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Discussion Starter #1
Obviously I know it matters, but,

I recently saw an ad for another e34 m5 and contacted the seller. Its in good shape and rode fine, but upon a compression test, one cylinder was 155, with the rest being 175-180. Now thats out of range, but the low is still "decent". Now I know its all based in relation to other cylinders, but my question is:

How much power would be lost from just that one cylinder? Is it not worth looking into? Any way to get the compression in that one back up? There was no smoke, no misfire.

Thanks for the help, I might scoop this one up based on replies :haha:
 

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Based on the fact that the car drove fine , I wouldn't let those leakdown results put me off an all round decent car .
 

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maybe...

only if you get a great deal (<$10k) because that engine is going to need a rebuild in the next 50k miles (in my opinion). absolute compression #s are less important than variation between the cylinders. Blown head gaskets will only have ~30 psi (I think), so whatever has happened was not catastrophic. But it is probably the beginning of the end for that motor. how many miles? If less than 100k then it's probably worth the risk.

Do a leakdown test. That should tell you what part of the system is leaking. If it's valve seats or something in the head, then it could be reasonably cheap to pull the head and repair just that cylinder's valves.
 

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Discussion Starter #4 (Edited)
Well Im more concerned about power loss currently. I would hate a rebuild, but as you said, 25psi loss doesnt seem dramatic.

Could adjusting the valves bring it up? I dont want to buy a car that is missing 25hp because of a low cylinder. (How much would actually be lost?)
 

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Discussion Starter #7
Well I dont have a leakdown tester, nor do I want to go to the extent of paying for a test if this is just a second car that I dont need.

I think Im going to pass.
 

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You don't need a tester, just an airline and a spark plug hole fitment (like the end of the compression tester) - you blow air into the spark plug at TDC and see how much comes back out the oil cap!
 

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Take out the other spark plugs . Turn the motor ? till the cylinder is on compression , Then stethescope (or steel / plastic rod ) 1st on inlet manifold , then on exhaust manifold - other end of course rammed in your ear ! you can decide if it is inlet or exhaust valve leaking. Same for the cylinder block down low for the rings.
Valves produce smoke when leaking usually no big oil loss . & often by throttling off !
Rings smoke / also out of the oil filler & usually use more oil.

Humble opinion by non mechanic (willing to learn ! )

mm
 
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