A compression test will tell you the compression of the engine. It is not to be confused with a leakdown test, which puts air into the cylinders and measures the rate that the air leaks out. A leakdown test is another test to determine the condition of the engine. It's also not a boost leak test which tests for air leaks upstream of the engine.
VW compression for all 1.9L diesel non pumpe duse (ALH AHU,1Z) and pumpe duse
New: 363-450 psi
Minimum limit: 276 psi
Max difference between cylinders: 73 psi
Note - it's normal to get readings of up to 500-550 psi on a TDI engine, this is considered normal. The specs above are on a new engine, after the engine is broken in you may see higher compression. Keep in mind that high altitude will lower compression test readings.
Excessively low engine compression can be piston/cylinder wear, bad valve, bad head gasket, or a cracked engine. Diesel fuel is an oil so the pistons and cylinder walls don't wear out as fast a gasoline engines with the accumulation of mileage.
For the VW Passat TDI compression test, see this article.
If you have a PD engine (BRM, BEW, BHW), also see the Passat article since some tips apply to your car.
Clean the area around the glow plugs. The glow plug wiring harness is
marked in red below, the green mark is for another writeup. The area around them tend to
collect dirt and sand, things you don't want falling into the engine. You
can use compressed air or wad some paper towels and use some carb cleaner on the
metal to clean up. Always wear eye protection!
Warm up the engine to operating temperature. The metal parts of an engine expand when they get hot, so ice cold and warm engines will have slightly different compression. Remove all the glow plugs. Refer to glow plug replacement for more details. The below picture is from another "how to" so ignore the red circle. It illustrates the glow plug harness and #1 glow plug removed. Note that the #1 spot was cleaned, the other spots were still dirty.
Take your compression tester and find the correct adapter. Remember: do NOT use a gasoline car compression tester because the ranges are completely different. If the compression tester isn't holding pressure, a common problem is the stem inside the adapter coming loose. Use a valve stem tightening tool or very thin pliers to turn the spring loaded stem inside the adapter clockwise.
Test by threading the adapter GENTLY into the glow plug hole. If it does not go in smoothly for at least a few turns, you may be stripping the threads - STOP and check it. In some cars, the glow plugs do not go straight into the cylinder head, the threads are at an angle, so beware of parallax (viewing error)! Please read 1000q: mechanics tips - parallax for more details on viewing errors. One trick you can try is to turn the screw/adapter/bolt/etc., counterclockwise to loosen it and get the threads seated, and so you can "feel" the threads, then turning clockwise to tighten.
Attach the compression tester and disconnect the fuel pump relay or plug. On 2003 and earlier VW diesels, just follow the wires coming out of the fuel injection pump to find the plug. In the above picture, removing the black plug at the center-bottom of the picture will keep the fuel shut off solenoid from letting fuel pass and keep the pump off. Mk4 cars are similar. You DON'T want fuel getting injected into the engine while you are testing it because the engine will try to run on its own! Again, make sure there are no sources of ignition or sparks nearby!
Make sure the car is in neutral or "Park" before cranking the engine. Note that you want all the cylinders not being tested to have their glow plugs removed so that they are not building compression. This absorbs the energy of the engine cranking, and can affect the compression readings and unnecessarily drain the battery. Use an assistant to turn the engine to "start" for 4-5 engine cranking revolutions so you can note how far and how fast the compression tester needle moves. Keep cranking the engine until the compression does not rise any further. Then do it again to double check. Move to the other cylinders and write down the values so you can compare them later.
Unlike a gasoline engine, adding oil is not recommended due to the high compression in the cylinder. You risk having the compression tester be damaged or personal injury due to the cylinder firing. Remember, a diesel engine can run on oil and the peak compression reached from the combustion process is in the thousands, way beyond the range of the compression tester. In addition, if you do a compression test and the results are bad, you can do the test again. There would be no point in adding oil since you already know there is some problem either with the gauge or engine.
If the readings are still unsatisfactory, replace the glow plugs and start the engine. Run it until the engine is fully warmed up and repeat the test. If the readings are still unsatisfactory, then there may be a problem with the compression tester (may be inaccurate/broken), a poor seal or stripped threads around the glow plug hole, bad fuel injector seals/washers. More serious would be bad valves, rings, pistons, or worn cylinders. At this point, you can be sure that something is wrong with the engine or compression tester.
Also keep in mind that two different compression testers may show different readings on the same engine. Professional grade testers tend to be more accurate then cheaper gauges which can show slightly higher or lower readings. In this case, don't worry about a slightly low reading within spec- as long as the compression is consistent across all of the cylinders and the engine idles well, starts in the winter, and isn't pouring smoke, it is acceptable and I wouldn't worry about it. Keep in mind that high altitude will also lower compression test readings.
For example, VW diesel engines have a spec of 360-450 psi with new engines. It is only considered a bad engine if the reading is below 276 or there is a difference of more than 73 psi between cylinders. That is a lot of allowable variance. To put it in perspective, gasoline engines typically can have no more than 15 psi difference between cylinders on engine compression of about 125-180 psi, depending on the car. If you need more detailed information about the condition of the engine, then a leakdown test or a better compression tester is recommended. Consulting a mechanic would also be a good next step.
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