It was hot today, ac on full, I was sitting in the car for about 15 minutes with it idiling, and the temp eventually went to about 110c. Driving around slowly did not lower the temp, I had to get to maybe 35 or 40 for it to cool down, or if I shut off the AC it cooled down quicker.

Fan clutch?

Are you using MB coolant with a proper mix of water? When was the last time you flushed the system? Did you try adding some Redline Diesel water wetter?

Yes im using MB coolant, not sure about the mixture, it wasnt flushed in atleast 60,000 miles, because I never did it.

I see temps up to 100-105 sitting in a drive-thru with the A/C running and 92f+ outside. I think it's normal. Temp will drop back to 90c or so once I get moving. Check your cap and make sure it is not leaking. Last year I had a cap crap out on a trip along the Blue Ridge Pkwy. and the temps went to 110 going up a long hill or when I stopped after a long climb. Would also blow water out when I stopped. New cap stopped the problem and brought the temps back to normal.

Hi Turbodiesel as you know I have an sdl also. I had this problem right after changing over to the mb coolant. Found out it was simple, didn't have enough coolant in the overflow tank and thus - radiator. I thought it was some complicated problem and didn't think to check it at first. If you are even a little low, the temps will definitely go up. Myine went toward 120 c and I shut it off immediately. Surprisingly, the temps went down within normal range in 15 minutes.

ps, all things running well, it shouldn't go above 90 with a/c on in traffic. Mine doesn't even reach 90c, maybe 87-88c worse case.

You should hear the cooling fan lock up at about 100-110, and yes, sitting in traffic with the AC on will get you up there. May take a minute or two for the temp to come down.

You can test the cooling fan by letting the car idle until the temp goes up (or pull of when it is hot if you can), the have someone shut the engine off while you watch the engine fan. It must stop within one or two rotations when it should be on -- any more and the visco clutch is bad. If so, replace it. You should also notice an enormous amount of air moving if it locks up, and hear it, it's rather loud.

Ditto for the electric fan -- should run a slow speed with the AC on and head pressure about 300 psi, high speed with engine temp over 100 C or so. If it doesn't run on low speed, check the resistor (white rectangular ceramic block down by the receiver/dryer) for continuty. They can go bad, leaving you with only the high speed and make the car heat up fast with the AC on.

And do replace the coolant cap if you haven't yet, it is due to croak. Make sure you use an MB -- they hold something like 20 psi, and lower pressure caps allow the coolant to boil in the engine, reducing heat transfer capacity.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Just went through this with my 350 SDL ... could be different, but there are actually a limited number of things that could be at play.

I had a bad bearing in both viscous clutch and coolant pump ... fan was wobbling a bit. Replaced both. Temp had been pretty solid at 82-95C ... was linear with load and outside air temp - immediately after new coolant pump and vclutch, started to become a bit erratic - would run over 100C easily and then cool down only when the efans kicked on.

Replaced tstat and vclutch (second one of these) and presto ... immediately returned to where it used to be ... solid, with linear movement as a function of abnormally high loads, temps, hills, ac, etc ... never above 100C .... just can't get it there no matter how hard I try.

So ... what is the answer? Viscous clutch? Don't think so ... I think it was the tstat! Would have preferred to have switched only it, but did not want to replace all the coolant again.

Easy to change.

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George Stephenson
1991 350 SDL (200K and she ain't bent, yet)
former 2002 E320 4Matic Wagon - good car
former 1985 300 CD - great car
former 1981 300 TD - good car
former 1972 280 SEL - not so good car
a couple of those diesel Rabbits ...40-45 mpg

I would seriously check into this to be safe turbodiesel. Most of these other guys that claim their vehicles hit 110 c. I don't think that's such a good thing. My car has been in traffic jams with the a/c on and NEVER gone that high except the time it was overheating. Also, these cars are not SDL's. They have different engines so I am not certain it's a fair comparison. Why should yours run any hotter than mine? Doesn't make sense. My temp guage and sensors were checked when I bought it. Nothing wrong there. Maybe have your car looked at, it's not worth a cracked head...

I don't know, but they should have fixed it if they do things the "right" way. I am not sure I would take it back there.

There are enough stories of tstats not working correctly that one should at least consider the implications.

I have noticed, over the years, variances in tstat performance - not just MB cars, but everything from American to BMW. The most baffling was the BMW. Point is. tstats are mechanical devices and a small, really cheap part becomes the cause of very expensive damage.

Short story on the BMW ... had a 1976 2002 - interesting little car that was the starting point for the hugely successful 3 series follow on ... easy to work on, too. Was traveling cross country and making good time in 1981 when, after pulling into a gas station for fuel, I pulled out and it ran right up to just under the red (temps were in 50Fs) - this, after cruising for 2000 miles at normal temp with normal flux due to load, etc. Previous year of ownership also with no issues. In the middle of nowhere, no other options ... had a gallon of water (I was always a cautious Naval Aviator) ... so, pulled off the "external tstat" (was mounted in it's own container and connected in the system just to the back of the fan) and after considering the impact of my actions, used a large screwdriver to remove the tstat part of the assembly). Back together, added gallon of water, and ran cold (duh) for another day. Got to Colorado Springs, CO and went to dealer, replaced it in their parking lot, added water and it again ran up to the red ... removed it got another and it ran up to the red ... dealer gave standard "blown head gasket" and "bad radiator" and "blocked cooling passages" stories ... and repeated them consistently in the face of the fact the car ran incredibly well with no tstat at all at any and all speeds. Gave them all the parts back ... went to local autoparts place and got the cheapest one I could find ... worked great .... but, following some good advice, I drilled holes around the perimeter of the valve, in order to ensure no matter what happened it would not stick. Turns out there had been serious production issue that affected several lots of tstats ... lesson learned - be educated and aware, don't always trust the system that stands to benefit from your problem.

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George Stephenson
1991 350 SDL (200K and she ain't bent, yet)
former 2002 E320 4Matic Wagon - good car
former 1985 300 CD - great car
former 1981 300 TD - good car
former 1972 280 SEL - not so good car
a couple of those diesel Rabbits ...40-45 mpg

I read all the tsat posts and went with a bosch 80deg tstat to solve my low engine temp problem. sofar sogood. rock steady just above 80degc on the dash gauge. not with a/c however cuz it doesn't work.
(course now my luck the whole thing will blow up tomorrow just
cause i've written this )

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1984 300D Turbo - 231k....totalled 11/30/07 RIP