Compression test.. what to look for

i'm sure this has already been asked a ton of times, but I was unable to find what i need to know by searching the posts.

what PSI am i looking for in a compresssion test? what's normal for a 4.0 OHV? what variation range in pressure is still considered normal and acceptable? what would indicate problems? anything i need to pay close attention or look out for? TIA

Compression should not vary anymore than 10-15 psi between cyclinders. It can run different on any motor. No motor will run the same compression. Most motors run anywhere between 95-210. If you have a cyclinder that is lower than the others than squirt just a little motor oil in it and run the compression again. If it doesn't build upi then you have a bad ring. Let me know what they are and I'll try to help with anything else. Oh yeah make sure to warm the motor up to operating temp before running the check. It makes the cylinders more even.

Originally posted by MSTNGGUY88
Compression should not vary anymore than 10-15 psi between cyclinders. It can run different on any motor. No motor will run the same compression. Most motors run anywhere between 95-210. If you have a cyclinder that is lower than the others than squirt just a little motor oil in it and run the compression again. If it doesn't build upi then you have a bad ring. Let me know what they are and I'll try to help with anything else. Oh yeah make sure to warm the motor up to operating temp before running the check. It makes the cylinders more even.

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Thanks for the info! It may be a few days before I get time to do it, but I will post the info as soon as I get it. I really appreciate the help.

Just to add to what Mustang Guy said about the compression. Anything below 95 psi means the motor is junk

yep yep. rings bad. thanks man. i always leave little things out. don't want to scare him yet.

When I had my compression test done at 100,000 miles, all my cylinders were between 155 and 160 psi. Just to give you some numbers as a reference point.

peace

Mike

If you check your compression and find that 1 or more cylinders is lower than the others (more than about 15%) there is a test you can do to see if it is rings or something else..

When we checked our last we had 160psi on all of the cylinders except 1 which had 60psi..

Put a little oil in the cylinder you are testing and then test the compression. If it comes up to normal or at least a bunch higher, it is most likely the rings that are toast. IF the compression doesn't change at all (like it did on ours) then it is headgasket or higher (cracked heads, bad vavles etc).. On our X it was cracked heads and recessed valves..

Thanks for all the tips and suggestions!

I finally got my compression gauge at sears thanks to a gift card Anyway, how do I disable the injectors so they don't spray fuel everywhere? My Haynes manual doesn't seem to have anything on doing a compression test, or at least not that I can find in the index. The directions that came with the gauge are of no value as far as getting started goes. (Anyone know what page it's on in the Haynes manual 36024?) thx

Nevermind, I just answered my questions on how to do it. I finally found the info in my Haynes manual. It's on 2D-6.