Mercedes-Benz W123 temperature fluctuation and cooling
Why does the W123 temperature fluctuate or rise too high?
The cause is usually the thermostat, air left in the system, a clogged radiator, a poor expansion tank cap or a weak water pump. Removing the thermostat does not fix the fault, because the temperature is then no longer regulated. A thermostat fitted the wrong way round gives the same symptoms.
What should you check in the cooling system?
1. Coolant level and air: if the coolant level drops, air is left in the system and the heater blows alternately cold and hot. Fill with the engine running and the heater on full. 2. The cap's seals and the correct pressure rating. 3. The cleanliness of the radiator. 4. The condition of the water pump: the impeller vanes can crumble.
Where can coolant go when no leak is visible?
Coolant can run down the side of the engine unnoticed from the water pump's lower hole. Other causes are a leak in the heater core and the hose connections. If coolant keeps disappearing, the cause can also be the cylinder head or the head gasket; grey oil on the dipstick points to that.
Is the temperature gauge reading always true?
No. If the gauge jumps, for example between 85 and 110 degrees, or shows hot on a cold engine, the fault can be a loose connection in the sensor wire behind the instrument cluster, the sensor or the instrument cluster. Normal temperature is about 82–95 degrees depending on how you drive.