compression test question

[kickitandholdit]

i know this question is dumb. i decided to buy a compression tester instead of getting it done. so i disable the ignition and pull all the plugs out and blah blah blah but what about the gas? just leave it alone or do i need to stop the gas when doing the test? also do i just put like a table spoon of oil in the spark plug hole and do the test again to see if the rings are good? and what differences in #s should i look for? like more than a 20psi difference between the cylinders is bad or what? and when i put oil in it to test the rings what psi differences mean what? thanks in advanced

 

[Old Dog]

I never disable the gas, never seemed to hurt anything.
As for how much oil, I'm not sure, if needed I just stick the oil can ( thumb pump style) in and give it a pump or 2. If the pres goes up it usually means bad rings, if not it means your leaking around the valves.
It's normal to have a 10 lb. difference between clys. I start to wonder whats happening if it gets over 30lbs or so.
Not sure what the pros say this is just my backyard way of thinking.

 

[kickitandholdit]

thanks for the input. i have instructions with the tester but having someones input can often make things easier

 

[Thick]

i know this question is dumb. i decided to buy a compression tester instead of getting it done. so i disable the ignition and pull all the plugs out and blah blah blah but what about the gas? just leave it alone or do i need to stop the gas when doing the test? also do i just put like a table spoon of oil in the spark plug hole and do the test again to see if the rings are good? and what differences in #s should i look for? like more than a 20psi difference between the cylinders is bad or what? and when i put oil in it to test the rings what psi differences mean what? thanks in advanced

Best bet is to buy a differential compression tester. Its a little easier and can help you diagnose issues. It hooks up to compressed air and you introduce about 80 psi to the cylinder at TDC (You'll know you are at TDC for that cylinder because it won't try to turn over) then you can see what pressure its holding and if it isn't holding pressure you can listen to find where its getting past.

If you hear air in the oil filler cap its probably the rings, exhaust its a stuck or bad valve, though the carb its probably a bad or stuck intake valve.

Never tried it on a jeep but it works great on airplanes.

I believe you can find the compression tester on ebay for about $30

 

[kickitandholdit]

ok i compression tested it w/o oil on the pistons. and they were all about 135 or 140psi

What psi is it supposed to be? i looked in the haynes manual and couldnt find it. i would test it again with oil on the pistons but... i lost my oil and didnt feel like going to get any more.. maby tomorrow.

 

[Thick]

ok i compression tested it w/o oil on the pistons. and they were all about 135 or 140psi

What psi is it supposed to be? i looked in the haynes manual and couldnt find it. i would test it again with oil on the pistons but... i lost my oil and didnt feel like going to get any more.. maby tomorrow.

should be 120-AMC 150 so you are looking pretty good.

 

[kickitandholdit]

should be 120-AMC 150 so you are looking pretty good.

awesome thanks for the info. so my jeep smokes a bit but the compression is fine... im confused again haha here goes more research

 

[Thick]

awesome thanks for the info. so my jeep smokes a bit but the compression is fine... im confused again haha here goes more research

burning coolant?

 

[kickitandholdit]

burning coolant?

if i was burning coolant wouldnt the compression on one of the cylinders have been low? and the dip stick looks normal and ill half to go check the coolant level.

 

[Old Dog]

Does it smoke all the time or only on start up?
What color is the smoke?

 

[Thick]

if i was burning coolant wouldnt the compression on one of the cylinders have been low? and the dip stick looks normal and ill half to go check the coolant level.

bad head gasket wouldn't necessarily cause a drop in compression..

 

[muffler bearing]

I am about to do a compression test on my 84 ......I was testing out the new freeze plug I put in and heard an awful racket coming from the engine and immedatley shut it down. Then started it and it as gone. I had a guy that insisted on comingto look at to buy it put a dollar to the exhaust pipe any it sucked it and blew it out. He stated it may have a bad valve....anyone had to deal with this? It does sound like it has a small skip now.

 

[kickitandholdit]

I am about to do a compression test on my 84 ......I was testing out the new freeze plug I put in and heard an awful racket coming from the engine and immedatley shut it down. Then started it and it as gone. I had a guy that insisted on comingto look at to buy it put a dollar to the exhaust pipe any it sucked it and blew it out. He stated it may have a bad valve....anyone had to deal with this? It does sound like it has a small skip now.

i dont know but im interested to see this answer haha

 

[kickitandholdit]

bad head gasket wouldn't necessarily cause a drop in compression..

so do i need to do a leak down test?

if i had a bad head gasket could i do a compression test and just see if one cylinders pressure falls or falls faster than the others? can i do this to see if i have a bad head gasket?

 

[Thick]

so do i need to do a leak down test?

if i had a bad head gasket could i do a compression test and just see if one cylinders pressure falls or falls faster than the others? can i do this to see if i have a bad head gasket?

Leakdown would be a good idea. I assume it would have some noticeable drop under constant pressure if you have a bad head gasket.

 

[fire3096]

awesome thanks for the info. so my jeep smokes a bit but the compression is fine... im confused again haha here goes more research

If it smokes on start up then clears up, it is going to be valve seals. if it cont. to smoke while driving, color of the smoke will help determine that.
black - fuel
white - colant (poss head gasket)
blue - oil (poss rings)

did i miss any?

that has been my exp. with that. and a leakdown test can REALLY help and the leakdown kit can let you change valve seals as well....easily done with spring tool - couple hrs.