thermostat possibly stuck?

[pollock]

Driving home Friday I was enjoying the heater and waiting at a stop light when the heater stopped blowing hot. I messed with the heater controls, tried different vent settings, etc, and it didn't seem to help. I was not using the auto setting, BTW. I don't think the AC compressor was running (not that it should have been, was not using defrost) but can't swear to it. Turned the heat off completely for a few minutes, then tried again and it was fine.

This morning the same thing happened. This time I thought to look down at my auto meter temp gauge. It was creeping up near 210°, which would be normal for my car on a non-cold day, but it was about 40° outside. Then as i continued to wait in traffic, the temp kept climbing. By the time I got the car moving again the needle was past 230° and climbing. I was starting to look for a place to pull over when it dropped back down below 210° and the heater started working again.

My only decent theory is that the thermostat is intermittently sticking CLOSED. Sound feasible? Seen that happen before? Any other ideas?

[ÆJackacks]

check that the rad cap is on properly/isn't leaking too, happened to me once, i guess since the cap wasn't properly tightened the system needed twice the heat to make enough pressure to get the thermostat to open.

If not that then sound like some other pressure leak somewhere, if the thermostat was stuck then it would continue to overheat, not overheat/cool down overheat/cool down, however your thermostat might just be opening at the wrong temp as well.

[gadget73]

Do you have automatic climate control? If so, it sounds like the interior air temperature sensor on the way out. If the stat was stuck closed, you'd get stupidly hot heat. Not to say that you don't have a thermostat issue, but thats not the heater problem.

210 is hot, 230 is really hot. Is your radiator and clutch fan in good shape? You should see 190-200 pretty steady if its in good shape, especially in 40 degree weather.

[Ike Clanton]

Every thermostat failure I ever saw as a mechanic failed open, not closed, for whatever that's worth.

[pollock]

I popped the radiator cap after work yesterday and it was low... Cautiously made it home (and had the same cold heater/hot engine occurence). Topped off the radiator last night with 50/50. Heater temp and engine temp were fine this morning. Hopefully the water pump was just cavitating in the air bubble, which would tend to cause both the cold heater and hot engine.

The water pump and radiator (a cop 3-row) are both less than a year old. Hopefully the pump isn't crapping out.

I haven't noticed any coolant leaks. I'm guessing I may not have gotten the heater core completely full of coolant at last service, or I have a very slow minor head gasket leak, or some combination thereof.

Will post updates if there are any. Thanks for the replies.

[ÆJackacks]

If the stat was stuck closed, you'd get stupidly hot heat.

really? when my rad cap "came loose" the car temp would rise with no heat coming, then all of a sudden the heat would just start working again and the engine rapidly cooled off, then the heat stopped working again, and the process restarted. The only thing that could control that in a non climate control system would be the thermostat or the rad cap no?

maybe when the thermostat closes it also completely cuts off circulation to the whole system?

it would sound like a better design, that way the heat actually doesn't kick in until the engine reaches proper temp when warming up.[2cents]

[ÆJackacks]

hmm now that i think about it, a thermostat opens at a certain temperature, not amount of pressure in the system right?, i have to rethink my theory . . . why would a loose rad cap cause that to happen?

[pvillese]

it would sound like a better design, that way the heat actually doesn't kick in until the engine reaches proper temp when warming up.[2cents]

I've actually had a newer vehicle that did this and several relatives as well. They used a bypass value in the heater hoses just before they connected to the firewall. I usually ended up removing the heater core bypass valve since they have almost always failed. I don't recall what year car we are talking about here but if yours does have such a bypass it could do this to you. My Cherokee had one and would randomly choose to work, so the heat would come and go. It shouldn't cause a drastic change in the temp though.

[gadget73]

really? when my rad cap "came loose" the car temp would rise with no heat coming, then all of a sudden the heat would just start working again and the engine rapidly cooled off, then the heat stopped working again, and the process restarted. The only thing that could control that in a non climate control system would be the thermostat or the rad cap no?

maybe when the thermostat closes it also completely cuts off circulation to the whole system?

it would sound like a better design, that way the heat actually doesn't kick in until the engine reaches proper temp when warming up.[2cents]

thermostat just cuts off flow to the radiator. Coolant still circulates through the engine and heater core though. Its also not pressure sensitive, the whole system has the same pressure. No idea what was up with your particular thing, but in a normal vehicle situation, if there is no flow to the radiator the motor just gets hotter and hotter, and that hotter water is sent through the heater core.

And there is a blend door on non-atc cars. Its just cable controlled instead of with the vacuum crap like atc has. The ATC does have the floor position lockout to keep the fan motor from running when the engine is cold, but its usually more trouble than its worth. I've considered a way to make that work without that same unreliable switch, but it involves some other parts and a relay and after working out most of the details I decided it was just too much work and complexity for no real gain.