During the past month, the engine temp on my '95 E320 Wagon rises to 115 C after idling for > 5 minutes. If the engine is revved a couple of times, the temp will almost instantaneously drop to < 100 C, but then will gradually climb again while idling. Temperature is okay while driving; around 95 C.

Auxillary fans work fine: low comes on with A/C (which is almost all the time in FL). High speed at approximately 98 C. (This is a bit lower than standard which I changed some years ago with a || resistor, based on input from here). Engine fan clutch seems to be coupled during these high temp situations. Car has 140k miles. Original radiator. Water pump, thermostat, and coolant cap replaced in April of 2002, 98K miles with factory parts. Coolant is flushed annually. The radiator seems evenly hot.

I'm thinking this is a coolant flow issue due to the fact that temps drop to normal when the engine runs above idle: perhaps a weak water pump or clogged radiator. Any thoughts or experience with this situation? Thanks.

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as ever
Paul

1989 560SL
1987 300SDL
1987 300TD
1983 BMW 633CSi
1972 280SE 4.5

Paul, it sounds like the thermostat is sticking.

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Fred Hoelzle

I had the exact same problem on my 94 E320 wagon. Turned out to be a stuck thermostat. You can confirm a stuck thermostat by feeling upper and lower radiator hoses during warm up after a cold start. If the lower hose remains relatively cool after 10 or 15 minutes, and the upper hose is warm or hot close to the thermostat housing and cooler closer to the radiator (or has cold spots on the under side) chances are good your thermostat is stuck closed. Replacing it is very easy--takes no more than 10 minutes. You can put it in a pan of boiling water on the stove to further confirm if it is indeed stuck. Be careful when you buy a replacement thermostat--you want one with a design opening temperature of 87 degrees C. (Many parts places will sell you the wrong one.....) Don't forget to drain some coolant before removing the old thermstat otherwise you'll end up with a mess on your garage floor.