Hello gentlemen,

This one has me baffled. My M103 engine is running waaay too cool (or at least appears to be) at around 76/77 deg C fully warmed up. Sometimes it will run at this temperature with the ambient at 81F or 93F -- no difference, sometimes not, but if engine temp does rise, it'll barely touch 80c. Gas mileage is way down due to this condition.

Occasionally I'll catch the needle "dancing" just a little.

Things I've checked:

1. thermostat - changed it out, no difference.

2. coolant temperature sensor - my '91 M103 (103.983) has this sensor on the head itself (two pole blue). This was recently changed due to an aux fan problem, so I doubt this is the cause. I've double checked the connections -- no problem there.

What should I check next? Does the coolant temperature sensor serve as the engine temp sensor? Are these two different sensors?

The "dancing" needle leads me to believe that whatever sensor/relay/etc that controls it is bad.

Please offer any insight you may have.

TIA!!

~Paul

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'91 300E, 212K miles