overheating

[brian19876]

i have a 96 grand am 3.1 v6 that is over heating and only get heat sometimes so i changed the thermostat and refill the system drove around for 15min had heat then no heat for the rest of the trip about 20 more min can't figure this out help please

[HeyDace]

Possible that cooling system is air bound, (did you open the bleed screws as you filled it up)? Is temp red lining or running normal? If running normal, possible blown head gasket. Watch the overflow tank when the engine is hot and running, goose the throttle and watch for bubbles in the tank. If that is the case then its head gasket time.

[brian19876]

The car runs normal for a while and then the coolant starts to back out of the coolant tank. Then the car starts to overheat. I opened the bleeder to remove the air from the system, but it still seems to overheat at random and I get heat at random.

[rixGAphx]

1. When you 'fill' the coolant, only fill to the bottom of the coolant bottle.
There's a line on the side (toward the front) that indicates fill level, tho the bottle is generally so cloudy with age that you can't see the level thru the plastic.
When you remove the cap, you'll see a little plastic 'dam' molded into the bottom directly benath the filler opening.
The coolant level should be right at this 'dam', maybe a 1/8" above the bottom of the curve.

The top of the bottle needs to be empty, so the coolant has space into which it can expands as it heats.

2. The plastic cap on the bottle is a pressure cap, it has an internal device that holds pressure to 15 psi, and then releases. The gasket on the side of the cap seals tightly to the bottle. It only needs to be as tight as an average lady can turn it by hand without straining.
Any tighter and the gasket can be damaged permanently.
If it's damaged, then air or coolant will leak out and the 15 psi pressure will never be achieved.

If you have coolant spewing out, then you have either 1) overfilled or 2) the pressure cap is bad and must be replaced.

3. If the heater only randomly provides heat to the cabin/defrost, then the valve in the heater line is malfunctioning.
Either the valve itself is bad/partially plugged, or the vacuum actuator that controls it is bad, or the dash control that sends vac to the actuator is bad.
My initial *guess* would be the valve itself is plugged, since bad vac is *usually* accompanied by other symptoms.

4. Have you had the system flushed?
Depending on the minerals in your local water, an 8 year old car can have a lot of mineral build-up (scale) particularly inside the tubes of the radiator.
A good chemical flush (I use Prestone) will dissolve the scale and provide maximum metal surface for transfer of heat.
Refill with 50/50 distilled water/antifreeze.

IMO, start by replacing the pressure cap on the coolant bottle, and filling the system to the proper level; don't forget to 'burp' the air at the little release port.

Good luck.
-Rick