Truck Repair: overheating, side exhaust manifold, fan clutches

Question
88 gmc truck 1 ton C3500/454 gas 4x2. For two months it's been runing up to 220' around town and over to 260' if I try to pull my 27ft RV. It used to pull a 35 ft RV that weighed more, with no trouble. I've replaced the radiator and had it checked for flow, put on a new water pump, 2 fan clutches, 5 thermostats, a thermostat housing, 3 new radiator caps, a starter[the mechanic told Carol it needed that while I was in the hospital],the intake manifold has been checked for plugs, the heads have had their comprssion checked..and it's fine, there's no water in the oil. Auto zone hooked up their "machine" to my computer and said my map sensor needed to be replaced but that shouldn't make it run as hot as it was while I was sitting there. I've been told to take the thermostat out, but the computer needs that if I'm not mistaken. The owner of the radatior shop said there was nothing wrong with my cooling system when he checked my radiator for flow that he could fing.... so why can't I hook up my RV and leave town. I can't make it a half mile before I start blowing hot and bubbling hot. I'm out of ideas and money. My truck isn't the only thing that's hot right now, and I'm so frustrated. If you have any ideas please send them along. Whatever it/they are will be greatly appreciated.

Answer
Hello Kathi,
I have two ideas.
First, I am not sure wheather you have injection, or carb. But I had several trucks in my fleet that were carbureted and did the same thing.
In the right side exhaust manifold of the pre-injected trucks was a heat riser valve controlled by vacuum, thru a temperature vacuum switch. The vacuum would close the valve till temp had the vacuum switch release the vacuum, and allow the valve to open.
The purpose of the valve in the manifold was early fuel evaporation. It would send the right side exhaust through a port in the intake manifold, under the carb, and out the left side.
When the tvs didn't work, the efe heat riser stayed closed, and "overheat it would".
See if you have that, and if so, unhook the vacuum line to it, and it will stay open.

The other problem I have had, is in the tail pipe, back about 8 inches inside the back end, there is a screen. Spark arrestor, is what it is.
Truck would run fine till you were really getting on it, and then flakes of rust from inside the system will stack up against the inside of the screen, and plug the exhaust system, and it will loose power, and overheat.
Then when you shut it off, the flakes fall off the screen, and it seems like everything is fine. I knocked those screens out of all my trucks.
It is possible for that same thing to happen inside the muffler, but harder to find, unless the muffler looks swolen.

Another possibility you don't want to hear is the hairline crack in a head that won't show up in a compression check, and allows almost no coolant loss, but hot compression gases can escape into the cooling system.
This can sometimes be detected with an exhaust gas analyzer heald over the open radiator cap while overheating to check for hydrocarbon, which is not normally in the cooling system. Could also possibly work in the coolant recovery tank when it is bubbling.

Good luck,
Van