The Stovebolt Site Forums The Shop Area Engine Shop Compression check for 235

I need a little help with my engine, I did a compression check this afternoon and they came in at anywhere from 52 psi to 73 psi. I think this is something like 28% difference, this doesn't look good. I ran the engine for about 30 minutes came home and did the check with the engine warm is this the correct way to get a good reading? also the plugs were really black with soot, they have about 100 miles on them, is it running too rich?

1954Chevy5Window,

Black sooty dry plugs are a good indication of a over-fuel condition. You didn't indicate they were oily, so It wouldn't appear you engine is 'pumping' oil.

I think your test procedure was fine. Unless you are trying to pinpoint a specific ailment, the compression check can give you heart palpitations for no real reason.

Your pressure readings weren't at the top of the scale, and they did vary quite a bit between cylinders, but how does the truck run!

Were you searching for a specific problem, or just checking out the health of your truck.

Both of the rigs you have listed are OLD...they deserve to be somewhat tired. If they start and run fine, case closed.

Keep them in tune, change the oil and drive the heck out of them. When they croak, then fix them.


Stuart

It starts good, but runs a little rough and doesn't seem to make the power it should. My burb has a 216 and will run circles around my 235 in my pickup. This is what has got me curious about my pickup. I think I need to check the timing next, huh?

i would check the timing yeahh thats the best bet mine truck did the same .. the best way for timing is do it by ear.. also check points

Your engine has problems. The compression readings are way low and too varied.

Warm your engine, pull the plugs and inject a liberal amount of oil in each cylinder. Check the compression again and compare readings. If this raises the compression, you have weak rings. If there is no change the problem is in the top end, perhaps a bad head gasket or burned valves.

You will never get this engine to run right until these problems are solved.

I forgot to include this quote from the truck shop manual reprint.

"Compression on all cylinders should be 110 pounds or better and all cylinders should read alike within 5 to 10 pounds for satisfactory engine performance."

Do a wet test too, I don't think a cylinder with only 52 PSI will even fire,
Scott

I may have not done the test properly does anyone have a description on how to do this properly? I re-read some instructions on the net and it talked about holding the accelerator wide open for max airflow, I didn't remember to do this would it make the difference?

1954Chevy5Window,

Some of the procedure has been outlined above. I think the general routine would be:

Warm the engine up, then pull all the plugs. This test is done with the key OFF.

Mechanically block the throttle wide open, as well at the choke. I like to use the screw in type of gauge, as it doesn't "pooof" as bad!

A remote starter switch makes the job easier. Crank the engine until the gauge reaches its max reading, and record it. Do this for each cylinder. Maximum reading should be obtained in 4 or 5 compression strokes.

As Cletis pointed out, if you run the test a second time after oiling the cylinders, and the readings increase, you can differentiate between valve problems and ring problems.


Stuart

Thanks for the info guys, I re-tested the compression with the throttle wide open they all came back anywhere from 102 to 112. I feel much better now, I bought a timing light and adjusted the timing and this helped the problem.

I also re-gapped the plugs, what gaps do ya'll use? I think the book called for .035 is this correct? Also should I adjust the timing right on the ball or should I use something different for the gas today?

Thanks again