1996 C36, 214xxx kms, engine rebuilt by previous owner 4 years ago, recent new water pump, thermostat, expansion tank and cap (6 months ago), hoses are in good condition and coolant level is normal.



Over the last week I have noticed sometimes the water temperature increases from the normal 80-85 range to up to 100. The last time this happened I was on the freeway and unable to pull over and then shortly after the needle went back down.


My initial thought was a faulty sender or guage, but I used an infra red thermometer to check the temperatures of the radiator and hoses and they seem to reflect the guage. There is a temperature difference between the top hoses and the bottom one, and both top hoses are similar.


The fans seem to be operating normally, and when the temperature rises the high speed kicks in.


So, this only happens sometimes, other times the temperature is in the normal range.


My usual mechanic is short staffed and I can't get in to him for three weeks, so I'm hoping for some insights from the forum members.

Even though it is relatively new I'd start by replacing the thermostat with a Behr, Wahler or a genuine MBZ. I've seen new thermostats which were defective right out of the box.

Also clean, tighten and examine the connection at the sending unit.

When checking temps with the IR, put a piece of electrical tape on the aluminum housings and aim at the tape.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

You also might try draining the system before replacing anything. Just to confirm there's no air pocket. Drain the system, then use a water hose and back flush the system to make sure there's no debris. Then refill the system. I would start filling the system, then turn the key on 2 position, engine not running, set the temp to max heat so the aux water pump is on then continue filling the system. This way it reduces air pockets. Once filled, idle car with heat on until operating temp is reached and see if you still have the issue after driving a few days.....or purposely try to force an overheat by idling with the ac on under load. This way, you're able to just shut it down if it starts to go excessively high or even get a water hose across the radiator to bring the temp down. I'm actually doing my 201 right now as I write this because I had to replace the expansion tank 😂

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1990 190E 3.0L