My 94 E320 is running slightly warmer than it should. Seems to warm up when sitting at idle, then returns to normal at higher engine RPM. All the standard items have checked good. (hoses, thermostat, fan clutch, aux fans, radiator.......)

At the end of last summer, I experimented with a thermostat rigged full open with a small brass bushing. Result was car ran very cool even in 90 degree heat with AC on. From this I conclude system cooling capacity is adequate. Tried two different new thermostats and symptoms returned. Checked thermostats in pan of hot water and noticed neither opened quite as far as the one I rigged full open with the brass sleeve.

I'm beginning to think I may have a partially damaged pump impeller. Could coolant flow be sufficient with thermostat rigged slightly past full open but not with standard functioning thermostat? Seems a damaged impeller might explain this.

Anyone ever have a pump impeller partially fail on a 104?

I'll probably just remove the pump and have a look........

Thanks for reading.

J. M. van Swaay

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1994 E320 Wagon, 230,000 miles
1995 E320 Sedan, 106,000 miles
1994 E500 Sedan, 79,000 miles