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Thermostat or something else

Temp gauge constantly goes up and down, between where it would always run when the car was warm and just below the red line, but never exceeds it (been running with heat on low and Windows cracked). Coolant will sometimes boil in reservoir, sometimes it wont. Oil is fine, no milky substances, coolant is dirty, but looks rusty/old and just needs to be changed as its been quite awhile, doesn’t look oily. Is that a solid thermostat failure or do you think there’s other possibilities? I don’t think air is trapped, i tried running with cap off for a bit, to no avail. Any help is appeciated.

Does the coolant look like this?

This is what happens when air gets into a cooling system with GM’s Dexcool.

There’s no way in telling how much damage has been done to the engine.


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A lot of Dexcool lovers here, not me dumped that for peak global.

If you are the least bit handy with a few tools you could , flush the system, change the thermostat, put if fresh coolant, and purchase a new radiator cap,for less than $50 in parts.

If you want to flush it right, find the block drain plug and drain the coolant from the block too, before you fill it with fresh coolant.


pontiac grandprix


I’ve got a Grand Prix and all the Factory Service Manuals (for 04 - 08) and I’ve got some experience with them.

I run Dex-Cool in 5 GM cars. I have used it for years and years, hundreds of thousands of miles, never a problem. It’s very good coolant. In the early days of that coolant GM had some problems, but not any more, not for many years.

The coolant in my Pontiacs looks fresh and like new.

Why take a chance my thought.
You really need an analysis of your cooling system, a lot of could be’s but impossible to tell from here.[quote=“Tester, post:2, topic:105504”]
This is what happens when air gets into a cooling system with GM’s Dexcool.

Analogy, I am blind in one eye and cataracts in the good eye, My Eye doc says a 5% chance of you being totally blind after cataract surgery is too great, we will let it go as you are functional now. A 5% chance of sludge was greater than a 0%, though do not really know the percent of risk.

GXP. The 5.3L LS4 motor.

I’ll pop the radiator cap off tomorrow and snap a picture, thanks. Not by the car at the moment unfortunately.

Dirty, but not that rusty color.

can’t see the fluid. Looks low to me but it could be the viewing angle. I think this thing needs a good cleaning. With all the dirt and gung it wouldn’t surprise me if it sucked in air.

Barkydog, gave good advice. Have the system checked to see what is needed. Has to be much less costly than engine damage from overheating.

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You need to replace the air with a fluid, I see no fluid in that picture. Running with the cap off won’t help remove the air.

You shouldn’t operate this engine until the cooling system is refilled.

And you should not continue running it with the cap off. The cap prevents boilover by raising the boiling temperature of the fluid above the operating temperature of the engine.

What year is your GP?
How long have you owned it?
How many miles does it have on it?
The moniker was used from 1962 to 2008, and a lot of changes took place over those 46 years.

When was the cooling system last flushed? Was it refilled with the proper coolant mix?
What other cooling system repairs and maintenance has/nave been done?
Are there any other operating problems being experienced? Any other recent problems that you’re aware of?

I think the entire cooling system needs to be evaluated. That includes

  • a visual assessment of the cooling fluid by someone who knows what to look for,

  • a test of the system’s ability to hold pressure (a leak test),

  • an assessment of the radiator’s ability to dissipate heat (some call it “thermal mapping”),

  • a test of the thermostat (simple; done with a Pyrex vessel, a hotplate, and a temperature probe)

  • a flow test of the radiator,

  • a flow test of the water pump,

  • a check of the radiator hoses,

  • a check to ensure that all air is purged from the system,

  • and, the real biggie, a pressure leakdown test of the engine’s cylinders… or a
    simple “lab test” of the coolant for the presence of hydrocarbons, although if you do have a headgasket breech someone who knows what to look for will probably see the consequences almost immediately.

All of these tests are simple and basic. And IMHO you need to know the answers to fix the problems. Odds are, the mechanic will know the answer without having to go through even half the tests.

Finding out the answer is critical. It’s possible that (with luck) the problem will simply be the T-stat, but if it’s more serious it could destroy the engine if left uncorrected. The sooner you get a good diagnosis is the better your odds are.

The 5.3 liter engine was only available on the last generation W body. The exact year makes little difference, the cooling system must be refilled and the leaks repaired.

If the coolant is rusty/dirty then you know the thermostat will be rusty/dirty. When the thermostat sticks closed or comes apart and clogs the water hose you will get a hot engine and hear a thumping sound. I suggest, based on your temp fluctuations, you have a shop flush the radiator and engine, put in a new thermostat, and check all the water hoses. Think about a split hose while on the interstate highway.

From what I can see, it appears that your cap is broken. Looks like half of it is still in the hole.

Did a full coolant flush, with sealant and everything. Replaced the radiator cap, which spring wasn’t even really attached well and part was indeed broken off. Since then there’s been no issues, thanks all!

Figures. The one part that I forgot to include in the list… the radiator cap!
Since the radiator cap is critical to allowing the coolant to maintain pressure, necessary for the engine to run right without boiling over, you likely just fixed your car.

Happy motoring. :grinning:

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