How To Do A Compression Test (VW 1.8L Jetta, Passat, Golf, New Beetle)

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.

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:

1. Rough Idle.
2. 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:

1. No start.
2. 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:

1. Ratchet wrench.
2. Appropriate spark plug socket.
3. Extensions for the ratchet wrench.
4. 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:

TEST 1: 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:

1. 1

   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.
   
   IMPORTANT: This step is important, so don't skip it.

2. 2

   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.

3. 3

   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!

4. 4

   Install the compression tester on the number 1 engine cylinder.
   
   NOTE: Hand tighten the compression gauge only! Do not use any type of tool to get it tight.

5. 5

   Have your helper crank the engine.

6. 6

   Write down the compression value.
   
   Include the number of the cylinder this reading belongs to.

7. 7

   Repeat steps 1 thru' 6 on the other cylinders.

Let's interpret your test result:

CASE 1: Your vehicle does NOT start and you got 0 PSI on all cylinders. This test result usually means that the engine has a serious internal mechanical problem.

The engine might have one of the following:

• Timing belt problem.
• Blown head gasket problem.
• Engine threw a rod.

Any compression value below 100 PSI (even if it is not 0 PSI) means internal mechanical engine trouble.

CASE 2: The compression values differ from one another. This could be normal or it could be causing a problem.

To find out if the low compression values are withing a normal range or not, go to: Interpreting The Results Of The Engine Compression Test.

CASE 3: All 4 cylinders have a compression value of less than 100 PSI. This indicates an internal engine problem.

This test result is usually caused by one of the following:

• Timing belt problem.
• Worn piston rings.
• Worn cylinder head valves.