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Old 05-15-2014, 09:49 AM
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rover67 rover67 is offline
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Join Date: Nov 2007
Location: Boulder, Co
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Originally Posted by Rzeppa View Post
No, not really. Compression tests are not done at engine operating temperatures, they are always done at ambient temperatures, in order for the results to be meaningful for comparison to specifications. Diagnosing a fault is a completely different scenario and as such testing while at operating temperatures is perfectly acceptable. Most of us do compression tests to see whether we're due for a rebuild or how well our rebuild went! Also, the old "put some oil in each cylinder" test is to see if our low compression is due to worn rings or a valve not sealing well.

Totally agreed about the expansion causing different conditions in compression, including exposing a fault that might not be observable at ambient temperature.
oil in a cylinder making compression go up means worn rings, not valves really.

either way i'm not sure i'd freak out over the variance or even the number itself. Sure it might be worn, but the thing you should figure out is why #5 is low. super easy to do like Robbie said and apply shop air while it's at TDC, valves closed. listen for air from intake, exhaust, and PCV. look for bubbles in coolant. you can do this with the bits from a cheap HF leakdown kit. If it has that low a compression the leakdown numbers are gonna suck anyways so figure out where the air is going. Just be careful.. when you put pressure to a cylinder at TDC it can come off of TDC and spin the motor.... which if you have a manual (i know the 80 is an auto) can move the rig, or if you have a wrench on the crank pulley that you used to move the motor, can hit you in the face.
Marco Capote
85 FJ60
82 FJ40
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