high temp at high speeds

88 XJ 4X4 4.0 A/C AW-4 Gets up to about 240*F at 70 to 75 MPH lower speeds its fine 210*F What can be done about it?

There are too many individual components to the cooling system to say for sure.
* Radiator can be clogged - internally and/or externally(fins)
* Hoses can be old and clogged or they can constrict when hot or under pressure.
* Pressure bottle may not be holding pressure.
* Thermostat may not be opening all the way.
* Fan clutch may not be fully engaging.
* Cooling fluid levels low or have air in the lines.
* Water pump may not be working correctly
* S-belt problems: too loose, too tight, or a pulley binding.
* Aux fan not working properly.

While not a full list of possible problems, start with the above items.
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Until you get it fixed either drive slower,
Or you could put the heater on full blast to dissipate some of the heat in the coolant.

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Rule of thumb, high temps at high speed is either poor or shot radiator or worn impeller on the water pump. I tend to go with the radiator first.

I had a similar situation in my '91. It idled fine but would overheat at speed. A new radiator fixed it.

same with my 92. a new radiator fixed it.

Uhmmm, new radiator didn't solve mine. Still overheats when driving faster than 75 mph.

luv2offroad said:

Uhmmm, new radiator didn't solve mine. Still overheats when driving faster than 75 mph.

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Did you check to make sure you still have the spring in your lower radiator hose? At high sustained RPM the water pump can suck the lower hose closed.

A/C delco told me a spring wasn't necessary unless you had to buy a custom hose and bend it around a corner. Like to stop it from kinking.....

If the hose is molded and formed for your particular application, suposively you don't need the hose.

duplicate post

I also had that problem, new rad fixed it

I did the waterpump first which did nothing so

Blaine B. said:

A/C delco told me a spring wasn't necessary unless you had to buy a custom hose and bend it around a corner. Like to stop it from kinking.....

If the hose is molded and formed for your particular application, suposively you don't need the hose.

Click to expand...

If the hose is brand new it's own strength is usually enough to hold it open at high rpms, however as the hose ages is loses some of it's strength. This could lead to collapse. The purpose of the spring in the lower hose is to prevent that collapse. There is a very good reason it was included from the factory.

True, I noticed the new hoses were much stiffer than the origional.

However the max pressure in the system is 16 psi if you use a 16 psi cap - so under full acceleration, that still doesn't seem like enough to close the hose.

And he's talking high speed.....So I'm guessing 2500-3000 RPM on the highway for that speed. That's not alot....

At high speeds the mechanical fan almost freewheels completely, because there's supposed to be enough air flowing through the radiator from the high rate of speed being traveled. He sould clean out his fins......Just use a regular garden hose with an attachment and try to hit it from both sides.

It's not an issue of pressure but an issue of flow.

If the water pump is trying to pull more coolant than can flow through the radiator the result is basically a vacuum. Being the only flexible item between the radiator and the water pump, the lower hose will naturally try to collapse.

This problem can really start to show up in combination with a clogged radiator because as the radiator becomes clogged it will flow less.

I'd like to see though, how much vacuum it'd take though....

I guess it is possible though, don't want to start a huge nasty debate

Most all lower hoses have a spring (though some may have some other kind of stiffening). It doesn't take many PSI to lift a car when put in a tire. The pressure differential in the system can pull a rubber hose shut, I've watched it happen. If you notice, the upper hose is smaller than the lower hose, the thermostat is regulating the flow causing a difference in pressure. A partially plugged radiator likely adds to the process. I've also seen the layers of an old hose separate and the inner layer collapse while the outside of the hose looked fine. I've seen brake lines separate and cause a blockage and they have a whole lot more pressure then a radiator hose.
High RPM overheating is most likely a restriction someplace. Possibly low pressure in system because of a pin hole leak or a defective surge tank cap, allowing the coolant to boil. Or air in the system, messing with the flow.