Temp gauge questions
 

Please can someone help me with my temp gauge.I have a 300sd 1982, the temp shows very hot under normal driving conditions,when I turn the defroster on and hot air comes out , the temp needle drops 1/3 inch. Shortly after it rises again.

Is the thermostat broken? Any advice .

Many thanks

When the heater fan comes on, it is cooling down the engine. This tells me there is a problem somewhere in the cooling capacity. It is probably a bad fan clutch. It could also be a bad water pump and when you turn on the heater, the auxilliary water pump is circulating the coolant enought to cool down the engine for a while. It could be a thermostat stuck closed but it doesn't seem likely to me. You could test this by simply removing the thermostat and seeing what happens. Put the stat in a pan of water, boil it and see if it opens. Is the auxilliary electric cooling fan coming on at all?

Make sure you have enough water in your system. That can cause fluctuation of the temperature needle.

It is normal for temp to fall as much as from 90 to 70C when you turn of the cabin heat at full. This is as a result of additional cooling that is offered by the heater core. (This is the reason why if your car ever starts over heating, you should turn on the heat at max and roll down the windows until you reach the nearest repair place. LOL)
Well after the drop from 90 to 70c it should slowly creep up back to the normal operating temp and stay there. If you see a lot of fluctuaion in temp, that can be also as a result of insufficient coolant or trapped air pockets. Make sure you have enough coolant in there.

Hope this helps.

MVK