I don't have an oil temp gauge and am curious about the effects of blocking radiators to help the car warm up in winter months. Trust me, when it gets down to -40 degrees you start getting creative on trying to keep the cold out / heat in.

There are two radiators on the car, one for coolant and another small one for engine oil. Everybody around here will put a piece of cardboard ahead of the main coolant radiator, and the effects are good.

Last year I was also experimenting with blocking off the oil radiator. But I'm a little concerned about doing that, only because I can't see the oil temp. I don't even know what a "normal oil temp" would be, say on a 20C/70F day. Or what is "too hot" for engine oil?

Does anybody have an oil temp gauge mounted and tell us about the relationship of oil temps to engine coolant temps? And safe operating ranges. Etc.. Thanks

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Cheers!
Scott McPhee

1987 300D

Engine coolant can only carry heat away from metal it is in contact with.

By definition, it isn't in contact with any of the engine internals. The oil is, the oil carries heat away from these components.

Too hot engine oil stops working as a lubricant, you are quite right that an oil pressure gauge AND an oil temperature gauge are both extremely useful items. By and large, lube oil temp should match coolant temp, the difference being motor car engines rarely have a thermostat in the lube oil cooling circuit

The old rule of thumb was that you should never block more than 75% of the radiator from air flow, after all the thermostat regulates the two coolant circuits, the idea was to block too much cold air getting into the engine bay (the full factory underbody tray set served several purposes) so again if you stick to blocking 75% you'll be OK.

Far better to concentrate on an appropriate visocity oil, good quality filters, and a sensible engine warm up regime.

Generally speaking, it is hard to have coolant and lube oil at wildly different temperatures, so provided your engine temp gauge is somehwere in the operating range, I wouldn't worry too much.

If you overheat the coolant, then you almost certainly overheated the lube too, a point that all non engineers miss.... everyone only cares about boiling the ***** and blowing a head gasket or warping the head, the overheated lube oil causes as much internal damage, and internal mechanical damage aside, it needs to be replaced immediately with fresh oil... this is why so many engine problems manifest themselves AFTER an overheating episode, which the owner thinks they got away with...

HTH etc

At those temperatures, I seriously doubt the oil thermostat ever opens up unless you start driving fast up long mountain grades.

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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

I did not know there was an oil thermostat, where is it?

So, safe to say the relationship of these two temps should be "proportional" and "close to each other".

You're right, the main purpose of blocking the main rad is to limit icy wind blowing into the engine bay, chilling the engine from all sides. It wouldn't really matter if you mounted the cardboard behind the rad to accomplish this same goal, but there's a fan there. So it goes in front.

It sounds like blocking the oil rad would be a "helper" at maintaining engine heat, but not a leader. Cardboard covering that rad is down low and doesn't do a lot for preventing cold air blowing into the engine bay, that flow is directed under the left front fender.

I think I'll continue doing what we do, and block the majority of the main rad with cardboard. I do leave an opening, near the bottom, to allow some flow through the rad down low where airflow tends to go under the block and under the body rather than straight onto the engine block. I will also block the oil rad with a similar 20% opening strategy. Of course, I have a good functioning coolant temp gauge and keep my eye on it. In the spring when it starts rising above 80 around town, the cardboard comes off.

I totally agree about the underbody tray, it helps so much with keeping road grime out, salt and water spraying up, foreign objects from taking out fan belts, oil pans, etc... and managing heat. I was shocked when I was under there one day and looked up near left front wheel and spotted a set of functioning louvers (connected to bimetal spring) that regulates engine bay temps by opening up the vent when hot. Of course this only makes sense (has a purpose) when the body tray is installed.

It would be so cool to have something like that in front of the rad. Some newer BMW's have such louvers as part of the grill, to help with aerodynamics at speed, more than for temp management.

__________________
Cheers!
Scott McPhee

1987 300D

Kerry is correct. No need to cover the oil heat exchanger. I seriously doubt oil temperatures will even reach the thermostat opening temp. I believe the thermostat is in the oil filter assembly. Not absolutly sure though.

Correct. The thermostat is in the oil filter housing. OM602 and 603 only. OM601 has no oil cooler and OM606 has an oil-to-coolant heat exchanger rather than a separate oil cooler. According to FSM for the OM603 engine, the thermostat starts to open at 110ºC and is fully open by 125ºC.

You should be able to check operation by touching the oil cooler hoses after the car has been driven. In sub-freezing weather I bet the thermostat never opens.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

The underbelly is more important for what you're trying to accomplish than covering the radiator. According to Engineers I talked to at M-B when the car was launched, a major function of the "sound encapsulation panel" beneath the '87 300D's engine is to control temperature in the engine compartment so that the engine doesn't run too cold.

Also as mentioned above, the oil cooler only gets flow if the oil is hot enough, so the thermostat in the oil flow will control that.

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Gone to the dark side

- Jeff

I've got the full belly pan set and agree this is first step for managing winter heat. But in our climate, it is not enough. It's arguable whether the pan or rad covering is more important for managing winter temps.


Where in the oil circuit is this thermostat?

If mine is there and functioning, you're right, there is no need for me to block the oil rad.

__________________
Cheers!
Scott McPhee

1987 300D