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After replacing two thermostats, the symptoms remain the same:

1. If leave the car at idle for 15 minutes or so, the water temp goes up into the normal range nicely, and the air blowing inside is toasty warm.

2. As soon as I start moving and air starts flowing around the engine, the water temp drops way below normal and the air temp blowing inside drops to a lukewarm state--sometimes more cold than warm the outside temp.

Suggestions?

Sure thermostat is not stuck open? Maybe try another one.

mitmaks wrote:Sure thermostat is not stuck open? Maybe try another one.

I've gone through 2 new ones. Gotta be something else.

Is your coolant full? Any air bubbles trapped in system, overflow reservoir full? Any coolant leaks anywhere?
I'd pressure test system (you can rent pressure test tool from local parts store)
How cold is it outside? Gotta remember it takes outside air and heats it up, unless it's on recirculate.
Last thing is I would check blend door to see if it fully opens/closes as it should.

is this a v6?

Possibly plugged up heater core.

I'm having similar issues (not trying to threadjack) where my car warms up and blows nice warm air. At about 3 miles down the road (@24F) it goes hot and I'm still getting hot air. Then, I think the thermostat opens and it goes back to cold/non-warm air from the heater. Pretty much at no time when I'm at speed does it blow air over 50-60F.
During the warmer days (about 2 months ago) I found a leak in my radiator and ran that stop-leak stuff through it. The leak lessened but I still catch steam coming from under the hood at times. When I swapped my headgasket (for no legit reason) I noticed some strange gunk inside my blocks water passages. I tried to get it out but couldn't get to it all. But it shouldn't STOP my water flow, I hope.

During the summer after I'd built my engine and was putting it in, I flushed the radiator, the hoses, bypass lines and such. And was having cooling issues.

I'm voting for a plugged heater core on mine and probably yours too.

you should both start with a thorough flush. There are systems you can buy to do this yourself but it's a lot tidier if you take it to a minit lube or someplace like that and it's worth the money.

Ask them to pressure test the system while they're at it and they will. That way you'll know it's clean and flowing well and if there are any leaks.

Thank you all for the suggestions. I agree that a thorough flush is warranted. I will likely try it at home, however with an outside temp of around 25F most days, it makes it uncomfortable at best.

QUESTION: I notice some of the thermostats available have what looks like is an additional small valve to the side. (See http://www.reparts.net/parts/gat-th30688g1). Is this type thermostat necessary?

Thanks again.

Demonic1 wrote:I'm having similar issues (not trying to threadjack) where my car warms up and blows nice warm air. At about 3 miles down the road (@24F) it goes hot and I'm still getting hot air. Then, I think the thermostat opens and it goes back to cold/non-warm air from the heater. Pretty much at no time when I'm at speed does it blow air over 50-60F.
During the warmer days (about 2 months ago) I found a leak in my radiator and ran that stop-leak stuff through it. The leak lessened but I still catch steam coming from under the hood at times. When I swapped my headgasket (for no legit reason) I noticed some strange gunk inside my blocks water passages. I tried to get it out but couldn't get to it all. But it shouldn't STOP my water flow, I hope.

During the summer after I'd built my engine and was putting it in, I flushed the radiator, the hoses, bypass lines and such. And was having cooling issues.

I'm voting for a plugged heater core on mine and probably yours too.

Yes, you are correct sir. Next time you probably wouldn't want to use "stop leak" crap

When was the last time you've replaced water pump? Old water pumps sometimes have worn out impeller and it doesn't flow coolant efficiently.

FYI...replacing the heater core is the number one, top of the list, worst job in the whole car. The entire HVAC console has to come out.

mitmaks wrote:When was the last time you've replaced water pump? Old water pumps sometimes have worn out impeller and it doesn't flow coolant efficiently.

Water pump replace less than 6 months ago.

You might try flushing out cooling system and see if it will improve heat output. If not, you're going to have fun pulling that heater core

I decided to bleed the colling system. I removed the radiator cap and added coolant until it was full; not more than a half quart. I turned on the car and let it idle for a while. (There were a few tiny bubbles that surfaced but as the engine warmed up, they went away. As noted before, the temperature gauge went up to the normal range. Air in the cabin was nice and warm--though not blazing hot like my other cars when the engine is at operating temperature.

I moved the temperature selection dial in the car from cold to hot and the cabin air temperature fluctuated accordingly from very cold (it's 7 degrees F outside) to quite warm--again not hot. I felt the radiator hoses, and they were hot to the touch. I felt the heater core hoses at the firewall and they felt quite warm.

So I took the car for a test drive. Within approximately 1/4 of a mile, the coolant temperature indicator went from normal operating temperature to bottoming out in the cold range; cabin air blowing was cool as well with the temperature selector all the way to hot. When I got home--with the car running--I opened the hood and felt the radiator and heater core hoses and all were lukewarm to the touch.

All these symptoms naturally point to a bad thermostat--which I've replaced twice in the last six months. Even if the heater core was partially blocked, wouldn't the engine temperature remain stable, or even in the hot range?