99 grand am overheating v6 3.4L


Old 11-16-10, 05:09 AM
Thread Starter
Join Date: Nov 2007
Location: USA
Posts: 36
99 grand am overheating v6 3.4L

1999 Pontiac Grand Am SE
V6 3.4 L
170000 miles

Original problem, car overheated and service soon light.
Service engine soon light directed me to misfire cylinder 1, replaced spark plug and corrected service engine soon light.

Now the overheating issue, replaced thermostat, no heat, and car overheats.
Flushed heater core both directions with garden. No heat until thermostat opens. Initial start car overheats and thermostat does not open. After warming car up and shuting off the thermostat finally opens. Its acts like once the thermostat opens it is fine but the first startup the car overheats.

Bleed cooling system Did get some air out. .Blew air thru bypass line and another line back to coolant reservoir to check for blockages, no blockage found.

Last night once thermostat open went on 20mile drive car acted fine thermostat seem to open when it should car did not over heat. The only thing I notice the vents would blow hot air and then cycle to cold and then back to hot.

Sat car for about .5hour topped off coolant fluid and initial start it overheated, thermostat did not open.

How does this bypass from top of water pump play in all of this?
Does this sound like an air lock?
Should I flush radiator?
Should I drill holes in thermostat?

Thanks for any help in advance.
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Old 11-16-10, 06:27 AM
Join Date: Jan 2006
Location: Owen Sound Ontario Canada
Posts: 541
Bypass is there so stat can be closed and pump wont cavitate.The bypass should not be the problem.
Sounds like an airlock. To me the air will be in the heater core or heater core hoses.
At that mileage a rad flush is in order but I dont think it will help you right now.
I would not drill the stat. I would fix the problem and not cover it up.

In the past I have installed a flush kit in the inlet to the heater core and started the vehicle with the garden hose connected to the connection and the water on. I would start the car and run it with the rad cap off. Just let it run watching the temp gauge. Should remain cold. When I was satisfied it will not overheat then shut off engine and disconnect the garden hose and put the cap back on the flush connection. Then go for a test drive.
That works for me in the past. Then I would add antifreeze or what ever you use to have the coolant chemistry right.
Question: How can you tell if the stat opens or not?
Old 11-16-10, 07:03 AM
Thread Starter
Join Date: Nov 2007
Location: USA
Posts: 36
temp gauge drops and car does not overheat. Can I disconnect both line to heater core on engine side and fill them with antifreeze to eliminate that air pocket.
Old 11-16-10, 03:04 PM
Join Date: Jan 2006
Location: Owen Sound Ontario Canada
Posts: 541
I can picture in my head what you mean. Worth a shot.
Another cheap test would be to put short, plugged hose pieces on the engine and run it like that with the heater core circuit out of the picture.
If it works well then it must be air in the system. I it still overheats then I have you barking up the wrong tree.
Old 11-16-10, 07:49 PM
Join Date: Sep 2005
Location: York haven
Posts: 111
Vehicles either use a closed system with a reservoir and are self purging, or the older ones use an air bleeder at the highest cooling system point, and no true reservoir.
So, as we do have an air bleeder in this system, I'd connect a hose to it, run this hose to the radiator and run her for a few minutes - you should see a ton of air leave (use a clear plastic hose).
Also, assure that the heater hose are correctly run - sometimes they can be easily switched.
Ensure that the thermostat venthole is "up"......if you have this design..

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