77 240d

Temp gauge creeps up to max at idle, goes back down to normal levels under acceleration.

Assume bad grounds, so checked temp with infrared gun.

Registers 160 deg at thermostat housing and 210 deg F at the back of the block next to the IP and first glow plug.

Just replaced water pump, thermostat. Was not doing this right after I made those repairs. This is new behavior.

Aux fan not spinning. Bad fan clutch causing it to run hot?
Thanks

Here's my guess...new thermostat is not opening completely. Do you have the old one to try?

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

AFAIK, 240Ds did not have a fan clutch. Just a direct drive fan driven off the water pump pulley.

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Milan Brown 1979 240D, rebuilt OM617.952 turbo diesel, rebuilt 722.315 transmission (my only daily driver)

Instagram: @maximed93

Correct.

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[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.

Hmm maybe fan clutch isn't the right word. I'm thinking of the small aux electric fan in front of the grill, separate from the direct drive fan. Isn't it supposed to help with cooling at idle?

Could be the new thermostat isn't working right. I'll check them out.

The aux electric fan is to help the A/C. Engine cooling is primarily by the engine driven fan.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Ok thanks guys.
I suspect a bad (new) thermostat. Woof.

Likely cause, but don't rule out a radiator that's partially blocked especially if it's original and previous owners have run straight water in it.

I went through exactly this last year with my SL. Had the OE thermostat from 1983 still in it and was having heat problems. Replaced with a new OE thermostat and seemed to work fine for several days, then started running even hotter than it did with the old one. Opening temp is 84˚C on that particular thermostat, so I did the boiling water test. Barely opened 1mm at 100˚C. Replaced with an aftermarket Behr (did that boiling water test first this time!) and problem solved.

I guess my point is: if the car was working fine before you did the work, it can easily be a bad thermostat. New doesn't always mean better.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Noted, thanks.
Unfortunately it was overheating before I did the work too - so I don't have a good benchmark.
I bought a cheap thermostat the first time. Bad move. I'll go with a better one this time and see if it resolves the problem.

Also a good point. It does appear original.

A clogged radiator can often be diagnosed by looking for hot and cold spots in the fins. A hot spot indicates flow. A cold spot indicates a restriction. An infrared point and shoot is useful for testing this.

Clogged radiators usually show up at speed as the engine builds heat but the radiator cannot dissipate it.

Good luck!!!

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

I had found that if you turn the heater on and the temperature drops. It was the radiator in my few cases. I just do not feel comforatable with this test as being really good at all.

May depend on if circulation through the heater core is active if there is little cooling system flow. I suppose if it did not pull the temperature down might mean something as well.

Some radiator shops may offer a flow test or the use of that hand held temperature sensor might be a good ideal as mentioned.

I would do the old thermostat test in a pan of hot water with a thermometer. If the old thermostat proves bad there is a fair chance the new one is as well.

Barry is exactly right. If you turn the heat & blower full on and temp goes back to normal, it’s a clogged radiator. If the temp doesn’t drop, it’s the thermostat.

Cool - i'll try it thanks.
The heat definitely works - but i havent looked to see what happens to the gauge when I turn it on. "Circulation problem" is a good way to describe what I think I'm seeing.