A compression test will help you to determine the health of your engine by measuring the pressure of the air that gets compressed by the piston rings against the cylinder head valves.
This info comes in handy when you're trying to troubleshoot a hard to diagnose misfire condition, a no-start condition, broken timing belt (although there are easier ways to find out if it's broken), or if you're trying to see if you have a worn out engine on your hands.
The compression test is a test that you can do without having to take your VW to the shop and in this article, I'll show you how to do both a ‘Dry’ and ’Wet’ engine compression test and more importantly, how to interpret their results.
Contents of this tutorial:
Symptoms Of Bad Engine Compression
Low compression in one or several engine cylinders will have a direct effect on idle quality. The symptoms you'll see will be:
- Rough Idle.
- Misfire condition and misfire codes:
- P0300, P0301, P0302, P0303, P0304.
No compression in all of the cylinders will result in a cranks but does not start condition. The symptoms you'll see will be:
- No start.
- Everything else works, for example:
- The fuel pump will activate, so you'll see fuel pressure at specification (if testing with a fuel pressure gauge).
- All of the COP ignition coils will spark.
- If the COP ignition coils are sparking, then this indirectly proves that the crank sensor is OK too.
- The PCM will still activate all of the fuel injectors.
What Tools Do I Need?
The most important tool that you're gonna' need is a compression tester. You can either rent this bad boy from your local auto parts store (Auto Zone, O'Reilly Auto Parts, Advanced Auto Parts, etc.), or you can buy one online.
Since the spark plugs need to be removed, you'll need some of the following basic tools:
- Ratchet wrench.
- Appropriate spark plug socket.
- Extensions for the ratchet wrench.
- Motor oil (for the ‘Wet’ compression test part).
In case you're needing to buy a compression tester and want to save money by buying it online, you can shop here: