Sticky Clam issue.

[Turbo4whl]

Most likely the air pocket moved through.

Filling many newly assembled engines through the radiator with a well sealed thermostat can create an air pocket in the upper part of the engine. The heater core also could have an air pocket too. One way to avoid the air pocket is to fill the engine through the open thermostat hole. Then add the the stat, cover and upper hose. Finish filling through the radiator.

Also you can sometimes remove trapped air by pulling off the heater core hose at the manifold. Easiest way I have found is to add the bleed hole to the stat if it does not have one.

Note: Diesel engines should always be filled after pulling a vacuum on the cooling system, then let the vacuum draw the coolant in. Air bubbles next to the outside of the cylinder walls in the cooling system of a diesel can cause cavitation. Any engine can be filled this way, but most, do not have a vacuum fill tool.

 

[HotRodPC]

Ohhhh, so it's T stat??? I was going to suggest just tell the hoe to go shower and wash that gash.

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[Galane]

Dual fan controller circuit cribbed from a 1980's Pugeot. At low temp it runs the fans in series so they're each getting 6 volts and run at half speed. At high temp it switches the fans to parallel so they each get 12 volts to run at full speed. It also connects to the AC clutch wire to put the fans on low speed when the AC is on, and if the temp go to the high setting the fans switch to parallel. Definitely want to use the good, high temp, (mo' $) ice cube relays. I tried lower cost one at first, not a good idea when they welded themselves closed.

The high and low temps are adjusted with the 10 turn pots. I put the temp sensor in a pan of water on a temperature controlled burner on my 1960's vintage stove (made by Nash Kelvinator). That and a thermometer made it easy to set the high and low temps right where I wanted.

IC2 can be replaced with a dual version.
The person who did this schematic made it with the quad because he didn't have any duals. With the quad the entire circuit can be duplicated to run another pair of fans. I put this together in a plastic project box to run a dual fan from some Ford, in a Mountaineer, but it'd work with any dual 12v fan setup.

What would be really nice is if someone could do a Gerber file for this to make PCBs, with fat traces to solder in relay sockets. I used a piece of bare fiberglass perf board, a small prototype board from Radio Shack (remember those?) and a lot of point to point wiring.

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[Grit dog]

^ Don't think so. But the conditions were warmer than your original post so harder to get that temp differential.
Under the same conditions as original post, you could disconnect the fans and see if the same differential exists.