinpoint
the exact problem with the engine. When the cylinder is compressed with
air, you can usually hear where the air is releasing from. Leakage past
the intake valves can often be heard at the intake manifolds through the
fuel injection. Exhaust valve leakage can sometimes be heard through
the tailpipe. Leakage past the rings can sometimes be heard in the
crankcase breather hoses. The most obvious leakage occurs when the
cylinder heads have broken or pulled, and the air leaks directly out of
the combustion chamber in-between the cylinder and the head.
While the leak-down test is probably the best indicator of engine
condition, it shouldn’t be the final word in your evaluation. I have
heard from many people about great running engines that for one reason
or another do not test well on the leak-down tester. It’s important to
remember that the leak-down tester does not test the engine when it’s
running – it only does a static evaluation. As with any air cooled
motor, it’s operating characteristics vary widely. Use the leak-down
test as one indicator and back it up with other tests and observances.
Carbon Deposits
I
thought it important to mention some things about carbon deposits build
up inside engines. Just about every single engine I have ever seen torn
open has had a significant layer of carbon buildup on the pistons and
the inside of the heads and valves. Particularly with today’s ever
changing formulations of gasoline, the additional carbon build up
appears to be a problem in almost all air-cooled engines.
Carbon deposits will form naturally inside the combustion chamber as a
natural by-product of the combustion process. Both engine oil and
gasoline are hydrocarbons, so burning either of them incorrectly can
result in a buildup of excess carbon deposits. These deposits are often
caused by excessive oil burning in the combustion chamber, which is a
sign that your engine needs a rebuild regardless. In addition, a rich
mixture setting can also introduce more of the black soot that creates
the carbon buildups in the engine. Short-trip driving and extended
idling (not ideal running conditions for an engine) can also increase
the buildup rate. While excess carbon deposits can be cleaned and
removed without a complete overhaul, very often they are yet another
sign that something else on the engine needs attention (like rings and
guides).
Carbon deposits can cause the engine’s valves to become shrouded, and
covered with carbon. In an opposite manner to porting and polishing the
heads, the carbon buildup actually disrupts the flow of fuel mixture,
and can restrict the airflow into the combustion chamber. Even if the
engine has had a relatively short number of miles put on it since its
last rebuild, you may discover that it has very low or zero compression
in one of its cylinders. Often the reason for this is carbon deposits.
When an engine is left idle for a long period of time, moisture has a
habit of getting into the combustion chamber, and gets absorbed by the
carbon deposits. This absorption results in the carbon becoming loose
and flaking off. Carbon deposits that flake off have a bad habit of
getting lodged in-between the exhaust valve and it’s seat. This creates a
compression leak in the combustion chamber.
It’s important not to drive the car for extended periods of time
(hundreds of miles) if you think that a piece of carbon might be lodged
in-between the exhaust valve and its seat. The reason for this is
simple. The exhaust valve (unlike the intake valve) becomes very hot,
and needs to cool by coming in contact with its valve seat. If the valve
doesn’t seat properly, then it will be thermally isolated from its heat
sink (the seat in the head). Prolonged driving in this condition will
cause the valve to become burned, and will develop a typical pie-piece
shaped notch in the valve. Valves damaged in this manner are basically
destroyed, and will not seat properly even if the carbon is removed. In
the worst-case scenario, the valve will become so hot that the head of
the valve can break off. Having the head of a valve dance around the
inside of your combustion chamber will usually destroy the piston and
send chunks of metal circulating throughout your motor. Needless to say,
this is not a good thing.
As mentioned previously, worn valve guides, or worn out rings allow
excess oil into the combustion chamber that vastly increases carbon
build up. Of course, the solution to this problem is a full rebuild, or
at best a top-end valve job. In addition, how you drive your car can
affect the build up of deposits. Short drives around town have a
tendency to increase the level of carbon buildup. Slow-speed, short-trip
driving has a tendency to not let the engine heat up to normal
operating temperatures. Excess carbon deposits can often be ‘burned out’
by driving on the highway for about an hour or so. This should allow
the combustion chamber to heat up enough to burn away the carbon
deposits.
If your engine has been sitting for an extended period of time, you may
want to try using a gasoline additive to your fuel. Berryman B-12
Chemtool and Techron both have good reputations for helping to dissolve
and remove deposits. One of the best things to do is to take your 911 on
an extended, spirited drive of at least an hour or more along the
freeway. Try to vary your RPMs, but make sure that you keep them
relatively high to help raise cylinder head temps. The cleaning process
combined with the heated cylinder heads should be enough to clean out
any excess deposits. When you return from your drive, run the
compression or leak-down test again, and you may be surprised at the
improvement in the numbers!
Next month, take a breath, we'll discuss the various costs involved with tackling a rebuild of your engine!
