Experienced an overheating problem on my '89 300E with 151K miles this past weekend which I believe a new thermostat has solved. However, the fan clutch just spins when I turn off the engine when hot and doesn't seem to spin above a certain RPM when revving the engine while stopped.

I thought the viscous fan clutches locked into place after the engine temp. reached a certain point. Shouldn't a fan clutch on a hot engine keep up with engine speed? Is there any way to determine if I am subject to failure soon? Don't want to overheat again.

Thanks for any input. I have a new viscous fan but will return it to the dealer for a refund if my existing one does not appear to be failing.

When sitting at idle for a while, the fan clutch should start to gradually spin the fan faster and faster.

The fan clutch freewheels below 92C and then starts to engage more and more and should be fully locked by around 95C, maybe a little higher. If it holds the engine temps below 100C and you can hear the fan pull air through when the engine is revved, then I would say that it is fine.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record