Why does my 90 Jeep Cherokee overheat when I go 65-70mph but runs cool when I'm idling or going 40-50mph?

Question

It is a 4WD 6 cylinder-4.0L. I replaced the water pump last fall. The thermostat is good. It was a closed cooling system and I replaced the pressure overflow tank. And now I've just installed a new performance radiator and converted my entire system over to be an open cooling system. The belt is good and tight. There are no coolant leaks. Twice I've run coolant system cleaner/flush through. I've also tried using "Ice Water" in the system to lower the temperature. And it runs fine driving around town or idling at stoplights. But when I get on the highway and get to 65-70 it starts running very hot and often overheats. It does this no matter what I do. The oil is burned up and needs changed immediately. The air filter is due for a change, too. I also need to replace the head gasket soon as it is leaking oil. I can't figure this out!! Please help!!! I'm already poor and can't afford to do much else to it.

Answer 1

My first thought is that you made a mistake by converting to the open system. The stock engine is built to have a closed, pressurized system to adequately handle the engine's cooling needs and I am thinking that your problems are due to this switchover. The open system is OK at low speeds, but at high speeds it's very obviously not handling it at all. If you go back to the stock configuration I'll wager you won't have this problem...assuming you haven't already damaged the engine by now, that is.

A 4.0 Cherokee engine's not a high perf engine and barely enough for the 4WD application...way too fragile to handle a lot of mods. Don't try to make it something it isn't, unless you enjoy rebulding engines periodically when they go supernova on you.

Good Luck

Answer 2

Replace the head gasket now!!!!!!! before you kill the engine. Is the fan working ? The more work you ask the engine to do the more heat it makes. That leaky head gasket may be leaking compression into the cooling system. Open cooling system ???
you mean you have no overflow tank?.Does the performance rad have enough capacity, the correct flow ? is it designed for a cherokee? I've seen trick aluminum rads that didn't cool worth a damn. Make sure the electric fan is coming on .

Answer 3

If it were a cooling system problem the situation would be the opposite - it would overheat at low speeds/when idling. Moving at speed would keep it cooler.
Sounds like the engine is just generating too much heat for the cooling system to keep up at that speed. If there is a small engine problem, it will tend to throw out exponentially more heat as engine speed increases. The fact that you are burning oil tends to point toward an internal engine problem as well.
I'm afraid the prognosis is not very good :(

Answer 4

I have run into this complaint before...Sometimes a thermostat opens but only 1/4 to 1/2 way, restricting coolant flow at higher rpms.. Double check the t-stat first...Also check radiator hoses for possible restrictions...If that all checks out ok, you may have combustion leakage into cooling system....

Answer 5

Change the stuff you mentioed. What about the water pump? Also, if you paitned the radiator, that will insulate it. Is the fan working? Probably not. Your fan probably has ceased to work, because driving down the road its getting air, sitting its not. Look and see if the fan is working.

Answer 6

Did you replace your coolant with a 50/50 mixture?
check you belt tension er, it may have lost it's ability to maintain tension on the belt at high speed.
check you fan to make sure that it isn't cutting out at high speeds.
hope this is of some help.

Answer 7

try do no longer wager first investigate computing gadget floor [stunning part of engine rear of distributor on cyl head [over the years I easily have stumbled on extra beneficial than in elementary terms some broken or unfastened] next have alternator output checked and battery load examined and be certain the alternator diodes are doing their activity [no stray alternating modern-day entering into your direct modern-day equipment] treatment as necessary.[renault/bendix jeeps have been very liable to this] next clean out the throttle physique and air pass passage with carb air purifier and a brass "toothbrush" then making use of a test gadget, try the throttle place sensor. alter or replace as required. test techniques that artwork in this variety are MS 1700 [production facility gadget] drb 2 up and all snap on scanners from the brick up [this methodology makes use of a stay information circulation devoid of long term storage], in case you narrow back the main important off the computing gadget resets until yet another "experience" occurs. so look ahead to "fastened" values and implausable readings. the diagnostic connectors are under the two yellow plastic covers on the suitable inner fender halfway up those comparable connectors have been used on all jeep and renault fashions from 1983 up [different than grand wagoneer and J autos with V8 engines] on the two gasoline feed back platforms and gasoline injected platforms alongside with yours and definite the gasoline feed back platforms could be study with a scanner too.

Answer 8

replace the things u sed u needa replace and bring it to a mechanic to have it checked out

Answer 9

hose off the outside of your radiator. and check your clutch fan.

Answer 10

bad intake system or damaged air dam..