RAJSAMRA20

Hello.

I have a Mercedes ML 63 W166.

The cars coolant temperature gauge stays at 80•c when driving local when driving normal.

When you drive on the motorway, the temperature drops to 65-70•c.

You can get the temperature to go around 90•c if you drive the car hard. The heaters do work but when the temperature drops, they go from hot to warm.

So to keep the car at 90•c I have to keep driving the car hard all the time. What could the issue be?

I have attached a picture of the car’s temperature after being driven on motorway for 45 minutes constant.

Thanks

LeadfootZ3

More than likely you need a new thermostat. In the old days they failed closed which overheated the car. Now they fail open, which is much better and the result is exactly what you are describing.

Black Forest

As mentioned above thermostat. Im just surprised it didn't throw a code yet.

ML_63

mine threw a code twice for thermostat and temps drop on eway guessi just need thermostat only ? Sorry to revive old thread didn’t wanna start a new one . 14ml63

BlackML550

95%.