anyone know the compression range for a *cold* compression test?

i know its like 85-100psi or something like that, but that is after engine has been warmed up.

just bought my rx with belief of a blown motor, but to my surprise i got it running

my problem is the engine wont start unless i de-flood it by removing the plugs and squirting in some ATF

the engine will run smoothly then, it even idles fine after the 2000rpm thing it likes to do for 10 minutes......

i assume this is caused by having either one of two things (maybe more suggest please)

i have low compression (not enough to start without help, but enough to run off of)

or

i have a faulty injector that is drowning my plugs before they have a chance to work.

i have not compression tested it as of yet, waiting to borrow my buddy's tester, but i would like to know the psi range of a (not-blown) motor ASAP, to debate a rebuild.

please help with any suggestions, comments

 

I've seen some people say that a cold test will actually yield a higher test result than testing it warm. I haven't compared the two and I haven't read of anyone doing so (doesn't mean it hasn't been done, if you could weigh the threads on this board, you would sink the earth). Go ahead and do the compression check, if it's below 90 or below 85 for only one or some of the compression faces then you should look into what the issue is. Sometimes a seal gets stuck, usually when crusted with carbon; sometimes a seal is cracked or completely broken. Have you read up on compression tests? Such a test can only tell you so much about its condition, but it is an important first step.

 

Pressure will always be greater at higher temperatures, so a good estimate would be:

100 PSI / 363 Kelvin = x PSI / 298 Kelvin
x PSI = 82.1 PSI

363 Kelvin = 200 degrees F
298 Kelvin = 77 degress F

But then again if your car has a stuck/broken apex seal you'll be seeing 0 PSI on two rotor faces anyway. How does it sound when it's running, does it sound like its missing?

 

I doubt the ambient pressure is going to make much difference. The temperature will probably make more of a difference, and cold air should yield higher compression numbers since it's more dense (you pack more air into the chambers with basically the same turning effort). I'm sure doing the test at -50*F wouldn't be great though because then your oil will be so thick it won't seal the chambers well, so for each individual motor I'm betting there's a specific ambient temperature at which you'd see the highest compression results (it will also depend on the kind of oil you're using, maybe somewhat if you premix, and possibly some other things).

 

wow I was wondering the same thing glad I came across this.

 

If you can get the car running and warmed up, then why are you worrying about testing it cold? There are different opinions about good compression range, but I think most everyone will agree that for an original motor, 85 PSI and up is pretty good, obviously the higher the better.

 

i really appreciate all replys guys, thanks. i have not done the test as of yet, (school, work, other things) besides i dont own a tester, and my (potential) mechanic is incredibly hard to get in touch with and, from my experiance so far, very unreliable.

I am planning on testing tomorrow, thunderstorm or not, if a buddy of mine pulls through. (I should really just buy my own tester).

I have read that a cold test would give higher compression, but i dont want it to lead me on thinking everything is fine, when its not, which is why i was asking for some numbers.

texFCturboII, i can get the car running and warmed up, however, i am cheating to add more compression (by injecting ATF) and i'm worried this will affect my results.

thanks again guys, ill keep you updated if anyone is still interested.