I am having an overheating problem with my 98 4X4 T100, 160K. It never gets hot while moving at any speed but when idling the temp will begin to creep up. It doesn't happen all the time however. Also sometimes if I shut the motor off and then restart in a few minutes the gauge is almost in the red. It goes right back down when I rev it up to 1500 RPM for a few seconds. I have had the fan clutch replaced but it didn't help. I am about to flush the radiator but my confidense is not high. Any ideas?

well since you changed the fan clutch, the next thing you can try is the thermostat. how hot does it get when you leave it standing? Does the inside of the radiator look corroded? Maybe trying a flush cleaner could help.

Is it an automatic? The transmission cooler is in the bottom of the radiator assy. You could have a leak between the 2. Are you losing coolant or xmission fluid. A pressure tester works wonders. If you have access to one of those infared temp sensors you can look for a cool spot in the radiator. That indicates a plugged section. Lots of possibilities.

If everything else seems ok and you are still having a overheating problem, you could wire up a supplemental electric fan infront of the radiator to increase air movement, (I replaced my engine fan with a flexalite fan, but it is not without faults.)

It cooling off once you increase engine RPMs has me thinking it's the water pump or at least a problem with the water flow. Large Air pocket? Water Pump worn? The infrared thermometer might help see if the radiator water has cooled of and it just isn't circulating into the engine.

Thanks for the responses. I had a water pump put in along with the timing belt shortly after I bought the truck about 20K ago. The more I read the more I think it must be a clogged radiator, or at least the flow is reduced. I am going to flush it but I have been having a bad time finding the drain plug. THe manual I have shows it (badly) as being somewhere on the passenger side near the front. Very crowed with things in that area. There is something that looks reasonable just in front of the oil filter, a bolt with an allen wrench thing. Could that be it? Help appreciated.

The coolant drain on the radiator is on the lower passenger's side.
The coolant drain on the engine block (3.4L V6) is on the passenger's side at the front side of the motor mount, right between the two front motor mount bolts. Its a brass plug with a 14mm head. You can reach it through the wheel well with a long extension.

I thought that I would add a note about how this problem was resolved. I drained the coolant and refilled it (properly) and that seems to have solved it. I didn't flush it because it seemed so clean. At least for a week it has not overheated. My guess is that there must have been air in there. When they replaced the water pump they didn't refill it correctly. I can't see what else it could be.

Im having a problem with over heating too on the trail (at low speed 4Lo) and on steep grade winding highway (goin at ~40mph) on warm days 80s. Its a 98, 3.4, auto tranny with 108k miles. I am close to taking it to the dealer and have them figure it out. Its going to cost me alot and they'll probably tell me to replace a bunch of stuff the truck doesnt really need, so I need your insights before they cut my arm and leg off.

I ve been searching the net to help me figure it out. Heres one that is informational:

http://www.aa1car.com/library/overheat.htm

heres what ive done so far:
thermostat - replaced (from autozone)
cooling system leaks - no leaks found. i dont lose any coolant or oil. the only time it loses coolant is when its overheating...spitting boiling water on the reservoir.
radiator cap - replaced (from autozone)
fan - fan works, fan clutch: works fine (doesnt spin freely)
water pump - checked and it seems ok. no leaks, gasket was still in good condition (replaced with a new bead of RTV silicone when i put it back), no corrosion or wobble on propeller, weeping hole not plugged
belts - replaced all the belts including the timing belt
heater core - no signs of leakage. heater works fine
radiator - i cleaned the outside. ive let it run with the cap off to see if the water is circulating- water just comes out, so i cant tell.... possible cause?

besides using an infrared thermometer, how else can you check if the radiator is plugged or not

spark plugs - replaced, new
air filter - clean, fender snorkel

I also plugged an OBD sensor to see if theres anything wrong..... nothing

What else am I missing? Help

You have checked all the stuff that is normal. Do a compression check. See if you are pumping stuff into the system