Compression test vs. Cylinder leak down

How, why and what do they mean?

When the time comes to purchase a used vehicle the first question that comes to most prospective buyers minds is, “How’s the compression?” This has been the standard engine condition test since the internal combustion engine was invented.

But what exactly is a compression test; and what do the readings tell us?

A compression test measures how much pressure the piston creates in the cylinder when traveling from bottom dead center (BDC) to top dead center (TDC) with the valves closed. The reading is taken at the spark plug fitting in the cylinder head.

Because we are trying to recreate normal operating conditions there are a few parameters that need to be met before performing the test.

• The engine must be at or near operating temperature.

• All the spark plugs must be removed. (It is advisable to loosen all the spark plugs ½ turn, and then start the engine for 15 seconds. Then completely remove the spark plugs from the cylinder head. This blows out any carbon that might get broken loose and caught between the valve and valve seat. If this were to happen you could get a false, low compression reading.)

• The throttle must be all the way open (WOT)

• The ignition and/or fuel system should be disabled

It is important to disable the ignition/fuel system for safety reasons and engine health, as well as operator health.

Most Porsches produced after 1983 have a single DME relay, which can be removed to disable fuel and spark. On earlier 911s the 14-pin engine plug in the left rear corner can be removed and then bridge connector 1 & 14 to crank the engine.

Insert the compression gauge into the proper spark plug hole and crank the engine 5-6 times noting the first and final readings. Repeat on each cylinder. More important than the exact number of times the engine is cranked, is the consistency between cylinders. If you crank #1 cylinder 6 times, all cylinders must be cranked 6 times. Another concern is the battery condition. The engine must crank at the same speed for all cylinders.

The most important factor in a compression test is consistency between cylinders. A good rule of thumb is to have less than 10% variance between cylinders. So if an engine has an average compression reading of 150-psi there should be less than 15psi between the highest and lowest cylinders.

Lets say for instance that we have the following compression readings:

1-150, 2-180, 3-140, 4-145

1,3 & 4 are all within the 10% specification but 2 is 20% higher than the average of the other cylinders. Does this mean number three is high or the others are low?

To get to the bottom of this question we need to start by examining the spark plugs. Do they all look the same? Is #2 badly carbon fouled? If you have one or more cylinders with high compression and oil burning, the high compression can be caused by the build-up of carbon on top of the piston.

In our test case, the high compression of #2 cylinder can be caused by excessive carbon build-up on the piston. Now to prove it!

Cylinder leakdown test-

While a compression test is a dynamic test (engine moving), a cylinder leakdown test (C.L.T.) is a static test (engine at rest). The compression test measures how much pressure the engine can produce while cranking; in contrast to the C.L.T., which measures how much pressure is lost in the engine. In a C.L.T. the engine is placed on TDC of the cylinder in question and using a similar type of connector as the compression test, we fill the cylinder with pressure. The tester then measures the volume of air needed to maintain a predetermined pressure in the cylinder. This reading is expressed in a percentage. Good cylinder leakdown readings should be below 5-8%.

The great thing about C.L.T. is that it deals with how well the cylinder is sealing and nothing else. The readings are not affected by carbon deposits, cam timing, or even engine cranking speed.

Another great feature of the C.L.T. is the fact that you can hear where the air is leaking out of the cylinder. When a cylinder has high percentage of leakage, first check the oil filler cap. Do you hear a hissing sound? If so, you may have pressure leaking by the rings. Is there air escaping out the exhaust? Is it escaping out the intake system? Then a burned valve may be the problem. If two adjoining cylinders have similar low readings and you hear leakage out the other cylinder, then a failed head gasket may be the problem.

Being able to pinpoint the exact source of the compression loss will tell you where the problem is; and not just that you have one. This knowledge will greatly assist you in the next step… the repair.