: Random overheating

12-30-12, 11:07 AM
For starters I have an 1991 STS 4.9 and it has new radiator, water pump, thermostat, and hoses. What it will do it will run 185 according to the display no problem, but I will come to a stop sometimes will lose heat and temp will raise up quickly. It has gone as high as 247 but as soon as I start driving again she goes right back down to 185 and rapidly. Doesnt do this all the time just every once in a while.

Also have that bubbling when I turn it off from the overflow jug. I am scared it is a head gasket issue because if I run the car with the radiator cap off the coolant will just start gushing out the top of the radiator unless I give it gas but as soon as I let off the gas the coolant starts gushing out again. It will eventually level off though. Is it possible I have too much coolant in it?

Was going to start with a new cap and possibly replace the thermostat and see if that makes a difference. Cooling fan is okay unfortunately it is straight wired which I will be fixing this week but I know that isnt the issue. Any thoughts or ideas would be appreciated. Also I will do a block test to see if it is head gasket jsut hoping not.

12-30-12, 02:47 PM
You have all the signs of a HG failure. I'd borrow a block test kit from AutoZone and run the test just to be sure.

12-30-12, 09:23 PM
Was really hoping it was just the cap I will get a block tester sometime this week to confirm before I do them though. Only odd thing is I never ever have to add coolant to it.

12-30-12, 09:31 PM
A radiator cap not holding pressure would cause it to overheat, but not "raise up quickly" and it would not go back down. It wouldn't cause bubbling in the tank either. That's a very bad sign. After that happens, open the recovery tank and smell inside. Does it smell like exhaust?

Running the engine with the cap off lowers the boiling point and WILL cause it to overheat and over flow unless this happens on a cold start.

Oh yeah, to answer your question, no you don't have too much coolant in it. On the 4.9 the radiator (not the recovery tank) is filled right to the cap.

12-30-12, 10:12 PM
Yea doesn't sound good at all to me. I do head gaskets on a wide variety of cars in fact just finished a honda today. I have tons of success doing them but am curious as to how hard this one may be. Looking at the car it doesn't seem to bad but looks can be decieving. and before I go and do the work I will actually test it first.

12-31-12, 11:06 AM
The strange part is that the 4.9 is not known for HG failures. I'd definitely do the block test first.

01-05-13, 07:40 PM
You probably have an air pocket in the cooling system. Trapped air will cause fluctuations in temperature and cool heater operation.


Remove the cap when the engine is stone cold and engine off. It should overflow from the neck. If not, its low. Have the front of the car higher than the rear, heater set to 90 degrees. Start the car, having someone holding the idle a little higher than normal to speed up the air escaping and slowly add coolant till the heater is warm and it just overflows the neck then quickly reinstall a new cap (AC Delco) Let the idle come back down and confirm the holding tank is at the proper level. Add if necc. Recheck the level in the tank the next morning. This engine could leak coolant thru the intake gaskets and would show up on the underside of the oil filler cap as a light brown milky substance. classic symtom of air trapped in the cooling system. IMO

01-08-13, 06:18 PM
This is a head gasket problem if I ever saw one. It's not as common as problematic engines, but 4.X head gasket cases ARE out there. I've done a number of them.

If your experience is as broad as you make it sound, it shouldn't be a problem to repair. As with many older aluminum block engines, thread repair on the head bolt threads will be required. Outside of that, there's little to be concerned about.

01-09-13, 08:26 PM
Take off the grill and plastic shield above the headlights. I had a phantom running hot condition and the entire drivers side of the ac condenser was caked with grease from a leaking cooler infront of it.