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Compression Testing Your Engine
 
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Pelican Technical Article:

Compression Testing Your Engine

Steve Vernon

Time:

2 hours2 hrs

Tab:

$0

Talent:

**

Tools:

13/16-inch spark plug socket, extension, compression tester

Applicable Models:

Porsche 944 Turbo (1986-89)

Hot Tip:

Remove the fuel pump relay

Performance Gain:

Information about the health of the engine

Complementary Modification:

Leak down test

A compression test is a great part of testing the overall health of an engine. By running the test, you can determine the amount of compression each cylinder is making. Large differences between cylinders, low compression or extremely high compression are all things you need to know to determine the state of your motor.

The compression test is relatively easy to perform. With the exception of getting the compression-testing tool, you can do it in your driveway in under two hours.

You will need to borrow, rent or purchase a compression-testing tool. This tool is really a single use only tool, so check around, as most auto stores will rent you one for a small amount or possibly loan it to you for a short time for free.

This is a compression tester.
Figure 1

This is a compression tester. Most local automotive stores have a tool rental program that will allow you to borrow one. The tool can really only be used for testing the compression of the cylinders. Unless you are a professional mechanic, you really want to try and avoid buying one.

You will need to remove the spark plugs, but only one at a time, as you run the test on each cylinder individually.
Figure 2

You will need to remove the spark plugs, but only one at a time, as you run the test on each cylinder individually. This photo shows a close-up of access to the plugs. You can see there is easy access to the numbers 2, 3 and 4 plugs (red arrows), while number 1 is partially hidden (yellow arrow). It is much easier to run the test on the number 1 plug, if you remove the turbo pipe. While it is possible to change the plug out without removing the pipe, if you are doing this for the first time, do yourself a favor and remove the pipe.

Remove the three fasteners securing the air box lid to the base (red arrows).
Figure 3

Remove the three fasteners securing the air box lid to the base (red arrows).

Disconnect the line running to the turbo pipe with a 17mm wrench.
Figure 4

Disconnect the line running to the turbo pipe with a 17mm wrench. Note that there are two washers (red arrows) that need to be replaced if you remove the line.

Disconnect the clamp over the pipe by reaching down and unclipping it.
Figure 5

Disconnect the clamp over the pipe by reaching down and unclipping it. Then remove the lid. Next, use a flathead screwdriver and loosen the hose clamps on both ends of the pipe (red arrows).

Remove the pipe from the engine (red arrow).
Figure 6

Remove the pipe from the engine (red arrow).

With the pipe removed, you can see the spark plug wire, boot and how much more room you have to work (red arrow).
Figure 7

With the pipe removed, you can see the spark plug wire, boot and how much more room you have to work (red arrow).

Gently wiggle and pull the wire and boot from the top of the plug.
Figure 8

Gently wiggle and pull the wire and boot from the top of the plug. There is a special tool for removing the wires that fits in the rings on the bottom of the boot (red arrow). If you take your time and care, you can remove the wire without using it. Do not just grab the connector by the wire and pull, as you run the risk of tearing or separating the cover (yellow arrow).

Using your 13/16-inch spark plug socket and an extension to clear the intake runner, remove the plug from the engine (red arrow).
Figure 9

Using your 13/16-inch spark plug socket and an extension to clear the intake runner, remove the plug from the engine (red arrow). The engine should be cool to the touch, especially when installing the new plugs. You want to make sure the plug is properly seated to avoid cross threading the plug in the head. Do this for all four plugs.

The compression tester will come with different size threads (yellow arrows) on the end as well as rubber gaskets to help seal up the test (red arrows).
Figure 10

The compression tester will come with different size threads (yellow arrows) on the end as well as rubber gaskets to help seal up the test (red arrows). Some testers will also come with accessories to reach recessed openings. With the plug removed, insert the threaded end of the compression tester. Make sure that the threads fit correctly and then tighten hand tight. You do not need to use a wrench. Hand tight will be fine.

With the threaded end inserted into the spark plug hole, connect the hose to the gauge.
Figure 11

With the threaded end inserted into the spark plug hole, connect the hose to the gauge. The gauge should be zeroed. If it is not, there will be a release valve on the side.

Remove the fuel pump relay (red arrow).
Figure 12

Remove the fuel pump relay (red arrow). On older cars it is located up under the dash. On newer cars it is located in the engine bay. Check with your manual and make sure you remove the proper relay. This will allow the motor to turn over without injecting any fuel into the combustion chamber.

If you have a friend helping, have them turn the motor over for five to six compression cycles or until the gauge reaches the maximum compression (red arrow) it is going to get to.
Figure 13

If you have a friend helping, have them turn the motor over for five to six compression cycles or until the gauge reaches the maximum compression (red arrow) it is going to get to. If you are working alone, the gauge should hold enough pressure to give you enough time to crank the motor and then get out and look at the gauge. There is a release valve on the side of the gauge. Release the pressure between tests and when moving to different cylinders. Remove the tester, and replace the spark plug. Then perform the test on each of the remaining cylinders. All cylinders should have the same compression numbers within 8%. Check with your manual to determine what the compression should be for your engine.






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