Engine cools down when interior heat is on.

[raynon2]

Hey, so the other day I let the car idle for a few minutes while I went into the bank. The heater was cranked and fan was on high. When I got back to the car the engine temp had dropped down to just above the 50 line. Yes, the engine was completely warmed up prior to this. Ever since then it hasn't done it as drastically, but if I'm idling with the interior heat on I can watch the temp. gauge drop dramatically. I know this is somewhat normal, but for it to drop almost all the way back to 50 is not. Anyone else have this problem? I think my thermostat may be stuck open slightly and therefore is not allowing the engine to stay at operating temperature.

[Thymeclock]

Hey, so the other day I let the car idle for a few minutes while I went into the bank. The heater was cranked and fan was on high. When I got back to the car the engine temp had dropped down to just above the 50 line. Yes, the engine was completely warmed up prior to this. Ever since then it hasn't done it as drastically, but if I'm idling with the interior heat on I can watch the temp. gauge drop dramatically. I know this is somewhat normal, but for it to drop almost all the way back to 50 is not. Anyone else have this problem? I think my thermostat may be stuck open slightly and therefore is not allowing the engine to stay at operating temperature.

I think you may be right. Especially if the car is more than 5 years old and/or has more than 30K miles on it.

[ontarian_frog]

How cold was it outside? On very cold days, idling the car and having the heater fan on full will cool the engine down. It's normal.

[Thymeclock]

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

[raynon2]

How cold was it outside? On very cold days, idling the car and having the heater fan on full will cool the engine down. It's normal.

Oh, yes, it was cold, about -22C that day. So yeah that makes sense.

[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.

[petrified.rabbit]

that could also be the instance the thermostat opens and the cold uncirculated coolant in the radiator gets added to the system.

[Thymeclock]

that could also be the instance the thermostat opens and the cold uncirculated coolant in the radiator gets added to the system.

But after it opens he shouldn't continue to see unusual fluctuations on the gauge. He said that "the engine was completely warmed up prior to this". If after driving for about 30 minutes you still don't have hot air coming out of the heater, it's a defective thermostat. "Quite warm" is not the same as "hot" - but only he can be the judge of that.

[camopaint0707]

Replace the thermostat and see if the problem persists.