Bad Compression In The 6th Cylinder - BMW Forum - BimmerWerkz.com
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#1 (permalink) Old 12-22-2004, 01:35 PM
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I had a mechanic take a look at my 2003 BMW 325i before I got it sent to the dealership. The car was running hard and misfiring. If I deacclerated to fast the car would stall out. In order to get the car running, you would need to give it some gas. And acceleration was sluggish at best.
He ran a compression test and found that the compression in the 6th cylinder was not correct. He is assuming that it is a bent exhaust value. What is the likelihood off an exhaust value bending? Any other possiblities? Has this happened to anyone else? Anyone else experience something similar? Thanks
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#2 (permalink) Old 12-22-2004, 01:51 PM
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It is possible to bend an exahust valve. This is commonly done by misshifting and over-reving the motor. e.g.: shifting into 2nd instead of 4th. While not a common thing it has happened before.
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#3 (permalink) Old 12-22-2004, 02:01 PM
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Quote:
Originally posted by Mach 5@Dec 22 2004, 12:51 PM
It is possible to bend an exahust valve. This is commonly done by misshifting and over-reving the motor. e.g.: shifting into 2nd instead of 4th. While not a common thing it has happened before.
Ben
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Wouldn't an over-rev bend more than just one of the engines exhaust values? Wouldn't that be more consistent through out all the cylinders instead of just one?
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#4 (permalink) Old 12-22-2004, 03:43 PM
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not necessarily, it may be too short of a poeriod to bend them all.

to test whether this is a valve at all (as opposed to stuck ring):

run compressionn test.
pour a little oil into the cylinder (a little means about 2-3 ml at most).
re-run compression test.

if it normalizes it is rings. if it does not it is valve.
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#5 (permalink) Old 12-22-2004, 04:35 PM
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Quote:
Originally posted by peterkulesza@Dec 22 2004, 02:43 PM
not necessarily, it may be too short of a poeriod to bend them all.

to test whether this is a valve at all (as opposed to stuck ring):

run compressionn test.
pour a little oil into the cylinder (a little means about 2-3 ml at most).
re-run compression test.

if it normalizes it is rings. if it does not it is valve.
[snapback]256479[/snapback]
I previously misshifted and bent all the exhaust values on a previous car. I know that the car was not over-revved and after trying your sugguestion, it does like to be a bent value. Is there anything else that may have caused the value to bend? Could the exhaust value have been bad?
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#6 (permalink) Old 12-23-2004, 04:22 PM
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do not know which one: in cases of overreving itcan be ether intake or exhaust, depending on the design (just like when the timing chain jumps or a belt breaks).

in your case, i do not know enough to advise you further beyond suggesting taking the head off and looking at the valve train.

sometimes bad seats in old burnt out valves can result in compression loss: this is unlikely in your car.
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#7 (permalink) Old 12-30-2004, 11:07 AM
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Just my experience, I had missshifted my 2000 323i BMW and I had bent ALL of my valves, intake and exhaust. The car was still driveable as long as I kept the engine revved. Valve job was pretty costy, good luck with it.

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