I have a 1991 560SEC that has started to smoke and use excessive amounts of oil. It does not foul spark plugs and runs fine. This started pretty much all at once. It uses two quarts of oil very quickly (approx. 250 miles) and does not smoke when it is two quarts low. Piston rings? Valve guides? Or maybe some type of crankcase ventilation problem. Any help would be appreciated.

My 1985 500sel euro ( pretty much we have the same engine) I had the same problem i re did the gaskets and had the heads reconditioned. how high milage is yours i did mine at 187k and it has 215 now with the millage goiong up it will burn more oil but the amount your burrning was like my dodge caravan nit need new gaskets also. It cost me about 3,000 dollars but i am good friends with my mechinic it may cost up to 8,00 depending whats wrong. But i would have it checked out as soon as possible beacuse you rellay need to take care of it it may be something very wrong or just something little like a bolt. ( exuse my spellin)

No, this is not a gasket problem. There is no external oil leakage. This is an internal problem, I'm just hoping someone can explain the crankcase ventilation system to me. I was hoping that it could be a ventilation problem rather than rings or valve guides. Thank you for your reply.

you might wanna contact mercedes benz they usally will tell u

It's never rings and as such it is never a breathing problem. If you aren't fouling plugs with that much consumption it must be even in each cylinder; which is good as if one valve guide seal was using that much oil it would be the result of a bad guide.

Valve guide oil seals are the answer. We usually pull the plugs and go for the cylinders with the most carbon on the plugs first. This is to try and find the bad guides first before replacing all the seals. It's a 5.5hr job to do all cylinders.

There is no diagnosis, you just do it. If you come across a bad guide then its valve job time.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician