1985 300TD 316,000 miles

Excellent original car that has always been serviced and maintained meticulously.

The car burns maybe a 1/2 quart of oil max between changes every 3000 miles{since new}. The oil cap moves slightly but does not bounce around or blow off when loose{ regular amount of blow-by}.
The car gets mostly highway miles. When traveling at 75 mph, the car performs flawlessly. If I decide to increase speed to 80-85 mph for prolonged periods, I find oil leaks when I stop. Oil leaks around the cap{new cap} and in one instance the grommet on the turbo down tube was pushed up slightly from the oil pan and leaked. Also oil will leak around valve cover gasket. There are zero oil leaks when speed is kept at 75 mph or lower. I'm thinking that my crankcase pressure is too much at higher speeds and has to vent somewhere. Again, everything is highly maintained so there is no plugged hose from the valve cover to air cleaner or oil soaked air filter etc.

My initial thought was excessive blow-by because of high mileage BUT there is minimal oil consumption, car starts instantly, has great power and there is only an issue above 80mph. And, I always thought blow-by decreases with higher engine speeds? Any ideas? Is there another "vent" that could be plugged somewhere else in the crankcase? What am I missing here and what would cause such high crankcase pressures at higher rpms?
Thanks
John

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1985 300TDT
2013 E350 Bluetec

Why should blow-by decrease with higher engine speeds? Blow-by gasses escape passed piston rings, so more piston movements, i.e. higher rpm, means more blow-by gasses. Is the gasket of the new cap also new?

it is true that blowby would increase with engine speed, heat etc.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

I've read a few posts that stated blow-by decreases with engine speed.....not claiming it to be true, thus the question mark after that sentence in my post. I am poorly informed then because your explanation makes sense. So, I guess with the increased rpm's and the high mileage; the engine cannot vent the blow-by gases adequately and it escapes from the cap and anywhere else it can?
Cap gasket is new.

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1985 300TDT
2013 E350 Bluetec

I will just keep driving and maintaining it then because it does run extremely well. Just a mess to clean up the oil after a long highspeed highway run. Of course, I could just slow down a bit and give the ole girl a break

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1985 300TDT
2013 E350 Bluetec

Blowby per stroke of the piston would decrease with speed, but the increasing number of strokes would more than make up for it. Sort of like calling a decrease in the rate of increase of government spending an actual decrease in spending.

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'83 300D, 126K miles.

T in a pressure gauge to one of the lines and monitor it. The vacuum pump vents into the crankcase on an 85 so it could be that a vacuum leak or a bad check valve is causing the vacuum pump to increase crankcase pressure also.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

have you opened up your oil separator to see if it's gunked up inside?

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Try running a draft tube made from cheapo heater hose, vent it under the car and see if the problem goes away.

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$60 OM617 Blank Exhaust Flanges
$110 OM606 Blank Exhaust Flanges
No merc at the moment

OK, so maybe a bad vacuum pump check valve. This is a good possibility because it is original and has never been rebuilt. It just doesnt make sense that the blow-by is that severe when there is almost no oil consumption. I would think that if the rings are worn badly enough to cause significant blow-by; then I would be burning a lot more oil between changes? Yes?

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1985 300TDT
2013 E350 Bluetec

If you are speaking of the small "can" inside the air cleaner then: Yes, I opened it. It had the normal amount of oil film on the inside. I resealed the cap with goop and bent the metal tabs down. It is tight and diverts all the oil mist from the valve cover back down the tube to the oil pan. There is no oil film on the inside of the air cleaner. Thats the way it should be, right?

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1985 300TDT
2013 E350 Bluetec

Sounds more and more like the vacuum pump if the tubing is all clear

__________________
$60 OM617 Blank Exhaust Flanges
$110 OM606 Blank Exhaust Flanges
No merc at the moment

Try using a draft tube to vent blowby to beneath the carand see if that helps at all

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1982 300D Turbodiesel, daily driver. Mods so far: Fram 8038 paper filter, 4 brake light mod, Gen II w126 (front) rotors/calipers, boost turned up to 12lbs, non-egr manifolds, water/methanol injection, 4-speed manual
1980 300SD Turbodiesel, project car, nearly ready to hit the street

1974 240D, New paint

Do I run the draft tube from the fitting on the valve cover to under the car and plug the connection on the air cleaner?

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1985 300TDT
2013 E350 Bluetec

Yessir!

Should only cost you $15 tops for brand new tubing

__________________
$60 OM617 Blank Exhaust Flanges
$110 OM606 Blank Exhaust Flanges
No merc at the moment