Despite the fact that its winter, this problem has been consistent since the warmer weather. I have exhausted all techniques and tests I can think of so wanted to see if anyone else has seen this happen.

I have a 195 degree t-stat and the truck runs at about 155-168 F max while I'm moving. If I sit still and idle for 10-15 mins, the temp will get to the 195, fans come on and it drops to 175 or so and cycles from there. The problem comes in when I'm moving. I have electric fans that switch on at 195 and off at 175. (they barely ever come on. Never come on while moving, but at idle will come on after a long time)

What I've tried....
I've tried 4 different brands of thermostats and all behave the same.
I've tried re-routing the heater core return to the water pump instead of the radiator.
I've tried a wide range radiator caps, 7psi, 13psi, 16psi, 18psi.
I've tried a 180 degree thermostat just to see what happens and it runs exactly the same only at idle cycles lower.
I've tried changing both CTS sensor and dash temp sensor. (I'm reading both actuals using the EBL Flash ECM.
I've tried drilling small 3/16 holes in the t-stat.

The only thing I can think of is that I did go with an aftermarket high flow water pump. Is it possible that the pump is so strong its forcing coolant past the t-stat and into the radiator which is in turn introducing ice cold coolant to the engine? The only idea I have left is to cover the radiator with cardboard but clearly this is not the end solution. I have seen grille covers on big rigs and diesel trucks but Im not sure if they are battling the same issue I am? (cooling that is too good?) I just cant understand the physics behind this as there should be ZERO coolant returning to the radiator unless 195 is reached yet I never reach over 168 unless at idle as explained above.

 

Well heres what i see. You are correct the thermostat shouldn't all coolant to flow when the temp is too cold (though some are designed to). Logically ff no waters circulating it should eventually overheat. Since it obviously isnt somethings wrong. However we are overlooking the heater core. The heater core does act like a bypass around the thermostat. We may be able to restrict your heater core to limit the amount of coolant that can bypass. Although in the future you may want to conciser a standard water pump when it needs replacement.

 

Thanks for the input. I think what I'll do as a test is to disconnect the heater core and put a bypass in. I am willing to try it, but I have doubts since at idle, I see normal temp range behavior. I'll add this to my checklist of things to try!

 

Well the thing is it cant just be bypassed it has to be restricted to allow less coolent too bypass past the thermostat. The reason why this all is is because at idle theres no airflow through the radiator this means that the heat dissipation of the radiator is much less and will heat up till the fans come on as it does. However when your moving its almost like the fans on all the time due to the amount of airflow through the radiator just from moving so the radiator is going to get cold. Now the motor shouldn't get cold though unless theres circulation and that circulation is most likely though the heater core. By slowing the circulation we can increase the temp of the motor. Atleast thats my guess as to how to solve the problem.

 

As FYI, I have identified the "running too cool" problem. Turns out that the GMB 130-1620P waterpump I am running is capable of 300% increase in pressure. This pressure was forcing my t-stat open slightly and then at temp, just thrusting past it. I was able to find the GMB 130-1620 (not ending in P) which is a OEM replacement. I had no idea the pump could do this, i've never seen it in my travels. We'll, Im proof positive that it can happen.

Anyway, thanks to everyone for the help and if you are looking for a pump to stick your temps no higher than about 165 buy that pump! :-)