My wife and I regularly make a 600+ mile round-trip in our 300TD to visit her mother. Sometimes I have to add a whole quart of oil for the return trip, and sometimes I don't. It seems like this oil consumption goes down on hot summer days. Last week my wife made the trip and the dipstick showed the sump was still a 1/4 of the way above the "add" mark when she got back.

Is it possible the car could be using LESS oil in hot weather? This seems to defy logic.

But I like it.

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'81 300TD turbo <<sold 12/05 to another diesel nut

Lower oil pressure with the engine running hotter? Just a guess.

Might have to do with the vicousity of the oil. Higher temperatures would make the oil thinner, which would change the pressure, and maybe that affects the rate at which its burned. If what I just said is stupid, forgive me ... I'm an English major.

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1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)

Hot weather causes the thermostat in the oil cooler to open. Lowers the temperature of the oil so there's greater viscosity and less consumption/leaking.

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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

Really? I would have thought even with the oil cooler the oil would still be hotter, since the engine is hotter, but I was just guessing. It seems odd to me that the oil would be more viscous when the engine is at 100C than when its at 80C. Interesting. Vehicular science sure makes me feel dumb ...

__________________
1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)

WW,

If you don't check the oil exactly the same way each time, the dip stick readings will vary.

What is important is to check the oil the same time after the engine is shut off and the engine at about the same temperature. This is so the same amount of oil drains back to the oil pan from the head, block and oil filter.

P E H

a thermostat i all the mb oil coolers? if so this is news to me.

i have a hard time thinking that with the oil running through the oil cooler it would be colder than on a cool day in which the stat doesnt open. i am thinking maybe it just is running back into the sump more thoroughly.
wierd though.

tom w

__________________
[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.

Interesting feedback, and some interesting theories. Thanks! The oil cooler explanation certainly makes sense, but I would think the cooler is flowing oil as soon as the engines reaches normal operating temps. At what temp does it actually begin cooling oil fully?

BTW, I do make a point to check the oil under consistent conditions, usually the morning after a hard run, before it's been started up again, and always on flat ground.

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'81 300TD turbo <<sold 12/05 to another diesel nut

Thermostat for the oil cooler is in the filter housing. Doesn't get discussed much on here and I don't think there's been much experimentation with it's operation.
However, your point is well taken. Perhaps once the thermostat opens it is sticking and allowing the oil in the cooler to drain back into the engine.
I'm guessing that as much as the cooling system thermostat cause problems, the oil thermostat probably malfunctions as readily, it's just that none of us are aware of it.

__________________
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

doubt that the oil could drain back into the engine from the oil cooler. it seems to me that it will always be under pressure whether it is open or closed.

my thought was not that complicated. i was just thinking of some sludge issues stopping oil from draining down from the head.

tom w

__________________
[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.

My father drove a 115 240 for years in western NC, while retired (drove the thing from 77 to 01 & it's still running). He would occasionally go to Philly to see my sister -- and found the engine maybe 2 qts down once when he got there. He was mystified because the same thing didn't happen on the way home. The only explanation we could ever come up with was that he had been making short trips so much when home that the sump had accumulated a bunch of fuel and/or water, either of which evaporated out during the long drive (or he hadn't checked the oil before leaving like he thought he had).

Any parallels?

and prob true about the water accumulating, etc. but the poster's car is more full after a trip. not less.

this is the type of mistake i make, so don't feel bad.

tom w

__________________
[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.

If the oil thermostat is not working correctly, is it possible for the oil in the cooler to drain down into the sump, leaving the cooler empty, giving the impression there is more oil than is actually the case once the oil in the sump fills up the cooler again?

__________________
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

The oil cooler is on the pressure side of the system and seems to kick in when the engine reaches about 90-100C or thereabouts.

As far I as I know...if you have no leaks in your filter housing/hoses there is no way for the oil to "drain back" to the engine becuase there is nowhere for air to get into the system to displace the oil.

In cold weather, the oil stays thicker longer, during warm-up, and that would make it linger in the upper cylinders longer, increasing blow-by and oil consumption.