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Pelican Technical Article:

Compression Test

Jared Fenton


1 hour1 hr






Compression gauge set, spark plug socket

Applicable Models:

Jetta MkIV 2.0L (1999-05)

Parts Required:

Compression gauge set

Hot Tip:

Disable the fuel pump before beginning

Performance Gain:

Knowing the condition of your engine

Complementary Modification:

Replace spark plugs

One of the most common tests that can be performed on an 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. Checking the compression is something that you should consider before purchasing a used Jetta.

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. See our article on Spark Plug Replacement for more information on removing the plugs. Although you might think that it's good practice to use anti-seize compound on the plug threads, sometimes this is a bad idea. The anti-seize compound can interfere with the proper grounding of the plugs.

Warm the car up to operating temperature and then turn it off. Wait about five minutes or so, as head temperatures tend to spike right after you turn the engine off. At this point, the water pump is not circulating coolant through the engine, 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 cylinder head to gall. If this happens, you're looking at a very expensive repair that requires removing the cylinder head.

After about five minutes, remove the coil packs and spark plugs from their holes. You'll also need to disable the fuel pump. You are going to be cranking the engine over several times, and you don't want raw fuel to be dumped into the system. Remove the plastic cover in the engine compartment covering the fuse box. Open the fuse box and remove fuse 28. This will prevent the fuel pump from operating. However it may also trigger a diagnostic trouble code (DTC). If this happens, you may need to use a diagnostic scanner to reset the DTC.

Having a helper around is useful, as you can watch the gauge while he or she cranks the engine. I also recommend that you attach a battery charger to the jumpstart posts to avoid running it down. Don't set it at the highest amp setting, 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 three cylinders.

In general, compression tests are somewhat 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 (as with a stronger battery) 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 180 psi, then the minimum acceptable reading would be about 153 psi.

In general, 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 180 psi range, and one cylinder is down around 120 psi, that should give you cause for concern. The important thing 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.

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:

A standard compression reading of about 180 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 180 psi X .8359 = 150 psi. The cylinders would be reading low if compared to sea level measurements, yet perfectly fine at this altitude.

You can determine if the rings are causing low compression readings by squirting about a tablespoon of 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.

Shown here is the location of the fuse box on the Mk4 Jetta.
Figure 1

Shown here is the location of the fuse box on the Mk4 Jetta. Open the cover (green arrow) to access the fuse box underneath.

Locate fuse number 28 (green arrow) and pull it out.
Figure 2

Locate fuse number 28 (green arrow) and pull it out. This will disable the fuel pump from running during the compression test. Note that the numbers on the end of the fuses refer to the fuse's amperage rating and not order. The glove box normally contains a paper showing the fuse locations.

Shown here is a compression tester kit with the pressure gauge and adapter hoses.
Figure 3

Shown here is a compression tester kit with the pressure gauge and adapter hoses. Most compression testers are available at your local parts store.

After letting the engine warm up, remove the coil packs and spark plugs from each cylinder.
Figure 4

After letting the engine warm up, remove the coil packs and spark plugs from each cylinder. Install the compression gauge in one spark plug hole and crank the motor over 12-16 times while holding the gas pedal down. Once the gauge stops moving, record the result for the cylinder. Now relieve the pressure in the gauge by pressing the relief valve (green arrow) and repeat the whole procedure for each cylinder.

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Page last updated: Mon 10/24/2016 03:04:44 AM