1985 300D running hot
 

I've got a 1985 300D which is running at about 110C which is pretty hot for winter. When we run the heater it runs about 10 degrees cooler. Any idea where I should start first? I'm thinking radiator since it runs cooler with the heater going.

How many kms on the car? Water pump may need to be looked at.

It's got 207K on the car. We just bought it and it's in mint condition besides this problem.

Likely to be the thermostat. Radiator and water pump are other possiblities, but the thermostat is less expensive and easy to change, so I'd do that first. The fan clutch is another possibility. Under what conditions does it run hot?

I've got an 1982 300d that I've had the radiator rodded out, replaced the thermo fan, replaced the thermostat, looks like I should have started at the water pump.

Lets start with first things first--check that the car has enough coolant...

-GH

I agree to check water, I agree that if it were stat the heater would have no effect.

First try cleaning the external fins of the radiator with air and water blowing from back to front.

Cleaning it professionally may not work since there is not much metal in the fins to work with.

Self flushing with the popular flush may do it too.

It could be the fan too....it should have resistance to turn when stopped and not wobble.

Tom W

Try a citric acid flush. Do a search for citric acid flush and follow the directions.

I would also think about replacing the radiator as it is over twenty years old and is probably clogged. You can get a replacement Behr from radiators.com for about $200.00 When I did the flush, it loosened up the clog that had blocked a hole in the rad... Replacing it is at most a 1/2 hour job.

It made a big difference in the 300SD took the temps from 100+ to 85

Make sure that you fill the system with Zerex G-05 and only to a 50% ratio. Any more antifreeze and the temps will go up due to the Zerex's lower ability to transfer heat.

Make sure that you 'burp' the system as well... park on an incline with the radiator facing uphill, get it up to operating temperature, turn on the defroster and gun it up to 3000RPM a few times. that should get any air out of the system.


Good Luck!