I feel your pain.....why you ask? Because I'm having a lot of the same thing happening to me.
Here is what I know and what I've done.
I have replaced my thermostat, and am right now in the process of doing a step by step on how to replace it, including pictures!!
It is a big job, but if taken slowly, and pay attention, its not too bad of a job. But vx still overheats in town driving with ac on, and even with the heater on
It takes a lot to get it to cool down.
Befor replacing the thermo, my theory was. While driving, the resevour was filling all the way up with. Coolant, and then blowing the coolant out the top of resivour
Which then left me low on coolant. However, even after thermo replacement, and even when the engine isn't overheating, when I stop and shut the engine off, I hear the resivour bubbles, and it fills up to the top.
I replaced my orange coolant because it looked extremly dirty, and even had little tar ball looking things in it. This tar ball residue also stained my resivour. So with this in mind, I wonder if the lines in my radiator are also covered in this tar all residue, which would constrict the flow thru the radiator.
My mechanic buddy told me that the orange coolant breaks down luke that, and gets all mucky. So now I'm running the green coolant.
But I'm still overheating.
I also replaced my radiator cap, which I suggest you do also. If the cap is bad, nothing else works proper
Still overheating, and by overheating, in my case, I can drive for hours withac on and. At highway speed and not overheat, or even get high. But if there is a stoplight I have to stop for, within a minute, I am getting hot. Then its windows down, heater on full blast,
And then even at highway speeds, it does not cool down where it should be. This is the point where I. Think I'm loosing coolant thru the resivour. Because I am always low on coolant when I check it after I get home.
Sorry if this is hard to follow. With this blackberry, its like I'm typing on a postage stamp! Lol
Hard to stay cohesive.....
Something else you need to check is that you don't have a ton of debris on the front side of the radiator. It wouldn't take too many butterflys plastered on the front side to restrict the flow of air over the fins.
But that is where my next step is, and that is to replace the radiator.
I may need to check the fan clutch again. I had the water pump replaced a couple of years ago (or it was SUPPOSED to be repaced, I ought the pump) with my timing belt debackle.
I don't really know anymore. I am watching this thread closely. OH, my oil is not milky.
That's about all I got right now, get a new radiator cap. Its cheap, and easy to do, and you will probably be repacing the one that came with the truck.
Hopefully my mini novel may shake something loose in someones head and we can get this problem fixed.
I will try and finishthe thermostat replacement how to soon, so you can do that. The shop here wanted 500 to replace mine!