My vehicle is a 1999 Pontiac Grand Prix and it keeps overheating.?

Question

I live in Maine so this has become a problem.
I have an 1999 Pontiac Grand Prix that cycles back and forth from the heater blowing cold air, warm air, and hot air.
When the temp. gague starts to go up, the heater blows cold air and the temp. often goes to 240 to 250 degrees. It stays there for a short time and drops again to a normal temp. and most of the time the heater will return to warm/hot air. While this is happening, the anti-freeze reservoir is at the proper level.
The temp gague just keeps going up and going down.
I have the anti-freeze reservoir at the proper level when I start the car. During the time I am having a problem, I have notice that the plastic jug (the reservoir) become full to the top with just cold air blowing.
The mechanic has changed thermostat, taken the water pump off and inspected that and says it is working, and changed the radiator cap, and bled some air from the system.
Also, he checked the heater core and radiator and those aren't plugged.
It's kind of confusing. I have had to add anti-freeze once because I drove too far and got things too hot but even then, the reservoir was full to the top again while the car has hot. I added the anti-freeze after the car cooled off and the reservoir was empty when I added the anti-freeze.
The fact that the temp. gague keep fluctuating seems to be why I lose my heat but I don't know why.
Any ideas of what is wrong? Thanks.

Answer 1

have your mechanic check the sensor that controls the operation of your electric radiator fan. your symptoms are typical for sensor malfunction. this problem is often indicated by your engine light being on. You can take your vehicle to several different parts suppliers and they will run a test with a scanner that can tell the reason for the engine light being. Autozone,Advance Auto, and Oreily are jest a few that will perform this test at no charge to you. There may be others in your area as well.

Answer 2

Replace the radiator cap again. The only way the fluid can get to the overflow jug is past the radiator cap, and just because it's supposed to be new, that doesn't mean it's good.

Answer 3

Sounds like a cooling fan issue to me, are the fans operating at the correct temp? are they operating at all? is this a problem that changes with the car speed the car is driving at (overheats in town....Ok on the hwy) If so it is the fans

Answer 4

sounds like your cooling fans are not operating properly.there two speeds on these fans.low comes on about 212 degrees and second speed is 225 to 230 degrees.more and likely your low speed is not working.check the low speed relay or swap it with the high speed relay and see if the fans come on sooner.

Answer 5

dunno but better figure it out fast...my grand prix was doing that and all sudden was driving and shut down..ended up cracking my heads.gave it to junk yard for 100 bucks...sucked.check the hose that goes to your thermostat..sometimes the wires in the hose get bunched up and screws with the temp.happend to me once.replaced the hose for 10 bucks and was fine.