Overheating , Don't know why.... please help !

I have a 96 Rodeo, it overheats (or gets close) after about 15-18 miles of driving, the weird thing is that when it idles for an hour or even more it wont heat up. I have replaced the thermostat, Water pump seems to be working fine, and the clutch fan has torque. Does anyone have any idea what my next step should be. Any input would be helpful

Thanks

Check that the radiator fins arent clogged with mud, dirt, bugs, etc. If you have a/c pull the radiator mounts loose and look between the radiator and condenser. Stuff can get built up in there and never cleaned out.

Other then that the radiator could be getting clogged internally. Several ppl here have replaced their radiators for that reason.

Thanks, I'll give it a shot

Are you using an OEM ISUZU thermostat? If not, get one in there asap! Aftermarket thermostats don't work well in these engines.

I need to get a OEM thermostat for sure, I was reading some of the other post and, The vehicle is 11 years old with the original radiator, you think its time ?

I do. I did mine in my '97 last year or the year before.

And you're sure you don't have a leak anywhere?

99% sure that I dont have a leak, its holding fluid. The only time I lose water/antifreeze is when it heats up, it boils out of the overflow. I have never actually overheated it, I turn it off when it hits 3/4 to be safe. But it just strikes me weird that it doesnt over heat when it idles but rather when I take it over 18 miles (mainly freeway time). Reading the other post I assume I would need to get a new radiator and an OEM thermo. I'm running a pressure test tomorrow to make sure its not a crack or a gasket, even though it runs great, but just to make sure.

If you're overheating on the highway, it's your radiator. Don't even mess with it, just get a new one. Like Surferfletch, I too replaced mine, last August in fact, primarily due to age.

Need to add: if your truck is an automatic, get that transmission fluid changed too, by dropping the pan, replacing the filter, and roughly 5qts of new fluid. When you overheat the engine, the transmission overheats too, these tranny's don't last long on cooked fluid.