Question about engine temperature

[dubbed]

Is it normal for the temp gauge to fall below 90 when the heater is on? I don't recall this happening when I first bought the car but think it started after I replaced the water pump or temp sensor. I'm thinking of replacing the thermostat to see if that helps.

[Njay]

Sounds like the thermostat, if it takes long to get to 90 or drops when you drive at a constant 80-100kmh

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[grtuned]

Is it normal for the temp gauge to fall below 90 when the heater is on? I don't recall this happening when I first bought the car but think it started after I replaced the water pump or temp sensor. I'm thinking of replacing the thermostat to see if that helps.

I think your heading in the right direction regarding thermostat, How long is the car taking to warm up to operating temp??
if its taking to long this may be due to the thermostat being stuck open.
Also i think its normal for temp to drop a little if you switch thermostat on ac to hot as water in heat exchanger may help cool down the engine untill it heats up to operating temp may be more evident driving on hwy with the air helping it cool down also.

[dubbed]

It doesn't really take long to get to operating temp, it literally stays around 75-80 if the heater's on but is dead on 90 when the heater's off. I'll try a new thermostat - won't hurt to replace it anyway.

[clark]

Definitely sounds like the thermostat. Very similar to what happened to me when mine went. Relatively cheap fix too!

[dubbed]

"Cheap fix" and "VW" don't seem to go hand in hand But in this case I might get lucky!