Do you know what the pressure rating is on the new cap? Radiator caps are designed as pressure/vacuum relief valves but if the spring is too heavy it might not function properly:
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I'm assuming you've tried it with the old cap installed?
After my commute home from work, so fully warmed up, with an IR thermometer gun, at idle just a few minutes after getting in the garage, I measured a 45 degree temp drop between the radiator inlet (top) and exit. For what it is worth......what is your truck getting? The heater core hoses were balanced, and were about 10 degrees cooler than the radiator input, blower not on. And my temp gauge has always operated right in the center for as long as I can remember. My normal operating temp is about 200-205F, taken from scan gauge.
Unfortunately, I don't have anything to read temp with. I've been eying a scangauge, I might have to pick one up at least for temp alone.Meat thermometer, aluminum foil & zip ties? I've used that before. I'd just really like to see some actual temperatures.
how far up the temp gauge go?
I have worked on a bunch of different cooling setups. The cooling system in a vehicle is fairly simple. Lets start with the basics, I am sure you already know that but outside eyes help.
Coolant warms up in the block until the thermostat opens and allows it to circulate through the radiator. In the bottom out the top. (or vise versa depending) the the water temp sensor is in the manifold usually on the cold side of the thermostat. So the gauge reads the temp coming out of the motor not at the radiator.
The fan pulls air across the fins of the radiator thus cooling it. The water pump combined with the expanding fluid creates pressure opening the radiator cap and allowing excess to flow into the bottle. When the system cools it sucks the over flow back in.
Here is my theory. You are not getting enough air movement across the fins. The fans are blocking air flow. HOWEVER, this problem occurred with the stock fan as well. This tells me the same issue.
Lets do a easy driveway bench test. With the truck cold take the cap off the radiator and put a thermometer in it. Start the truck and record the temp and mark the time when you start. When the temp starts to rise with the thermostat opening mark the time as well. Let it idle until it gets up to operating temp and record it.
From there rev the engine at about 2k RPMs with the throttle assembly. In neutral record the time from start to when the fans kick on and the temp. Then see how low the temps get till they turn off. Check these temps against the fan settings and factory settings.
You may have the fans coming on too late and not cooling enough. OR they are pushing the air, flow is important. Furthermore most electric fans don't work with stock radiators. Most upgrade to a thicker radiator dissipating heat faster.
The fact of the matter is the coolant is not cooling. All of this points to something with the radiator. It could be clogged. It might be the water pump not circulating enough coolant as well. Either way you need a baseline.
The water pump..... is this the factory unit? original? replacement? Aftermarket? Do you hear any noises? Belt squeals, bearing screech?
My parents had an older jeep grand Cherokee that had what we called the super fan. when we would tow it bumped up the fan speed by almost triple. It was loud.
Were all of the changes / replacements you mentioned doing done at the same time, or did you do them one by one with testing and driving in between?