what is causing the temp. increase of the engine E300 Merc?

[inaayat]

The temperature of my engine is gradually increasing day by day. When i switch on the heater the temperature goes down and even when the car is idling it stay normal.

i locked the fan clutch and still no difference.

Now What.

[kumar]

My vote goes to poor circulation of coolant as when turning the heater on only hot air should blow out so at this point, it should have more than enough coolant in the system if its making it through the heater core

it is best recommend to start with changing thermostat of the car,see if you find any difference.