Jeep running hot only up hills and at full speed...

I looked all over the site and couldn't find a problem quite like this one.

When idling this 88 cherokee runs at the normal operating temperature - about 195. But when driving at full speed or especially up hills, it likes to run real hot. Not overheating but real warm, about 220-240. If taken back down to idle, it'll just come down to normal operating temperature. It usually will only hit 240 for a second or two when climbing and then drop down again.

My cooling system is the closed loop but I have an aftermarket aluminum pressure bottle that feeds into a second open loop type reservoir and there isn't an abnormal amount of exchange between the two bottles. In fact, the pressure bottle is always kept completely full and when the system needs to purge or bring in coolant for temperature adjustment, it exchanges through the extra reservoir.

Any ideas here folks? This is really perplexing and troublesome. Why would it run at normal temperature when idling or traveling at slow speeds but want to run hot at highway temps?

I had a similar problem with my '99. It runs hot on the highway but normal on street and idle. I swapped out the radiator and it's now running normal at around 200*. So maybe you have a plugged radiator as well... :dunno:

You may have a slightly modified cooling system but this is more than likely an OEM tech problem. There is a current OEM thread with a title essentially the same as yours.

have you checked the thermostat? replace it, it could be stuck closed while doing that flush the system. then fill it with a 30% distilled water and 70% coolant and burp the system. Check to see if your electric fan is clicking on when it should if not then hard wire it to a switch so you can control it. if that doesnt work then id suspect the radiator. Are their a lot of bugs stuck in it? do you have a winch or lights blocking airflow to the radiator? usually if it overheats at low speeds its probably the fan clutch. if it overheats at highway speeds its probably radiator related.
in my case i replaced every single thing in the cooling system. The radiator was the main problem, a new CSF 3 core solved almost everything. ran perfectly cool in traffic or crawling and perfectly cool at highway speeds but as soon as i got on an good uphill on a control road it would run super hot, ended up being my transmission. A separate trans cooler fixed that problem.

mxracer624 said:

have you checked the thermostat? replace it, it could be stuck closed while doing that flush the system. then fill it with a 30% distilled water and 70% coolant and burp the system. Check to see if your electric fan is clicking on when it should if not then hard wire it to a switch so you can control it. if that doesnt work then id suspect the radiator. Are their a lot of bugs stuck in it? do you have a winch or lights blocking airflow to the radiator? usually if it overheats at low speeds its probably the fan clutch. if it overheats at highway speeds its probably radiator related.
in my case i replaced every single thing in the cooling system. The radiator was the main problem, a new CSF 3 core solved almost everything. ran perfectly cool in traffic or crawling and perfectly cool at highway speeds but as soon as i got on an good uphill on a control road it would run super hot, ended up being my transmission. A separate trans cooler fixed that problem.

Click to expand...

Radiator is a 4 core replaced a couple of years ago. Electric fan is modified to turn on at 200. No winch, no lights. New 180 thermo back two months ago.

Usual suspects in highway overheating:

Lower radiator hose missing the spring and collapsing, restricting coolant flow. I prefer to use the "universal" lower hose with the spring built in.

Could be restricted flow through the radiator--clogging.

Could be a stretched timing chain.

Extensive article on FlowKooler's website is a good read: http://flowkooler.com/cooling_faq/

Same problem and symptoms I had, expecially climbing long hills. I went through all the above. Turned out to be a clogged cat I discovered when installing a dual exhaust system. Replace your cat and I think your problem will go away.

Also, a faulty thermostat could be running at open loop. - Not giving the coolant enough time to cool as it flows constantly through the block.