1. If my oil rings are bad would it show up in a compression test or a diganostic's?
2. My car smokes a bit but only does it when i use the gears to slow the car down (engine braking) and then give her about half throttle straight after I get a puff of smoke. Can someone explain this if possible?
Andy, I am not a mechanic, but I would suspect that a compression test would not be able to determine oil ring leakage. It would be a measure of the total...If that was low (compression), and you were puffing oil on the deceleration, the assumption could be made it was an oil ring...Is it using lots?..I see you have 177K on it..I am at 140K, but have no issues..
It was using a huge amount until i got the oil seporator changed now it use's about 1 litre every 800-900 km. It does not smoke when slowing down using the gear but does when I accelerate after this. If I just pull away slowly, no smoke.
A compression test will tell you whether you have a complete seal at the oil rings, intake, and exhaust valves. It does not isolate any one of the three.
Having said that, as a general rule, when compression is low the first culprit is rings, not valves.
There are 2 fairly obvious signs in addition to a low compression test: blueish exhaust (oil burning off) and fouled spark plugs.
If either or both of those are present (and it sounds like you have at least the exhaust) and you have low compression it is time for new rings. While in there it probably makes sense to replace the valve lifter springs.
I am starting to think that maybe BMW have an acceptable level of compression, because I have had the test done and they said everything is ok, could this be correct?
I have the compression test results, could somebody tell me what the figuars would read in an engine that is running perfectly so I can do a comparison against mine.
Last edited by andy777; 3rd November 2007 at 15:47.
I am starting to think that maybe BMW have an acceptable level of compression, because I have had the test done and they said everything is ok, could this be correct?
I have the compression test results, could somebody tell me what the figuars would read in an engine that is running perfectly so I can do a comparison against mine.
If I recall, Lscman has posted on proper technique for doing compression tests as well as interpretation...Search.
You will need to ensure that the test was done properly- as with everything you have a BMW shop do, you should not assume "BMW" on the building ensures they did it right...
According to some other posts, BMW says acceptable is 175-205 psi. What were your results?
d-
My result range between -545 / 432, so I now have no idea whats going on can somebody tell me how read these figuars.
TEST1
1: -545
2: -75
3: 202
4: -40
5: 432
6: -406
7: 10
8: -354
It could be that you are sucking in oil from the exhaust guides(bad seals).
Start looking for new replacment cat. converters if that has been going on for awhile.
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I can safely say that I have never in my life seen a negative compression number. Doesn't matter what scale you use, compression is always measuring pressure, and that pressure is always measured relative to atmospheric, which means it must be positive.
Go back to whomever did the test and ask them what the heck they measured, get their baselines that they compare against.
Regardless of understanding or not understanding the scale, the rule of thumb is that you should have no more than 5% variance across all cylinders.
and then give her about half throttle straight after 
I get a puff of smoke. Can someone explain this if possible?
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