Engine temp drops with heater running...

[DCandDGK]

Warmed up my car this morning and while driving to work decided to turn on the heater for a bit. Temperature dropped down to the 1/4 mark on temp gauge. Turned heater off and temp went back up to half almost immediately. Any ideas what that would be about? TIA

Bo

[Castor Troy]

Sounds like your thermostat is stuck open.

To heat your car, a valve opens that sends coolant through the heater core in your heater box, which is used to warm the air. The heater core is basically another radiator.

If your thermostat is stuck open, the coolant is being sent through both the radiator and the heater core, and that is more cooling than the engine needs.

[wildbill]

running under temp implies open thermostat. the thermostat opens and closes to roughly control coolant temp

[DCandDGK]

Good to hear. Thanks for the input. Doing a timing belt kit swap tomorrow anyway.