This is the second post of mine regarding this topic - the first was over a year ago. Drove my '78 NA 300D 2200 miles this week, mostly highway miles averaging 65-70 mph. I used over 4 quarts of oil during this trip and I can't figure out where it's going. I notice little to no smoke while driving - no puff on start-up or acceleration, although I did notice some smoke while idling this morning after I started the car and let it run for a few minutes. There is some blow-by, no oil in the air filter or the vacuum pump line, and no external leaks.

The car runs great - starts up fine, even when cold. Recent compression tests indicated 300-320 psi across all 5 cylinders. I guess the only explanation is that I am burning and just can't see the smoke.

Shouldn't I see more smoke with this kind of consumption?

Should I be concerned about this - considering the car runs great?

Any other ideas as to where it's going, or is this 'normal'?

ryan

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RG Newell

1984 300D
1972 250
1986 560SL
1991 300CE

I would bet that it is burning it and doing a very clean job of it. Diesel's will run on almost any oil with very little smoke. Many people filter and process used engine oil to use as fuel, as well as transmission fluid, hydraulic fluid, vegetable oil, etc.

It sounds like your engine has major wear in the cylinders and thus the blow by (from the compression rings) and the oil consumption (from the worn oil rings).

Just my guess. Good luck. KRW

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1991 350SDL 350k+ miles

There is no such thing as a normal oil leak.

It can be burning it at that rate and you will never know it by looking at the exhaust. Highway speeds do speed up the rate of consumption in these too. You can have a car that burns almost no oil around town and put it on the highway and it will burn a quart every several hundred miles.

Personally, I feel that a quart every 500 or so miles isn't the end of the world, but if you wanted to try something you might look at the valve stem seals as being suspect at this point. On a NA engine they can cause some leakage.

My '74 burns oil at twice that rate and it runs great otherwise...I just add oil and keep driving.

The type of oil you use can also have an influence...what kind are you using?

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz

Take your hand (when the engine is off) and feel under the air cleaner housing. You should feel a connection from the air filter to a small tube that collects blow by oil. Sometimes when you replace your air filter these tubes do no line up an oil leaks down the right or passnger side of the engine. This was happening to me during 700 mile drive home when I initially bought my vehicle Two to three quarts of oil every fill up!!!!. Simply connecting these tubes and I burn maybe a half quart of oil every 500 miles. Also the air cleaner bracket is notrius for breaking, due to vibration the arms on the bracket fail.

You won't notice any smoke in the exhaust until the oil consumption climbes to one quart every 200 miles.............and, at that point, the visibility is still very limited. Serious exhaust smoke occurs at one quart every 100 miles or less.

The easiest thing to check is the valve stem seals. Cracked or old seals can let quite a bit of oil through worn valve guides. If the oil control rings are shot, there's no inexpensive way of solving this, so a careful eye on the oil level would be the most prudent choice.

Running straight 40 grade diesel oil should be tried in warmer weather. Consmption will usually reduce to half. Not always the case but often enough to try it. As an additinal bonus I believe older engines are better protected especially on the highway with the heavier viscosity. This of course does not adress the cause of the problem.

Thanks for the responses - I am always amazed at how helpful folks are on this board!

I read thru several of your posts and was wandering - did they read all of my original post? Until, I went back an re-read my post and noticed that I forgot to mention that I just recently (within the last 3000 miles) replaced the valve stem seals!

And I concur - my oil use definitely goes up with highway driving...

I used a synthetic oil (5W-40) during the winter and then switched to regular 15W-40 for the summer.

BTW, I know oil leaks are not normal, but I don't have any leaks..

If I had a ring problem, wouldn't that show up with poor compression readings? My compression numbers were not that bad for this NA OM617 engine...

Brian, thanks for the advice regarding the smoke issue - I was wandering when, and if, you would start noticing smoke.

ryan

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RG Newell

1984 300D
1972 250
1986 560SL
1991 300CE

The compression rings determine compression basically. The oil control rings are more to control the oil by scraping it back off the walls. I guess they also limit the amount of splash penatration up between the pistons and walls as well to some extent. If the piston ring grooves are worn the rings can actually set up a pumping action with the oil trapped in the excess side clearance of the ring groove.

Are you sure about this? Could be wrong, but i always thought oil ring grooves remained somewhat "back-pressurized" by ports in the piston groove behind em, pushin and circulating oil onto walls via crankcase splash'n pressure as it were. Believe i seen this configuration of ported oil ring grooves with one set of pistons or another, aint sure on what though - coulda been the moto guzzi I rebuilt several yrs ago.

Prof, what y'all aint tellin us is mileage and history of oil consumption long as you've own the 300D. Did replacing valve seals make any difference? Was it really, really low mileage car thats probly sat undriven a dozen yrs when you bought it notso long ago?

In the latter case (low miles, long off road time) I seen pitted bores result from condensation. Rings and compression will be fine, performance will be excellent - yet walls will collect oil on pitted bore surfaces that burns oil on each stroke.

Doc, this could be the dilemma with yer oil burning 240/8 too. Nevertheless knowin diesels, I'll betcha they'll last another 150k miles easy. Y'all just gotta keep feedin em oil.

Its an old engine so its going to burn some. I wouldn't worry about it, maybe try some 15w50 and see if that makes a difference. These old engines are really working on the highway so burning some oil isn't unexpected.

The oil scrapper ring probably doesn't work as well as it did when it was new so some oil gets past, and every original gasket is probably rock hard. That engine could run for another decade like that, so just keep the oil full and drive it. As long as it runs good and starts easy its fine.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

Nah, years by themselves are meaningless. My daily driver '79 240D has burned an unwavering respectable 1 qt per 1200 for the last 100k miles long as i've owned it. And that aint bad for not having an oil/vapor collector - plus having the usual oil leaks evidenced by wet sump bottom half coming from god knows where.

And yes in spite of what F/I says - there IS such a thing as "normal" oil leaks. Anybody actually seen one 616 or 617 diesel that doesnt leak something from somewhere?? Professor if yer checkin for leaks, gotta crawl underneath and examine weep hole under the bell housing too. Could be pushin oil through back crankshaft seal that you aint noticed.

At one time most cars burnt oil. If you think of yourself as living in a different era your engine is normal. The advice to just keep driving it is pretty sound. Usually does not impact reliability.
Worn piston grooves can create oil pumping. The oil rings travel from one side of the piston groove to the other at each and every stroke reversal. If there is too much clearance the oil that enters that area is squeezed out onto the wall. I agree the holes in the ring grooves lessen the transfer ratio somewhat.
Also when the grooves are worn the rings tilt more than normal from the excess side clearance. Quite a few gentlemen on reconditioning these 616 and 617 engines have run across this problem. Either change the pistons or machine the grooves wider to allow the fitting of a spacer ring with the new oil ring.

Yeah but wouldnt 300-320 lbs compress spaek for tight rings and near-spec clearances between pistons and walls?

Sure you would, if its the oil ring. Its probably also leaking some, and I'm sure the head isn't perfect. Even though the valve steam seals have been replaced who knows maybe some is getting by those as well.

Old cars leak oil, old European cars leak a lot!

Whats MB's spec for this engine? Probably a quart every 200 miles or something like that.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200