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Compression testing is one of the most common procedures for checking the relative health of an engine. After all, engines are air pumps, and without compression, they tend not to work well. If you ever want to know where you stand in terms of the overall health of your engine, a compression check is the first place to start. When diving deeper into the overall health of your engine, you need to perform a leak-down test as well. A compression test determines the relative compression between individual cylinders, while a leak-down test determines the sealing ability of the valves, head gasket, and piston rings.

Gareth Compression Test Coils Off


Tools Needed to Perform a Compression and Leak-Down Test:

The Compression Test:

The best practice is to have the engine slightly warm when you plan to do a compression test. The reason for this is that the piston rings expand when warm. In turn, this gives you a more accurate idea of the compression within each cylinder when the engine is at operating temperatures. Performing a compression test when the engine is cold is generally a less precise approach. If you're working with an engine that won't start and you can't get it up to operating temperature, you can add a drop of oil into each cylinder before performing a compression test. Adding just a drop of oil will help to lubricate the piston rings. This is a non-issue if you can run the engine first.

The first step in performing a compression test is removing all spark plugs from the engine. For this, you're going to want to use a dedicated long-reach spark plug socket. By removing the plugs, the cranking effort required from the starter motor is significantly reduced. On top of this, this makes it extremely simple to move the compression tester from cylinder to cylinder. At this point, it's a great time to have a battery charger hooked up while you're continuously cranking the engine over. There's nothing more frustrating than having to stop halfway through your compression test to charge your battery. 

Gareth Compression Test Spark Plug Removal

Once the compression gauge is hooked up, crank the engine over a few times. If you have an extra person around, have them watch the compression tester gauge for the peak reading. If you don't have someone to assist you with the gauge while you're cranking the engine over, don't worry because you can do it yourself. Five to six revolutions of the engine are all you will need. Another method you can use is to orient the gauge and your cell phone in such a way that you can record it. Then you have an easy reference that you can go back to later. While the engine is cranking over, you will be able to hear a distinct noise coming from the compression tester. I use this as the cue that the cylinder went through one full revolution. 

My compression numbers were all pretty consistent from cylinder to cylinder and well within the specifications of BMW. The tests across all six were:

#1 = 200 PSI
#2 = 200 PSI
#3 = 210 PSI
#4 = 195 PSI
#5 = 195 PSI
#6 = 205 PSI

Gareth Compression Test Gauge Reading

The numbers above tell me that my engine makes good compression in each of the cylinders. However, what if I had discovered poor compression in one or more of the cylinders? Compression is everything in an enginepoor compression can cause misfires as well as poor ignition. Poor compression can also have an effect on engine performance in regard to the balance of power across each cylinder. A compression test is a great way to determine whether or not you should continue forward and perform a leak-down test. 


The Leak-Down Test:

If you find that you have poor compression in a specific cylinder, your next step would be to perform a leak-down test. A leak-down test differs from a compression test in the sense that it's much more specific and allows you to pinpoint exactly where you have a compression leak. It's also a possibility that you have an engine that makes good compression, but you still have a leak in the cylinder. To complete a leak-down test, you will need a leak-down tester, an air compressor with sufficient CFM, and a way to determine if your engine is at top dead center (TDC) or not. A leak-down tester forces compressed air into the engine. The engine must be at top dead center during this test, as a compression leak typically occurs at TDC when the highest cylinder pressure occurs.

Gareth M52 Leak Down Test

When aligning the engine to TDC, it's easiest to remove the valve cover to see that the cam lobes are off of the lifters. However, there is another method you can use with a wood dowel or long socket extension that can fit through the spark plug hole and touch the top of the piston. Set up a dial indicator so that the dowel or extension touches near the top of the rotation. Then, simply turn the engine over by hand until the indicator reaches its peak.

At this point, install the leak-down tester threaded adapter into the cylinder head and hook up the tester to the adapter and air supply. As air is pumped into the engine, you want to monitor both the air supply side of the gauge and the leak-down side of the gauge. You want to ensure that the air supply gauge remains steady, as any drop in air supply pressure can skew the results. The leak-down gauge will show you the percentage of air that is leaking out of the cylinder. The nice part about this test is that the leaking air only has a few places to escape from. Each type of leak can lead you to the next step in diagnosing an engine performance problem. Here are a few examples of leaks that you can detect with a leak-down tester:

Leak-Down Testing Quick Symptom Guide

Test Result Common Causes

Noise through exhaust

Burned exhaust valve

Bent exhaust valve

Noise through intake

Bent intake valve

Noise through both 

intake and exhaust

Multiple bent valves

Shifted valve seats

Noise from the dry sump tank 

or crankcase breather

Scored cylinder walls

Broken piston ring

Burned piston

Cracked engine block

Bubbling in coolant

Blown head gasket

My E36 leak-down test results were pretty strong, and I'm satisfied that the engine is still in good health. It really shouldn't require any major work to bring it back within specifications. For reference, under 5% leak-down is amazing, 10% is good, 15% is just acceptable, and 20% means you'll be spending a lot of money in the near future. The exact results of my test were: 

#1 = 12%
#2 = 8%
#3 = 11%
#4 = 10%
#5 = 13%
#6 = 11%

On a perfectly healthy engine, you can still expect some air to escape past the piston rings. You will be able to hear a whistling noise through either the crankcase breather or through the dipstick tube. Overall, the leak-down test will give you detailed information to determine the health of your engine and help pinpoint a failure during the diagnostic process. Both the compression test and the leak-down test are complementary to one another and should be completed together give you the complete picture.

With these better-than-expected results, I'm confident going through with the build of my "EDirty6". If you'd like to keep up with my build, you can do so right here on the blog and by subscribing to our YouTube channel.  

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Written by :
Gareth Foley

Gareth is the BMW Make Level Marketer for FCP Euro and has been with the company since 2012. Gareth's BMW obsession started with a hand me down E39 528i when he was 17. From this car he learned how to do his own repair work while also learning more about BMW. When Gareth was at CCSU studying Marketing he had the opportunity to go to SEMA with the college car club. This is where he developed his love of the automotive industry. Since joining FCP in 2012 Gareth has sought out to develop one of the broadest and most accurate BMW replacement parts catalog. he can be reached at gareth1@fcpeuro.com

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