I need a little help here. I have a small mystery I am trying to figure out. I have a 2006 Chevy Uplander 3.5L. For the last few months the vehicle has been acting weird with overheating.
During idling or driving it will randomly creep up in heat and then drop down after a minute or two. Sometimes it gets into the red, other times it just goes up a few notches. It never stays there and always comes back down. Then, Sometimes it dips slightly under normal.
At first I though it was the thermostat, and still think it might be. I couldn't find the location and got some tips on where it is located from a friend who manages an auto-zone. I still couldn't find it. He said it as under the throttle control housing, but I couldn't find it. I did however check a bleeder and found very little gunk in it and took that out and put it back. I also found some brown sludge under the radiator cap, but there was nothing in the hose running to the engine block and none going too deep into the radiator. I was running the car on idle and could feel hot on the upper and lower hoses to the radiator. It seemed like everything was flowing fine.
I burped the radiator in a crude way, by simply taking off the cap and letting it run. For about a minute or two it was like a boiling pot with some coolant coming out and plenty of bubbles, but then it stopped and no more coolant was rising to come out. I ran the vehicle around town and didn't get a single issue with it. Then I make a sharp turn and it rose up slightly and then went back down. I then parked on an incline (front end higher than back) and was holding the rpms around 3500 it started to creep up again. Until it was near red line. Then as I pulled forward and drove home it went back down and was normal the rest of the ride.
Since then the issue has persisted in much the same manner. It will run just perfect for a while, and then randomly creep up and go back down. It doesnt matter if I run the heater or the air. It doesnt matter if I have been driving for 5 minutes or 50. Ive driven for an hour without issue and then another day just starting the car and letting it idle for a minute causes issue.

From my belief I would say that it could be either the thermostat or air in the coolant. It is worth noting that our heat in the vehicle has not worked for a few years. Of course we are in the south and heat is not a big issue. Neither the back nor the front heating vents blow hot.

 

I'd suggest flush, fill, and bleed the system. If you suspect the tstat, replace it also.

 

And don't forget to replace the radiator cap. Don't assume antifreeze cools the engine as well as water.
Overuse of "coolant" in the mix is a common mistake. From the outside, rinse the radiator fins out with
a garden hose. Sometimes mud in the cooling fins affects the system's ability to cool properly.

 

I appreciate the responses.

I can assure you that there is no mud in the fans and that they are spinning freely. I would also like to change the thermostat but I cannot locate it. I cannot find anyone who can tell me where it is and I dont want to spend 250 bucks for a mechanic to fix a 10 dollar part. I may have to, but I will go down fighting if I must.

We've been using premixed coolant, not straight. Most of it is peak performance brand from the local dollar general. I live in a very rural area, so brand selection is minuscule.

 

Trace the top radiator hose to the engine, at the end of the hose is the tstat.

 

I did. I followed the top hose to the metal joint it connects to. There was no thermostat or thermostat housing. Its where one of the two bleeder valves are (in the metal joint). I took off the hose and it was dry with no sludge, and no drainage of coolant. Which is why i am assuming there is too much air in the line. I have a thermostat sitting here waiting to be used, but I cannot find the thermostat in the vehicle. The lower house also had no thermostat on it. I looked around ,and while I did not take off the throttle control housing I did check under it (after removing the air intake hose) and could not see anywhere the thermostat would be. Its all essentially solid metal under there with no radiator hose going under the Throttle control housing. I might be blind, and thats a good possibility, but I didn't see anywhere the thermostat would be.

Pardon the bad editing on the image .

 

The bleeder valve on the drivers side, is the tstat housing. This is along the same as a 3.1. it's a bitch to get to, but you might need a horse shoe shaped wrench, and I think it's 15 mm

 

You will need to remove the air intake (maf sensor, tube)

 

The tstat sets horizontal. I'd also suggest you install a fail open tstat. The reason I replaced mine in the Lumina 3.1, was it failed open, in the winter no heat.

 

I appreciate the information. Would you mind detailing the steps a little more? Im no mechanic, but I have spent my share of hours keeping my cars running).
I took off the intake and the maf and removed the radiator upper house. I didnt see a thermostat there, even when looking around the bleeder valve. If its in there, its in there good and hidden from me. I would definitely appreciate any additional information you can provide.

This was the thermostat I bought Its nothing fancy but I figured it would do the job.