Performing Compression Test

Joed,

1. Before you begin, make sure the battery is fully charged.

2. Full the fuel injection fuse.( If its Injected) This cuts power to the injectors so they don�t spray gas all over the place.

3. Unplug the coil connector, you could get a nasty shock if you don't.

4. Remove all 8 plugs and examine them carefully for signs of a problem. If your valve seals are bad in any cylinder, the plug will be fouled to some degree. Signs of oil on the plugs could indicate a ring problem.

5. Screw the compression gauge into the plug hole

6. Push the throttle wide open to open the throttle plate, and keep it at WOT (wide open throttle) for as long as you�re cranking (if you didn�t pull the MPI fuse, there would be a bit of a mess, now).

7. Turn the ignition to the ON position, and hold it there for a count of four revolutions (do not turn the key on-and-off four times). On each rotation, the gauge needle will jump slightly less than the previous one.

It may take more than 4 revolutions to get a stable reading; crank for up to 10 seconds or until the needle stabilizes, whichever comes first.

Note: If your battery is weak, the engine will turn over slower and give you lower readings.

8. The last number the gauge jumps to is the compression for that cylinder.

9. Repeat for each cylinder.

All a "wet test" entails is a bit of oil in the low cylinder.

10. Add one capful (1-2 tablespoons) of oil to the cylinder with low compression.

11. Let it sit for a minute to allow the oil to flow down and coat the sides of the cylinder and piston. If there are any gaps in the rings, the oil will seal them for the next few minutes.


Crank the engine for 4 revs, remembering to floor the throttle.

12. If the reading improved significantly (more than 30 psi), your rings are probably worn.

13. Replace the plugs, preferably in the same cylinders they came out of. You may want to clean or re-gap them first, or put in new ones. A dab of anti-seize on the threads would be a good idea.

14. Remember to put the MPI fuse and coil connector back. You�re not going anywhere without them!

I've never heard of taking all the plugs out- just one at a time...

Everything I've read states to pull ALL plugs, then test one cylinder at a time.

I did my test cold just to get a relative comparison.

Here's my results:

1 - 120
2 - 130
3 - 140
4 - 140
5 - 130
6 - 125
7 - 120
8 - 140

I thought #8 was bad because of oil fouled plug but those results do not seem to comfirm that.

Joe

Thanks Jimbo...

I did have a bad wire... you can read about my recent puzzle in the backfiring thread. Any input would be appreciated.

Joe

The main reason for pulling all the plugs is so the engine will crank a bit faster (i.e. no compression) during the compression test. If you are only trying to make sure the cylinders are all within relative tolerance to each other, then I think either way is fine. Just make sure you test each cylinder the same way.

Make sure you ground the coil wire to the block.

Avoid overheating the starter. Allow it to cool every so often.

I'd hook the water to it since you are cranking the engine over. I have seen a dry run impeller and it ain't pretty.

Testing compression with all spark plugs removed is important, it takes a huge compression load off the starter, you will be doing a lot of cranking and all plugs need to be removed for this test so why would you wish to test with the plugs in the other 7 holes. Pull all and you will get better results and your starter and battery will perform better in the test.
If you are fouling one plug with good compression you probably have either a Valve Stem Seal failure or an intake manifold sucking oil into that cylinder past the gasket.
If the cylinder had a bad plug wire it can certainly foul out the plug.
Hope you got it figured out.