Heater works, then doesn't

[Scott-o]

2002 Infiniti QX4.

It's getting cold here (34 degrees). Drove our Q this am. Heat worked fine (auto temp set on 67). After about 15 min of driving, I feel there is no longer warm air coming out of the floor vents - it is cold air.

I noticed if I turned the auto temp all the way up (85 degrees) I could get some warm air again, though certainly not as hot as you would expect.

Anything obvious I can check?

Thanks in advance.

Scott

[nickelghandi]

Check your needle for the temp gauge. Is it about halfway or slightly above? If it is anything below halfway after 5 minutes of driving then it is likely you have a thermostat stuck open. If it gets bad enough you won't have hot air at all at any time and a check engine light may illuminate indicating that the engine isn't as warm as it thinks it should be after a set amount of driving.

When my thermostat started to stick open, I had similar issues. The heat would come on for a minute once the engine was warm, but then running the heat cooled the coolant down enough to where the secondary valve closed for the heater core. Then it would again start to heat, then again stop.

Eventually it got to the point where I had the light and no heat at all. I replaced the thermostat and it has been fine since.

[Scott-o]

Certainly sounds like you had the same symptom, and that makes sense that it could be the thermostat sticking.

If it keeps doing this, that is what I'll suspect.

Thanks!

[Blackjak]

This is most likely an issue with the thermostat however make sure your coolant tank is full. If you are low on coolant your heat will be intermittent due to the lack of consistent flow of coolant through the heater core.

[x2002Qx4x]

Had this problem a couple years ago. Ended up being a combination of air in the cooling system and the water control valve at the back of the engine being faulty. After replacing the water control valve, the heat worked decently all the time and then after bleeding the cooling system, everything was back to normal.