Because the t-stat is removed so the coolant is running through the radiator as fast as the water pump can push it, so it never gets cooled.
A t-stat cycles open/close constantly to keep the engine in the correct temp range, which is 190-200*F, little temp spikes to ~210 +/- a few degrees is normal and to be expected.
With the t-stat closed the water pump cannot push coolant through the radiator.
The DII t-stat is designed to start opening at 180, and with the small DII radiator your truck should be in that 190-200 range while cruising the highway.
A/C on or off makes no difference on the highway, if it does your radiator is bad.
So, put a t-stat back in it, do not drive it until you do because you are going to have a expensive boat anchor if you do.

 

The faster the coolant mass flows, the better the cooling.

If the coolant is held longer time in the radiator to cool it, it is also being held longer inside the block, heating it more ! -No sense !- It is a closed circuit.
The faster it flows, the better the cooling.

Then a 180F thermostat would be always open at 190-200F, same as a free flow no-thermostat, and the pump will have no problem to push coolant.

 

Not sure where your thermometer is attached. We base our refernces on the engine coolant sensor, which is poked down thru the roof on the intake, and reports to the ECU. Where you stick something, or where you point an IR thermometer may not reflect true conditions.

Not sure how you inspected the rad. Looking down inside, even with a snake camera, won't show plugged up horizontal tubes. The true test is how hot or cold they are with relation to the others on the radiator. All the tubes are "fed in parallel" and should be within 10F top to bottom.

Thermostat is one hard working gizmo. It opens/closes constantly, until the temp gets to the "fully open" spec and above. On a D2 that is 205 per the RAVE. Once 212 is hit, electric fan comes on to help keep things from going too much higher.

Put a stat back in. Go by a parts store . Let them check your codes and read live data with one of their high end scanners. You'll get something to compare to your other thermometer. The dash gauge will continue to show 50% below 240F usually, which leads owners to believe everything is just peachy in happy valley.

 

If faster water flow was all it took to get better cooling, you would have a large number of people installing smaller pulleys on their water pumps. If you walk in front of the air conditioner, good. If you can linger, feels even better. Vehicles have thermostats for a reason.

The coolant has to stay in the block long enough for the heat transfer to take place. Aluminum transfers heat to the water, water moves to the radiator, transfers to aluminum fins, fins transfer to the air flow, and higher speed air flow will help remove heat faster. You don't even need a radiator fan at 50 mph.

 

I understand that you dont understand, and I am trying to explain it to you, and you are wrong, faster flowing coolant does NOT cool faster.
Think of it this way, there is a window a/c unit blowing cold air, you are hot, you run past the a/c unit, are you now cool or are you still hot?
You stop and stand infront of the a/c unit for a min, are you now cool or are you still hot?

Again, the t-stat cycles open/close, it opens/closes pretty quickly, quick enough to keep the coolant at a constant temp.

Why do you think a larger radiator cools better than a smaller one? The coolant stays in the radiator longer.

So, you can either believe me, a guy who's been working on cars for over 30yrs, or you can turn your engine into a boat anchor, the choice is yours.

 

Makes a great man cave addition

 

Thanks again, SavannahBuzz.
The thermometer probe is inside the hose exiting the block at the stock temperature sensor location, with no impairment to coolant flow towards the throttle valve and the reservoir. Reacts very quickly to temperature changes.

 

Can you guys link me to the 180 t-stat on Amazon or where you recommend purchasing it?

 

stat = Motorad 439-180 Thermostat : Amazon.com : Automotive

I guess the simple explanation of the thermodynamics (a subject that has made more than one engineering student opt to change to industrial management) is that the coolant must travel through the engine block in a quantity and speed to pick up as much heat as possible, then travel through the radiator to get rid of it. Stopping for brief periods increases the effectiveness of the process. You can wave your hand thru a candle flame. Go slow, and you absorb more heat than you might like. Or consider the oven, where SWMBO has been baking lasanga. You can reach in the oven with a pot holder and drag out the pan. If you leave your hand inside too long it will get more heat transfer than you want. Same thing in reverse for the radiator cooling the liquid down. You can at the store reach inside the freezer to grab a box of ice cream. Leave you hand in there long enough and it will get colder.

The balance of all this is worked out by the automotive engineers. Otherwise, we would have one size radiator fits all. I once had a Dodge Pickup with a 225 CI six engine. Radiator got swapped out to a larger one that went with a 400 CI V8. While I never overheated in Atlanta traffic (yes, I was there when I-285 was being built), I also took a long time to get warm in winter, and on very cold days would have to muff the radiator, it was a little too big.

 

Guy; you came here because you have the problem not us, we didn't cannibalize our cooling systems and them come to you.
So you can either listen or continue doing things your way, which looks like it isn't working.

"Happily running half a year without thermostat" has it dawned on you, that half a year was winter.

"Thanks, Spike. How can coolant not be long enough in the radiator ? Makes no sense to me." it does not surprise me that it doesn't makes sense to you, but he is absolute correct. The thermostat is not there just to open and close, it regulated the flow of water threw the cooling system. If coolant was just racing in one side of the radiator and racing out the other side; when is it being cooling? Guess what if you put i100% coolant it will overheat too, bet that doesn't make sense to you either.

" because the is a clear design goof and want to get rid of risks." Simply put, your better idea of a cooling system sux, put a thermostat back in iand put your hoses back, hook the heater core back and your temps will drop.