Can Closed Thermostat Bog Down the Pump?

When the thermostat is closed, only the bypass coolant circuit is open, which includes the cabin heater, exhaust heat recovery exchanger, EGR cooler, and throttle-body de-icer. Those are smaller-diameter passages than the ones through the radiator when the thermostat is open, so there is probably a bit more resistance.

However, I haven't seen any other posts linking water pump failure to a closed stat, and I'm not sure how likely a stuck stat would even be in a 2013. The one in my 2010 still seems to be doing the job.

Maybe you could do some data logging of the coolant temperature and pump RPM?

If the temperature rises steadily in warmup and then sort of levels off at the thermostat opening temperature, the stat is probably ok. And I would generally expect that the pump is run at moderate or low RPM when the temperature is lower than that.

You can look in this thread for more on how the water pump is controlled.

 

Since the engine was rebuilt, is there a possibility that someone dumped some head gasket sealer into your cooling system in the past? Now you've got occlusions/flow restrictions in certain portions of your cooling system circuit???

Just spit-balling.....

 

I would want to begin by collecting as much information as practical about exactly what is happening.

The P261B code should have an available freeze frame from the moment it was set. That should include how long the engine had been running, the coolant temperature, the desired RPM requested of the pump by the ECM, and the actual RPM feedback from the pump.

That would at least shed light on some questions like whether the engine was warming up, above or below thermostat opening temperature, and whether the reported low pump RPM was just somewhat lower than the request, or it wasn't spinning at all.

Some data logging could fill in an even more detailed picture, but even the freeze frame already saved would be a good start.

 

On the pump you took out did you by chance undo and pull the impeller out of the frame of the pump assembly just to look at it just for the heck of it see if it's cracking flaking did it spin in its bore by turning the impeller by your hand freely It won't really spin it'll move and stop moving stop moving stop kind of but you won't feel any real resistance and one's where the plastic is splitting and deteriorating you'll barely be able to turn the impeller sometimes and that will give you horrendous codes of out of whack RPM for the pump well kind of obviously we've had this twice cheap fake pumps. Maybe adding to the problem