This article is one in a series that have been released in conjunction with Wayne's new book, 101 Performance Projects for Your BMW 3 Series. The book contains 272 pages of full color projects detailing everything from performance mods to timing the camshafts. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any 3 Series owner's collection. The book was released in August 2006, and is available for ordering now. See The Official Book Website for more details.
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'.
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.
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.
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|
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.
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.
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!