Engine cools down when interior heat is on.

[raynon2]

Is the heat that you get from the heater as hot as it should be?

Yeah, that was kind of the odd part. Heat was still blowing quite warm.

[Thymeclock]

Yeah, that was kind of the odd part. Heat was still blowing quite warm.

If it's not quite as hot as it should be, it's a bad thermostat.