Hi guys. I purposely waited to see someone else reply first, but I wanted to add just a comment for your consideration. This same set of symptoms occurred to my '88 Grand Caravan a few years ago, but it took many years before this problem developed. The thermostat had been replaced long before that, and it had worked just fine for years. In my 12-mile drive from work to home, after the first two miles, the gauge would go to 3/4, down to max cold, then 15 to 30 seconds later, back to 3/4, then to not quite so cold. After five or six cycles like that over a couple of highway-speed miles, the gauge settled down at half scale and stayed there like normal.
What I found was that the replacement thermostat didn't have the little "dangle" by the red arrow in this photo. The coolant temperature sensor for the dash gauge was close to the source of heat generation in the engine, so it responded right away to increasing temperature, but the thermostat was a ways away. It took some time for the heat to migrate over to the thermostat to make it open. When it did open, coolant started to flow and the cold coolant in the radiator flowed in where it went through the engine and made the gauge's sensor see "cold" again. This repeated over and over about once per minute until it stabilized, then it was normal after that.
I drilled a 1/16" hole in the plate, similar to where yours is shown. That allowed just enough flow for the heat to reach the thermostat faster and cause it to open gradually, then stay open. I don't know why it had worked properly for many years before this problem developed. Since then, I've read of this happening at least twice. With one of them, the bleed hole was already there with the little piece of metal hanging in there. They cut off that piece to expose the entire hole, and that solved that one. These little bleed holes are much too small to act like a thermostat that's stuck open. Warm-up remains normal.
Replacing the coolant temperature sensor won't solve this, especially since the fan is already cycling on and off normally. It only reports what is occurring. Low coolant level, as mentioned, can cause this too. Also consider a leaking cylinder head gasket. When combustion gases sneak into the cooling system, it can pool under the thermostat causing it to not open. Thermostats must be hit with hot liquid to open. Hot air won't do it. If the liquid bubbles or splashes onto the bottom of the thermostat, the thermostat might open, but then, like mine, the coolant that was sitting in the radiator for a long time will be cold, and will suddenly rush in and circulate up the coolant temperature sensor, making it read "cold" again.
If this pans out, feel free to think I'm a genius. If not, at least it won't be overlooked as a possible cause.
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Saturday, July 30th, 2022 AT 5:50 PM
