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E30 (1982 - 1993)
God's Chariot. The E30 was produced primarily from 1982 through 1991. The cabriolet was the one exception which was produced through 1993.

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  #1  
Old 03-02-2013, 05:57 PM
DaveD DaveD is offline
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Mein Auto: 325ic
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
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Old 03-02-2013, 06:41 PM
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hornhospital hornhospital is offline
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Stuck valve.

By the way, 90 dry compression is awful. Should be in the 150 psi area. !80 would be really good.
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Last edited by hornhospital; 03-02-2013 at 06:43 PM.
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Old 03-02-2013, 06:47 PM
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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
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Old 03-02-2013, 08:05 PM
DaveD DaveD is offline
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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.
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Old 03-03-2013, 02:12 AM
tim96 tim96 is offline
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do you have the throttle open when doing the test? if your gauge is accurate something is up, 90 is low
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Old 03-03-2013, 06:50 AM
DaveD DaveD is offline
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If valves are sticking, is there a good fuel or oil additive I can use to clean both intake and exhaust valves? Thanks
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Old 03-03-2013, 07:07 AM
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Quote:
Originally Posted by DaveD View Post
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
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Old 03-03-2013, 07:08 PM
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downhiller downhiller is offline
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could also have a bad head gasket.
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Old 03-04-2013, 09:37 AM
FactoryWheel FactoryWheel is offline
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Originally Posted by downhiller View Post
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.
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