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HomeTech Articles > Time to Rebuild? - Part II

Pelican Technical Article:

Time to Rebuild?
Part II

This article is the first in a series that will be relased in conjunction with Wayne's upcoming book, How to Rebuild and Modify Your Porsche 911 Engine.  The book will be 224 pages of full color projects deailing everything from performance mods to timing the camshafts.   With more than 350+ full-color glossy photos accompanying extensive step-by-step assembly procedures, this book should be a staple in any 911 owner's collection.  See for more details.  Book due out in March 2003.

Although this article is written specifically with the 911/911 Turbo engine in mind, the principles of testing and evaluating your Porsche engine are universal to almost all engines.

In last month's article, I discussed performance and oil consumption issues.  This month I'll take a look at compression and leak-down tests, and discuss the merits and potential downfalls of each of them. We'll also talk a bit about how carbon deposits can affect your engine.

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 911 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, 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. Now, disconnect the cable from the capacitive discharge (CD) unit (1969-1983 911 and all Turbos). If you’re working on a 1965-68 911, then simply disconnect the power line (+) from the coil. If you’re testing a 1984-89 911 Carrera, then remove the small square DME relay from under the passenger seat. 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 (for 1969-83 911s). 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



     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. For the complete procedure on adjusting your valves with the engine inside the car, take a look at Project 18 in the book, “101 Projects for Your Porsche 911.” Along the same lines of thought, premature camshaft wear can also lead to variances in compression readings, however, this type of wear is not normally common on the 911 engine.

     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. The 911 engine doesn’t require any special leak-down adapters, so you should be able to take your 911 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.

     The 911 engine has a few problems of its own, specifically related to carbon build-up. 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. The horizontal layout of the 911 engine in a boxer configuration also lends itself to being susceptible to problems with carbon deposits. It is not uncommon to find a 911 engine that has not been run for a long time that has low compression. 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. The boxer orientation of the 911 engine means that the exhaust valves are located at the bottom of the engine. 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!

Want more technical articles like this one, but don't like viewing them on the computer screen?  Pick up a copy of Wayne Dempsey's book, 101 Projects for your Porsche 911.  See for more details.  Now available! 101 Projects
Comments and Suggestions:
SlingShot Comments: The article mentions the DME relay is under the passenger seat .
I'am working o a 84 Carrera , I found the DME relay under the Driver seat.

Did Porsche mix it up once and a while ?
October 9, 2017
  Followup from the Pelican Staff: Possible your model year was different than what is shown in the article. I appreciate you mentioning this as it will help users in the future. - Nick at Pelican Parts  
whizbang Comments: Hi. I'm getting ready to do a compression test on my 1969 911E. I found the CDI box, but the fuel pump relay continues to elude me. I can't find it on the SL33 wiring diagram, and when I look in the engine compartment where it would logically be located !, I found what look like 3 relays near the RPM transducer. All are silver. Any hints on how to locate the fuel pump relay, or maybe there is another way to keep raw fuel from getting dumped into the system while cranking?
Thanks for your help!
September 19, 2014
  Followup from the Pelican Staff: I cant recall the exact location. I opened a post in our forums. A Pelican community member may be able to answer your question.
- Nick at Pelican Parts
Dragonyen Comments: My 1996 Porsche 993 convertible has bad transmission oil leaks and engine oil leaks. My engine and transmission works wonderfully, no problems. My car mileage is less than 90,000km. My mechanic strongly recommends a complete engine rebuild to solve the above mentioned problems. Your views are appreciated
January 16, 2014
  Followup from the Pelican Staff: You may require a full engine reseal. I would have you mechanic review the procedure with you, to out you at ease. - Nick at Pelican Parts  
opalized Comments: ona leakdown test i pressurize the cylinder at tdc to 100 lb pressure, and then close the valve to the air compressor, and the pressure gage leaks down from 100 to nothing in about 14 seconds, is this cylinder dead??
no noice in the intake or exhaust pipe or dip stick, and all cylinders do the same thing, although one leaks down in 12 seconds from 100 to nothing???
April 4, 2010
  Followup from the Pelican Staff: If at TDC and it leaks all the pressure, yes. There is a major pressure loss in the cylinder. You'll have to locate the source of the loss, valves, gasket or piston. - Nick at Pelican Parts  
stallion Comments: I drive a 996 C4 each day got 55K on clock 1991 when I start it, it smokes big cloud, then stops, some times it does this and some times goes months with out doing it, its very embaresing when it does any cures?
March 10, 2010
  Followup from the Pelican Staff: This could be a failing air oil separator (AOS).

/techarticles/Boxster_Tech/09-ENGINE-Air_Oil_Separator/09-ENGINE-Air_Oil_Separator.htm?utm_expid=607614-4.Tu1b6PKkRKiR6eNB7gUS0Q.0& - Nick at Pelican Parts
bjm12001 Comments: I recently did a leak down test on my 89 Carrera ~50K miles & not driven alot and 5 of the cylinders were around 6-8% & cylinder #2 was around 65%. I did the test with the engine both cold & warm with no change in readings. The thing that has me baffled is that sparkplug in #2 is perfect light gray ash color....what's up with that? Thanks
February 2, 2010
  Followup from the Pelican Staff: If at TDC and it leaks most of the pressure, this is problem. There is a major pressure loss in the cylinder. You'll have to locate the source of the loss, valves, gasket or piston. - Nick at Pelican Parts  
RHINOSPEED Comments: Hi, I find a little mistake, i think, when You check the leak in the cilinders, You have to rotate the engine crankshaft 120°, not 180°, its a 6 cylinders engine, not 4 cylinders, just like You check tha valves lash. ¿Its not?-
November 4, 2009
  Followup from the Pelican Staff: Yes, that's correct, it would be 180 degrees for a four-cylinder motor and 120 degrees for a six-cylinder engine. - Wayne at Pelican Parts  

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