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Old 02-09-2007, 03:52 AM
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Join Date: Mar 2003
Location: Los Angeles, CA
Posts: 445
excessive oil burning 300SL

1992 300SL w/ 180k miles.

Car burns 1 qt every 750 miles. Even now when using 20W50 oil. No external oil leaks or signs of blowby. Don't suspect a bad cylinder because all the spark plugs look in similar condition and are not badly fowled. I don't notice any smoke from exhaust even when decelerating. How well does an engine like this respond to replacing valve stem seals?

I recal some MBs were plagued with poor quality valve stem seals from the factory. Would this car/engine be one of them? Does this car have a common problem with excessive oil burning from some other cause?

I once replaced the valve stem seals on a 1990 2.6L 190E w/ 250k miles and the oil consumption problem went away. But this was a known problem on these engines.

1972 450SL
1982 300D Turbo
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Old 02-09-2007, 07:01 AM
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If this is an M103 engine, the problem could very well be bad valve stem seals. Have you done a compression test or leakdown test.

I would probably skip the compression test and do a leakdown test and listen for air escaping heard through the oil filler hole. If you do here air escaping through the oil filler hole then you have a cylinder problem, if not it is likely the valve stem seals or valve guides. If this diagnosis proves positive, I think I would just pull the head, do a valve job and replace the valve guides. The bottom end of the M103 engine is very long lived and it would be worth the effort IMHO as long as the cylinders pass the leakdown test.
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Old 02-09-2007, 09:49 AM
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I believe the 300-series R129s were fitted with the M104. Sounds more like the head gasket, but you would see oil pools on the garage floor (and anywhere else you park).

The only other item I would look at is the front timing cover seal. Common area of leakage.
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Old 02-10-2007, 12:23 AM
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Join Date: Mar 2003
Location: Los Angeles, CA
Posts: 445

Thanks for your advise. The compression test or better yet leak down is a good idea before digging in on engine work. BTW, the engine is the 104. Does any one know if these 104s were known to have early failing valve stem seals as were common on some of the early 90s MB straight sixes (e.g. 103 engines)?

The suggestion about the bad head gasket puzzles me. I did't think/know that a bad head gasket manifests itself with external oil leaks. The car shows no signs of a bad head gasket and there are no external oil leaks. Also, the front timing cover seal is not leaking and appears to have already been replaced (signs of non-MB sealant evident). I will ignore the bad head gasket suggestion unless someone else suggests otherwise.

1972 450SL
1982 300D Turbo
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