Dry Compression Test
If your VW starts, I suggest you perform the compression test with a slightly warmed up engine. This will help to get the most accurate result of the health of your engine.
In case you're wondering why, it's because both the engine block, pistons and piston rings, and cylinder head valves will expand when heated. This expansion will create a slightly better seal than when the engine is completely cold and this will have an effect on your compression tester readings.
Now, if your VW doesn't start, don't worry about this, you can still do the test and get very useful data that'll help you find out what's going on.
Before you start, take a look at the whole article and familiarize yourself with all of the steps. Please remember to always think safety first, since you'll be working around a cranking engine.
OK, to get this show on the road, I'll first explain the test steps. At the end of the test steps, you'll find two possible test results that will help you to interpret your specific test results.
Let's get started:
Disable the fuel system. You can easily do this by simply:
- Disconnecting the fuel injectors from their electrical connectors.
- Or, disconnecting the fuel pump relay.
This step is important, so don't skip it.
Disable the ignition system. This applies to you if you're not testing all of the 4 Cylinders. You can easily do this by simply:
- Disconnecting the COP ignition coils from their electrical connectors.
Remove the spark plugs. As you're taking them out, be careful and don't drop any of them on the floor, or you could cause the spark plug's ceramic insulator to break, and this will cause a misfire!
Install the compression tester. Thread the engine compression gauge into the spark plug hole for the number 1 engine cylinder. Hand tighten the compression gauge only! Do not use any type of tool to get it tight.
When ready, crank the engine. It's best to have a helper crank the engine for you so that you can keep your eyes on the compression tester's gauge. Once the needle on the gauge stops climbing, have your helper stop cranking the engine.
Write down the compression value. Record this compression reading on a piece of paper. Include the number of the cylinder this reading belongs to. Now repeat steps 1 thru' 6 on the other cylinders.
Interpret the results. After testing all cylinders and having written down all of your compression test readings, now you need to interpret the results.
Interpreting The Results Of The Engine Compression Test
CASE 1: You got a reading of 100 PSI or less (less being 0 PSI) on all of the cylinders you tested. This test result indicates you've got serious engine mechanical problems.
It's absolutely rare for the engine to get to the point of 0 PSI compression on all cylinders, but if it were to, this would usually means a broken timing belt.
What is common, if you have a very high mileage engine, is low compression across the board. Low usually means anything under 120 PSI (although the service manual says 100 PSI is the minimum). If your engine has reached this point, it's also smoking from the quart of oil it's burning every few days. The other symptoms you'll see is that the idle will be very rough.
CASE 2: One or two cylinders gave a low compression value. This might be normal for your VW's engine, since each cylinder will not give the exact same pressure value.
What is NOT normal if the pressures vary by 15% or more. That's right, the individual cylinder compression readings of each engine cylinder can not vary more than 15% and this is how you can find out:
- Grab a calculator and multiply the highest compression reading that you recorded by 0.15.
Let's use the following compression readings to explain the point:
- Cylinder #1 175 PSI.
- Cylinder #2 160 PSI.
- Cylinder #3 165 PSI.
- Cylinder #4 95 PSI.
- The next step is to do the math: 175 x 0.15= 26, 175-26= 149.
- So then, 149 PSI is the lowest possible compression reading that any one of the rest of the engine cylinders can have. Any compression reading below this and that engine cylinder will misfire.
- This means that cylinder #4 is the one causing the misfire.
The next step is to do a ‘Wet’ compression test on the dead or low compression cylinder and this test is explained in the next page.