Heat gets hot, goes cold, gets hot again

[diem]

It just keeps happening. It doesn't constantly happen but on any given trip it will get cold and then return to hot within a few seconds.

Just replaced:

Head Gaskets (fel-pro)
LIM Gasket (fel-pro)
Thermostat (gates)

There are no leaks. The level is showing FULL. I properly bled the system and no air comes out when I turn the brass bleeder valve. It has been doing this since I purchased the car before changing all the gaskets.

The temperature will fluctuate from 195-205 and occasionally start rising (heat goes cold) and immediately drops from about 220 and the heat begins blowing hot again. I'm at a loss.

[Scrapper]

sounds like your computer is telling it open and close thermo to early. have you changed thermo sensor?

[diem]

Originally Posted by Scrapper

sounds like your computer is telling it open and close thermo to early. have you changed thermo sensor?

Are you referring to the engine coolant temperature sensor? If so, no. I've had no codes and the car warms up properly so I assumed it was alright.

[skeeter123]

sure sounds like an air pocket blocking coolant from flowing thru the heater core. Try bleeding it again, heater on full blast. You certainly wouldn't be the first to have to bleed a Grand Am repeatedly to clear out the air pockets. And, its a cheap option. Oh, and the computer can't "tell" thermostat to open, thermostat is a mechanical device that only opens/closes based on temp of coolant it's exposed to.
Hope this helps.

[diem]

That's what I thought.

It appears to have been massive air pockets in the system. The procedure described on a major grand am site would not work, using the bleeder valve. I had to run the car with the cap off and heater on it's hottest setting. It took 3 times of blowing out massive air bubbles but for the first time since owning the car, the temp needle stays on 200 and doesn't not move. I couldn't believe it.

[Scrapper]

how you get that when it will show moter on dash not the check engine lite either the lite when you see moter on dash what will you do just drive it until you get home and see whats wrong?

[xeroinfinity]

typical of these to get air trapped in the system. a real pita to bleed.
though we have a procedure in the Tips and Maintenance subforum.

[diem]

Well, it got cold again sitting at a red light. Took off and it became hot again. Temperature didn't fluctuate though.

Bled it some more today and it seems fine. It's like there is never ending air in the system. Used the method described with the hose going into the coolant tank for about 15 minutes. There seemed to be less and less bubbles. I'm going to try it again in the morning for good measure.

I used a block tester and there are no exhaust gases going into the coolant and the headgaskets were just replaced.

[thephantom1492]

I wonder... maybe you actually also dropped dirt or dislodged some and it plugged partially the heater core/radiator?

[xeroinfinity]

these newer models are a pain to get all the air out. Especially when the block or heater core is drained.
You'll just have to keep an eye on your coolant level. And keep your heater on as much as possible.

[GregGP]

Is the water pump in good shape?

[thephantom1492]

the water pump is also something I suspect too...

[thephantom1492]

also, maybe your new gasket ain't well installed or your engine block is disformed (I read it's common for the V6) causing the gasket to not seal well...

[diem]

So far so good guys.

The water pump isn't leaking. I'm not losing coolant and I'm not getting exhaust gases into the coolant, so I'm not sure it's a gasket sealing issue. It could be though. One thing I've noticed is when completely cold, I remove the cap from the tank and a good amount of air escapes and the coolant level rises about 1/2"

I just can't fathom that combustion gases are entering considering I couldn't get a positive reading with a block tester.

[skeeter123]

Sounds very odd... when system cools off, it usually contracts, sucking air into top of reservoir. Mine will collapse the radiator hose when it's cooled overnight. If you're holding pressure after its cooled down, at least you know you don't have any leaks. But its still odd. Are you sure air is escaping? Could it be getting sucked in instead? (but then why would level go up?)
Anyway, sounds like you could have a coolant passageway blocked, and coolant boils in there and creates air pockets which block flow. Maybe a good chemical flush would clear it out....
Also, I've seen where some on this forum recommend drilling a few holes (2-3 mm) at the edges of the thermostat, and when re-installing it make sure one of the holes is at top-dead-center. This lets air bubbles get past thermostat better.
You could also check the smaller-diameter hoses going to/from the reservoir to make sure they're not clogged.
Hope this helps.