i started to notice a problem this last winter 4 wheeling, truck would run fine until i locked in the hubs and put it in 4 high. a little background, truck is an 87 F 150 lariat, 4x4, with the 4.9L, 4sp tranny. the truck temperature gauge would start to rise soon as i would engage the 4 wheel. it wasn't so bad in the winter bening cool and all. since then i have replaced the water pump, thermastat, and hoses. 190 thermostat. here now in the summer it just plane overheats when in the hills. out on the highway its fine, on the hardpacked dirt roads its fine, but when i engage the 4 wheel it overheats. is it the computer, a sensor, any suggestions would help

 

radiator probly is clogged. My 78 f250 -300 did almost the same thing kinda but got REAL warm pulling trailers. I replaced the water pump. hoses, t-stat-160* ans still bad My radiator got a hole and I replaced it with a new 3 core HD one from a local good ol boy shop and it runs 160* even ideling 20 minutes in 103* weather.

 

There is more of a load on the motor in 4x, thus more heat is generated. A radiator shop can clean out your radiator which I'm sure is probably clogged from buildups. Do you have a shroud on your fan?

 

yes it does have a shroud around the radiator, and i should have mentioned that when i replaced the pump, i flushed the system. i live in the desert country of the southwest, would the 190 thermastat be right

 

go get that radiator rodded out

 

Indeed, flushing your radiator can't touch what a shop does. Its been a long time since I've had a radiator cleaned out professionally (rodded out like KO mentioned) but it didn't cost much at the time. Your 190 t-stat should be fine. If for whatever you can't get your radiator rodded out you could try using "water-wetter". Many have had good luck with it dropping temps.

 

Just a couple of things that have not been mentioned-basic things that are usually taken for granted.
Air flow over the radiator: The cooling fins can clog also. It doesn't take much to start restricting air flow and limit cooling capacity. Put a bright light on the fan side. From the front you should be able to see light over every square inch of the cooling fins.
Use distilled water in the cooling system: The minerals suspended in tap water bond to each other and your cooling system under heat and pressure. I didn't put much stock in that 'till I saw it in my wife's car; 6 months between problems and I found silica granules. Re-mixing with distilled cost me a whopping $1.30 extra.