W124 overheating problem
 

1992 300TE 164,000 miles. Overheats when idling with the A/C on. Goes from 85C to 110C in about 4 minutes. Rev the engine and the temperature goes down. Car drives down the road fine. Temperature is in a normal range when driven down the freeway 80 to 90 degrees C.
Checks done so far
1. The radiator is about 1 year old and a an OEM behr.
2. Just for kicks I looked for cold spots on the radiator and found none.
3. Thermostat opens, pulled when hot and it opens up. Approximately 1/2 to 3/4 of one inch open. Thermostat also looks new.
4. Engine fan clutch passes the rag test. With the engine hot, holding a rag to the engine fan I cannot stop the fan from moving when the car is idling.
5. One auxiliary fan was replaced and the crud that builds up behind the fan shroud in front of the A/C condenser has been cleaned out.
6. Both Auxiliary fans are working when the car is overheating.
7. Radiator cap holds 18 PSI.
8. No oil in the radiator or water in the engine oil.
9. Water pump looks old or possibly original equipment.

Right now I want to replace the water pump for possible circulation problems but a head gasket also comes to mind? Can anyone think of something I am forgetting to check B-4 I replace the water pump?
Thanks John

Sounds like water pump to me. Its unable to push enough flow at idle anymore.

Whats ambient temp?

Ambient temp was between 85 and 90 when the tests were run. John

On my 94 e 320 my fans have a seperate relay for low speed on the electric fans and a relay for high speed , when my low relay fuse was blown i had the same symptom as your descibing, replace fuse and all was well again , the relay is in fuse box mine was one green, one blue relay

There is a separate relay and I will check. Thanks John

fyi, only water pumps i've replaced on those was because they were leaking, not insufficient cooling. never seen that problem in any mb water pump. if the impeller came off, i'd be real suprised. look elsewhere. good luck, chuck.

I assume your car has a viscous fan clutch, i would guess the engaging temperature is too high. I did solve the problem on my 260E by shortening the pin by 0.2/0.3 mm.

Checked the relay and it is bad. Thanks for the tip. John

Do make sure the aux fans both turn clockwise. Replacement fans are commonly found to be wired backwards, despite the color coding on the wires. Sloppy techs miss that all of the time, I just rewired one two weeks ago for an overheat with the a/c on.