My '91 350SD runs a bit warm (some would call it HOT!) here in the Las Vegas summer heat - with daily ambient temps of well over 100F - especially in stop-&-go traffic. Out on the freeway it stays around 90C/100C, but in city traffic it will sometimes get pretty high on the gauge, probably around 115C - especially after a LONG day of driving all over town.

I just had the radiator flushed and refilled, but they used the green stuff (after I specifically asked for Zerex G-05, darnit!). I drove the car again today and the temperature only went slightly over 100C, but it has been up to 115C before. When I picked it up from the mechanic, I let it idle in the parking lot for about 20 minutes - maybe that's where the heat built up???

I'm looking for advice on where to start my search for the problem - I'm not opposed to a complete cooling system overhaul, but naturally I'd like to avoid the expense unless it is determined to be ultimately necessary!

I've already corresponded with several of you about my problem, and many thanks for all your help!

with my car a few weeks ago. The previous owner was not too big on maintenance, the car needed quite a bit of work so it took some time to solve the running hot problem as it involved multiple causes. What I think happened is the fan clutch began to fail causing the aux fans to activate more often and run longer than necessary to cool the system. The fans eventually shorted out and the car always ran hot.

I replaced the fans, rewired the burnt harness and replaced the relay which helped to keep the temps between 100°-110°C which is still too warm. Then I replaced the fan clutch which was the root cause. The fan now circulates so much air flow, the temp seldom exceeds 90°C even with the A/C on and the ambient temps 95°+F in stop and go traffic. The aux fans almost never run anymore. If you can rule out the usual culprits like radiator and thermostat, I would test the fan clutch and confirm the aux fans work.

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2001 E320
1991 350SD 265,000 - SOLD
2000 VW Beetle 1.8Turbo - restored
1997 E320 - SOLD
1988 190E - SOLD
1969 Mustang - SOLD

Hey, thank you very much for the insight! It sounds like you were in the identical situation that I am currently in. I will check all the fans and the fan clutch, and I hope that is the root of my problem, too.

I would LOVE to see this thing running 90C all the time - that would certainly make me feel a LOT better!!

my car had a blown head gasket when I bought it likely due to running it too hot for too long. I was fortunate there was no damage to the head but the oil intrusion into the cooling system took some time and effort to flush away.

Prolonged, excessive heat is definitely the enemy so anything you can do to get it to run in a cooler operating range should be the priority. Let us know how you make out!

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2001 E320
1991 350SD 265,000 - SOLD
2000 VW Beetle 1.8Turbo - restored
1997 E320 - SOLD
1988 190E - SOLD
1969 Mustang - SOLD

I took the 350SD out today for a test drive - I just had a GreaseCar WVO system installed and had a little difficulty with the car stalling when I switch from diesel to WVO. That is a seperate issue, but I think I may need an aux lift pump to boost the WVO... That's for another thread, though...

Anyhow, I drove the car for about 30-45 minutes including mostly highway driving and at speeds up to 95mph. Just to avoid excessive heat I kept the A/C off, and if the temp gauge got up to 100C, I hit the defrost and blew some heat off the engine. So, it never went over 100C on the drive - however I'm sure it would have if I had used the A/C and/or not used the defroster to blow off some heat.

The good news is that the car ran GREAT on WVO - smoother than it has ever run on diesel! But again, that's for another thread...

The other news (not necessarily bad news, but still...) is that I will be taking it to my mech for a complete cooling system check in a few days. I'll post my results here when I have them.

Have a good rest of the week!

Well, I just drove the 350SD to a gig tonight and although I did not use the A/C (just to be safe) the temp gauge never went above 80C in about a total of 35-45 total minutes driving! And it was 100F outside and I was in stop-&-go traffic all the way! I am still having it checked out, though - that was just too good to be true! I bet if I took it out for a longer drive in the real heat of the day I would see that gauge up a bit higher, so I'll still have my mechanic look at it.

Check other threads on this forum and the owner's manual. Anything below the redline (120C) is ok. It's when you go above this that your problems will start to occur.

I would suspect that head gasket went because of driving past the redline, not due to driving at say 110 which is completely within MB spec for the model.

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1982 300TD 210K miles ("The Replacement" aka "The Anvil") - SOLD
1979 300SD 245K miles (never ending project)
2007 Pinarello F3:13
1995 Ducati 916 (SOLD, sniff)
1999 Ducati 900SSie (SOLD)

Well, that was really easy ... I found the problem with my cooling system on the 350SD! One of the two secondary fans - the passenger side fan - behind the grill has seized and will no longer turn. It was even smoking when it was trying to turn. Once it is replaced, my cooling problems should be solved - but I'm still having my mechanic give it a thorough cooling system check including flush and replacement of the green stuff coolant with MB coolant.

Hopefully this will help reslove the temperature problems as well as the problems with the A/C blower, too.

Hey, I don't want to be the voice of doom, but be very careful. When I had cooling problems on my 91 350SD three years ago, I had it flushed and it got better.

Then I noticed a weakness driving around town. It was the passenger side electric fan - once, I replaced that it was better.

Then I would notice on a highway drive, the car was cool and then would heat up after getting off the highway - naturally, the fan clutch.

It's been good until just last month - the fan clutch gave out. In the process of doing the work, I gave her a complete flush, new water pump, new fan clutch and new t-stat. Additionally, I cleaned the outside of the evaporator thoroughly - along with the radiator. The fan clutch had spewed some oil.

The water pump was done because in order to get the fan clutch off, I had to pull the water pump from the car - it was that siezed. Since I had it off, I just invested in a new pump.

Filling her was a ritual - slowly adding and burping through the radiator cap and the top radiator hose.

Just an FYI.
Richard

Thanks Richard!

Sounds like my situation is very similar to your experience - and thank you for the advice. I will communicate all this to my mechanic and I will have the repairs done as a precaution - especially if they look to be immediately necessary.

Also - I didn't think you sounded at all like the voice of doom! Just the helpful voice of someone who has been through this before me! Thanks!

Rusty

Alright - I think my overheating problems are solved. After new auxiliary fans, fan clutch, t-stat, all new MB coolant, and no EGR my temp gauge rarely goes above 80C - I've only seen it up to ALMOST 90C running the A/C full blast. Whew... it's nice to have a motor that's not running so darn hot! The only problem this presents is now it takes longer for my WVO to heat up in my GreaseCar system! Any suggestions? Has anyone used a Vegtherm with any success (and w/o extensive polymerization?) I'd like to be able to switch over to WVO as soon as possible since I have tons of it and it's free (and it doesn't pollute like petrol fuel.)

Now I just have to work out my A/C-heater blower problem - but that's not such a dire problem. And it's also a subject of another thread so I won't elaborate here.

Thanks for all the help and advice with my overheating problem, though!

Hello MB Enthusiasts: I have a weird issue going on with my '91 350SD. 1st, let me state that the updated cooling fan (plastic) and clutch have been replaced 10 years ago. Last week I had the A/C blower motor and resistor inside the car replaced with OEM MB parts. Now I have noticed it getting up to 100C with the A/C running. Flushed the cooling system used OEM Mercedes Blue Coolant 50/50, and the radiator AUX fans are not running. I powered them directly from an alternate power source and they blow like all get out. I tested the female supply plug and it comes on @ 13VDC about a minute or so after the A/C is turned on. But when I plug it back into the Fan(s) they don't turn at all. Cleaned connectors, etc, still no AUX fans. HELP please. Could it be a fan that's drawing too much amperage? Then I would suspect that the red fuse would blow. If not maybe the ballast resistor? I don't know, that's why I am asking for help.. PS the car runs at 80C all the time without the A/C on. Maybe 84 to 86 at higher speeds of 80MPH and above. Appreciate all input I can get (pun intended)