Topic: Compression Test

[sharpxmen]

I noticed oil in the intake when i took it out to install my pacesetter header, so immediately i thought at least piston rings. Went and got myself a compression tester and tested it at cold (i know is not the best way but the intake and exhaust headers are off at this time). here are the results:
cyl1: 150psi
cyl2: 145psi
cyl3: 125psi
cyl4: 155psi

i will squirt some oil in cyl3 and see if it makes a difference - at this time i am hoping is a valve in cyl 3 so that would make it easier, but on the oter hand i'm thinking that if i do end up taking out the cylinder head might as well throw in a set of moly rings as well.

What do you guys think?
hoping to get some input from the guys that have gotten their engines rebuilt to any extent, any suggestions appreciated - thank you. (I should mention that i never taken apart an AMC engine, so i'm not familiar even with the compression readings on this engine but the difference on cyl3 raises a question).

[aw12345]

Anything less than 10% difference is condidered acceptable.
If you have a compressor at home get a leakdown tester is much more definitive than a compression test. It will show where the compression leaks of too and how much same story there less than 10% leak is acceptable

[sharpxmen]

just  did the test again. different results, but i guess it was pretty hot today so that might have made the difference. this time around the results were
1 - 155psi
2 - 155psi
3 - 140psi
4 - 160psi
all these at cold, engine has not been started in couple of days.

i then squirt some oil in cyl 3 and it got to 150psi - hmm, i guess must  be the rings on that one (i hope the piston is ok). so max - min = 20psi difference, 12.5%. if i compare it to cyl 1 and 2 then is 9.6% -  - should i do the rings or not? i think i'll pass on the rings this time around, get all the other stuff finished since I  seem to keep adding to the list and do it on the next one - i don't think it's anything major at this point that i need to worry about, i'll put some more miles on it and see how it goes (it's on jackstands at this point anyway).

[aw12345]

Try a leak down test, those things are about 40 bucks at harbor freight
The big difference is that it does n ot get affected by carbon built up and you can actually hear where the air goes. Just make sure that you disconnect the pcv hoses from the air filter and valve cover. it will tell you for sure if its valves or a broken piston ring, a broken piston ring would give you lower compression though and quite a lot of blowby

[sharpxmen]

what's a good compression test at cold as far as values go for this motor (mind you at cold since i can't start it right now) - trying to understand where am i at with it. The FSM says not to have it under 100psi warm and from what i know you get higher readings hot than cold - i'm a little in the dark here other than that 3rd cyl to compare to.

EDIT: the engine is in good shape, i guess i'm just a little paranoid - i don't need to add any oil between changes and i never seen it go down, just that i didn't see that much oil in the intake before that's all and got a little scared. According to chrysler fsm i guess it's fine

[aw12345]

100 psi os considered a minimum to get a cylinder to fire and contribute power.
How much above that number is not as important as how far apart the numbers are.
For all practical purposes a leak test it a better choice to determine the state of your engine. You pump a cylinder to 100 psi then look at the second gauge and see what number that one gives you as long as it stays above 90 your engine is good
In other words anything less than 10% leak is good ut will also tell you where it leaks from