Compression

Kudos to the person who can answer this question: I have a 1991 325ic. I checked the #5 cylinder compression by cranking, but not starting; compression =90 psi. Then, I checked the compression with the car at idle; compression =45 psi. What would cause the compression to decrease while the engine is at idle? Thanks dave

Stuck valve.

By the way, 90 dry compression is awful. Should be in the 150 psi area. !80 would be really good.

compression on a running motor will always be less than static. this is because the piston is traveling at a faster rate of speed.

think if it as a revolving door. the slower it moves the more people can fit in each compartment. when it spins faster, fewer people get in.

rule of thumb is the initial static compression reading is about what to expect for an idle compression.


but 90psi seems rather low. what did the other clinders measure?? did you do a wet compression test as well??




df

Thanks guys. When cranking, without starting, all cylinders are at 90 psi. I added some oil into the cylinder, with no increase in compression on #5. Thanks again.

do you have the throttle open when doing the test? if your gauge is accurate something is up, 90 is low

If valves are sticking, is there a good fuel or oil additive I can use to clean both intake and exhaust valves? Thanks

Originally Posted by DaveD

Thanks guys. When cranking, without starting, all cylinders are at 90 psi. I added some oil into the cylinder, with no increase in compression on #5. Thanks again.

all cyl @ 90...check

#5 no increase with wet compression?? hmmmm....not a good sign. what about the rest of the cylinders??

the throttle can be @ wide open (wot), but i have done both ways on many vehicles and not had any issues with the results.

is the battery up to spec??

what are the issues that you are having with the car that led you to do a compression check in the first place??

did you check the running compression on the other cylinders for comparison??




df

could also have a bad head gasket.

Originally Posted by downhiller

could also have a bad head gasket.

Next up should be a leakdown test. That'll tell you if it's the head gasket, intake valves, exhaust valves or rings.