Silhouette with Intermittent Overheating, possibly thermostat or head gasket?

I have a Oldsmobile Silhouette with 190k miles. Here is a brief relevant history:

120k, radiator was dented. Slow leak. Topped off every couple oil changes, no issues.

188k: One time the temperature gauge spiked very high when I came to a stop for a minute. Previous speeds were not out of the ordinary or high… city driving.

Now: Cold may have factor. Car spikes in temperature intermittently, but more so when driving slowly. Temperature stabilizes to normal if I turn off the engine for a minute and restart. I have seen the overflow tank bulge during a more serious overheating event.

Strange observation: When cruising at 30 mph and watching the temperature gauge, the heat blows ice cold normally. When temperature gauge rises to a few ticks above “normal” (mid-range), I can feel heat and the temperature shoots back down to a tick or two below normal.

Strange observation: Grade of road or hard braking seems to sometimes trigger relieving the overheat. The temperature gauge doesn’t go past the red, just right up to the red. If I run the car at idle during an overheat, it seems to recover.

MY best guess and hope is that the thermostat is intermittently sticking. What do you think? I’m considering replacing it myself. The problem seems to be better when the temperature was over 37F one day a couple weeks ago. Most of these symptoms have been in this 0-25F weather we’ve been having.

Thanks for your help.

I believe that when it was above freezing temperature outside, the heat was blowing constantly warm, but I cannot verify this at this moment.

How old is the thermostat?
I change it every other coolant change, along with the radiator cap.
The cold heater suggests there’s air in the system.

You never said what year this is. I can’t say about the 1st gens ('90-'96) but on the 2nd gens ('97 and up) replacing the thermostat in this van is a complete PITA. If you look at a shop manual procedure, it has you pulling the exhaust manifold to do it. It can be done without that - but it ain’t easy.

Before I thought about trying that, I’d have a shop check the coolant for exhaust gases - because it sounds to me like you have a head breach that is pumping exhaust out into the cooling jackets. It’s a pretty simple thing to check and can save you a lot of trouble.

Have you fixed the leak in the radiator yet? If not, the cooling system will have a difficult time maintaining temperature, even if the thermostat is working perfectly. The cooling system needs to be fully pressurized for it to work correctly. Also, since you noted a problem with overheating when idling, make sure the radiator cooling fans are coming on.

Has the water pump ever been replaced? If the impeller vanes are significantly eroded by corrosion, they won’t pump enough coolant at slow engine speeds.

The leak is very slow. This is a 2004 with a GM 3400 engine. A friend told me these were notorious for lower intake manifold gasket failures. Whether it was replaced already, I’d have to check.

Building upon the intermittent cold heater symptom… It seems like there is a bubble that every now and then gets trapped and blocks the flow. When the heat kicks back on it is instantly hot air blowing. It goes from the two extremes. There has been a couple times where the temp gauge went all the way up just below the red (no hot engine light yet, though)… then when I hit the brake it went instantly back down to normal operating temperature. I think that might’ve had to do with overfilling the overflow tank naively.

There is a vid on youtube of changing the thermostat without taking the exhaust off as you said… seems like a pain but I’d do it if that’s all that was wrong.

I’m nursing this car along… as you can tell by running a damaged radiator for 60,000 miles. I will need to to pull a history from our mechanic in order to answer some of these questions, as this is a hand-me-down vehicle.

Also, it doesn’t overheat when idling… I just meant when I’m in gear and rolling on and off in traffic. If I start it up cold and let it run for 30 minutes it won’t overheat.

Fans come on when overheated and it appears to cool off quickly when in park.

I think, but this could have been a dream, when I revved the engine up to 3-4k in park there was a weird release or something bringing the rpms back down. Again, not confirmed.