On my 85 300D, when I remove oil fill cap, I see visible exhaust smoke coming out through the hole. When I place my hand over the hole, I feel the pressure build. With 250,000 miles, I expect some play in the valve guides, and I would expect some visible exhaust. Performance of the vehicle is top notch...does not consume oil, runs smooth, etc. I havnt checked compression in a year and a half...probably time. I have always adjusted valve clearances every 15,000 miles, and a couple of years ago I put in new valve stem seals. Regarding blow by, at what point do you start thinking valve job, when play in valve guides results in low compression? when mileage economy goes down due to compression loss? general performance problems?

Thanks,

Mark

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1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)

Valve guide wear has much to do with oil consumption but nothing to do with blow by. Blow by is when the cylinders/pistons/rings are worn, allowing combustion gases to "blow by" the rings into the crank case. This is what causes the "action" when you pull off the oil filler cap.

On anything but a near new engine you will have some smoke and a little oil blowing out the filler hole with the cap off. If it's spraying like a paint gun, then you have some blow by. My fresh pistons, bores and rings in my daughter's car are so tight that I get nothing blowing out the filler hole with the engine idling. It's amazing. But, some smoke is nothing to worry about as long as the engine starts, runs well and doesn't use too much oil.

A gas engine just won't keep running when it gets worn out, it will foul plugs and other things that prevent it from going down the road. A diesel on the other hand will keep going as long as you can keep oil in it and it has enough compression to start.

It sounds like your engine is good for many more enjoyable miles. You will know it's time to pay attention to it when; it won't start due to ridiculously low compression or you can't carry enough oil with you to get where you're going.

Good luck,

Thanks a lot, Larry.

My car consumes almost no oil (half a quart in 10,000 miles maybe)....oil changes have been done every 4,000 to 5,000 miles by original owner up to 127,000, and religiously by me since I owned the car, so I would expect that lower end is in good shape. I expected that some blow by is normal, since all cars are set up to handle that (flame trap tubes, blowby tubes, egr, etc.). Its good to hear that what I observe is normal.

Just a further note on that....At Hatch & Sons in Wayland, MA (a very reputable Mercedes restoration shop/dealer) the mechanic there applied heat-resistant caulk to a metal baffle on the interior of the valve cover (after complete degreasing). He did this on my car for the previous owner. I believe that is to ensure that blowby gases travel the full length of the baffle so more oil will drain back into camshaft space rather than into air cleaner (??). This caulking material shows no sign of coming loose, but I would hate to see a piece get dislodged and plug something up!!

Any thoughts??

Thanks again,

Mark

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1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)

Yes, caulking the baffle will keep much of the oil out of the oil separarator in the air cleaner. Good modification.

Just keep what you're doing. Depending on how you drive it, highway or city, your oil change frequencies might could even be a little more often to stretch that engine a little further.

Have a great day,

agreed...since I bought the car I have changed oil every 3K (rotella), except when I had used synthetics (5K, or 10K with new filter at 5K).

Thanks

__________________
1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)