Just picked up a 1991 300D with 78,000 miles on it. Everything is great except the operating temp.

Symptoms: will settle at 60, maybe 70 on highway. Will occasionally reach just shy of 80 (e.g. after getting off highway). Origially assumed bad thermostat, but fan spins freely when engine shuts off whether hot or cold, so I'm assuming bad fan clutch.

I guess my main question is this: am I doing any signficant harm by letting it go for a couple of weeks at these temperatures (i.e. 60-70)? I just am not able to take it in right away.

Thanks.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 159k miles
06 E320 CDI "Rutherford", Black on Tan, 178k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 75k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

I don't think that's hot enough for the clutch to kick in. Anyway, your problem is not too high of a temp, but too low.

I'm thinking your thermostat is opening at too low a temperature.

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Michael LaFleur

'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)

Yes, I know my problem is temp too low. I'm wondering if the 60-70 range is harmful.

Unless I'm mistaken, the fan clutch on a cold engine should make the fan stop spinning as soon as the engine shuts off.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 159k miles
06 E320 CDI "Rutherford", Black on Tan, 178k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 75k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

You will not damage your engine at these temperatures. A 60°C thermostat was available for these engines...Robert

Only says you...
The 80* t-stat is the correct one to use.

Interesting!!!!

When I go to the parts counter at MBZ of Fresno and ask for a thermostat for a 617.950 engine, I am asked if I want the 60°C or the 80°C device as there are part numbers for both of them. Only says me???

Again, to adress the original question, you will not harm your engine by driving it with 60°-70°C coolant temperatures until you have the time to take it in for a new thermostat...Robert

When I first picked up my 300D I noticed that the temps. were never up to the proper level. It would rarely go above 60 unless I was on the road for extended periods. It also took forever on the initial warm up.

Some preventive service and a few beers later I found that some idiot had gutted the thermostat and put a bolt through the fan clutch.

Long story short... I put in the correct BEHR 80C thermostat and a new fan clutch. Now the car warms up quickly and never rises above 85 to 90 in any situation. The car runs a hell of a lot better warm that it ever did cold.

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AJ

1985 300D (SOLD)

Very important key point there. In the short term, its acceptable.
If its opening at 60-70*, its already begun to fail. It needs to be replaced with the proper temperature t-stat asap before it fails completely.

Bad thermostat. Fan clutch is operating normally. It should only bring the fan to an immediate stop when it's engaged, which would be when it's hot. I don't think your coolant temperature is getting hot enough to engage the fan clutch.

It won't do any significant harm to run it that way for a few weeks, though it will affect the operating characteristics of the heater and may cost you a few mpg until you get it fixed.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.

Agreed, Forced Induction....Agreed....

But I still find it interesting why Dad's 617.952, which logged 194,000 miles with no thermostat at all, had such minimal cylinder/skirt wear that it could have been re-ringed back to standard.

Here in Visalia, the summer temps are well over 100°F and when I was working at the dealer, we put the 60°C thermostat in everything, since either one could be easily obtained. We never had a complaint about a reduction in fuel economy or a power loss either.

Since both a 60°C and 85°C thermostat is available, I would suggest the 'correct' device is the one that gives you the most satisfactory heater performance, based on your climatic conditions...Robert

Dr. Bert-you're talking about a 617 with less than 200,000 miles on it. Properly maintained, they should go about twice that far before overhaul. I'm assuming it spent most of its time in an area that doesn't really get cold. Try that in northern Nevada and the results would be different.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.

odds are good that you will find no thermostat in your car. also good odds, are you will find a cooling PROBLEM once you get in there. it's possible there is a bad thermostat, as they do fail open often, but odds are not in your favor here. if you JUST picked up the car, it's likely it was "patched" to sell. I'd take it in PRONTO, and have it noted if there was a thermostat in there or not. and go back to the seller if it looks hinkey!

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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 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

I don't find it odd that the engine had to be removed and tore down with less than 200k miles since it had an improperly setup cooling system.

Thats because customers that take their car to somebody to be repaired rarely keep records of fuel economy and wouldn't notice 2-3mpg less.

"Suggest" all you want, that doesn't make it the correct choice.

Skippy,

Dad insisted on an overhaul, even though there was nothing found to indicate one was needed. Our summer temps are above 100°F and in the winter, it may go as low as +28°F, but usually, it doesn't get much below freezing here in the valley.

Again, I suggest you use a thermostat. However, I think 194,000 miles is proof sufficient that you will not destroy your engine by driving it at 60°C to 70°C as some would postulate herein.

When I bought my 1978 300SD from the original owners, it had a 60°C thermostat in it and when I went to the parts counter and asked for a thermostat, they gave me a 60°C, so that is what I used for the 87,000 miles that I put on the car before I rebuilt the drivetrain at 311,000 original miles...Robert

Dad insisted on the overhaul at 194,000 miles. There was nothing found to suggest ot needed an overhaul. That's just how Dad is.

My 1978 300SD had the 60°C thermostat in it when I bought it at 224,000 original miles. I put a 60°C back in it, because the dealer offered me both a 60°C and 80°C for the 617.950 by application. I drove it another 87,000 miles running it 'dangerously cold' (as you say) until it rolled up 311,000 miles.

Althoough it was still running quite well, I decided it would be a good time to go through the drivetrain.

Again, when actual results argue with theory, believe the results...Robert