Strange overheating

I have a question on overheating. I have a 91 cherokee that appears not to have the closed system. It has a resevoir and the electric fan sensor is located on the thermostat housing. The fan does kick on when it's suppose to. Anyway, the problem that I had was I was driving down the freeway and my cherokee started overheating. If I drove slower, the temp wouldn't get as hot. Does this sound like air in the system? My daily driving isn't that far and I rarely get up to 45 mph. On the freeway I was doing 75. I figured that with all that airflow, there should be no reason for it overheating. It seems to me like the thermostat is opening up also b/c the upper radiator hose seems to get hot. Could this be a plugged up radiator? I'm planning to change the hoses and get the system flushed next weekend, but would like to make sure doing that will solve the problem.

I have a 1988 and had the same type of overheating. I had changed out the thermostat, thermometer probe, water pump, and flushed out the system. It was only after changing out the radiator did I find relief. I changed out the radiator with a (1991 model, open system) three pass radiator.

So. If you have changed out everything, I suggest changing out the radiator too. And get a three pass radiator; the factory gives you a two pass OEM. Three pass will cost you about $135.00 delivered.

P.S. If your radiator does not have a radiator cap it is a closed system - if it has a cap it is an open system. both have resouvours (tanks)...

It could be your radiator, one thing though, if your upper hose is warm(hot) than you know the T-stat is working, I would lean more towards the water pump. How long has it been in there. You may not be getting enough flow at higher RPM's to cool the engine.

[QUOTE=Andyjk1] If I drove slower, the temp wouldn't get as hot.

This sounds like the radiator. At slower speeds the engine is not producing as much friction which causes heat. If it's a 1991 it's getting a little old. If you have not changed out the radiator, it's time to. The waterpump could also be weak, but they usually leak or give you some kind of other external indication when they are going bad, a plugged up radiator probably will not. I am assuming the temperature is in the 230+ range. I had the same problem years back on a Ford pickup I bought used. The radiator was plugged with rust and dirt. Flush out the block and replace the radiator and that will probably do it.
tb