Hi guys, I could really use some help/advice before I contact my mechanic about a problem I've been having. I have a stock 99' Jeep cherokee sport 4.0L with 142,000, well maintained miles. Over the last week it has been running between 210-230, Highway-city. When stopped at a red light for any significant amount of time the temperature will quickly jump to 250 and the check gauges light will come on. I stopped the car and shut it off in a parking lot one of these times and it boiled over from the overflow reservoir. I just flushed the radiator, and changed the thermostat, but to no avail, its still doing it! I have no idea what the next step I should take is. Please let me know if i can give you any more information on the problem, any help would be greatly appreciated, Thank you all in advance!~~Jay

 

fan clutch!!!! if you are heating up setting still then you are not getting enough air flow over the rad. make sure you electric fan works as well. ( i believe it only comes on with the a/c)

 

The fan is working fine, and i don't think that's the problem since its not a gradual heat up.... It jumps from 210 to 250 in 1 second or less, and then after i get going anywhere from 30 seconds to 2 minutes in will go back down equally as fast. The small electric fan was kicking on earlier in the week when it was doing this, but now it doesn,t seem to be coming on at all and i thought it was because of the sudden jump in temp. What temperature does the fan normally kick on at??

 

I can also hear the main fan running loudly as my rpm's increase when its running warm above 210.

 

210 is normal temp. you may want to check that fan clutch any way just to rule it out. with the engine stopped. see if it spins freely. more than one revolution and its bad. running down the road you have plenty of flow. but standing still you are relying on the fan to move enough air, ya know. even though it is spinning, it may not be going fast enough at an idle.

 

Ok, ill go check that right now. could it be air pockets?? since i just flushed it out and changed the thermostat? Would that maybe explain the sudden jump?? The only reason i didnt think it was that, was because its doing the same exact thing as before that was done. be right back.

 

air pockets are not typical on an open cooling system. the temp sensor will only read if it is submerged in coolant, it will show a drop in temp if the sensor is surrounded by air.

 

the main fan does spin freely!!! and more than one rotation. So what does that mean?? Do you think that's the problem?

 

I'm still just wondering about why its such a sudden jump in temp?? If it was the fan clutch wouldn't it be a more gradual temp. rise since its not blowing enough air to cool down the engine at idle.?? I just don't wanna replace that and not have the problem fixed........Then again, i guess it needs to be done eventually anyway.

 

if it spins freely with just your hand turning it. imagine what its doing at 6 or 700 rpms. when you're running down the road, air is forced through the rad. (i'm sure you know all this.) when you stop you, you no longer have that forced air flow. if the fan is not spinning fast enough then you gradually heat up. once you start moving again, forced air flow takes over and cools it back down. get more than one opinion please. make double sure before you go and buy one.

 

I figure i'll give my mechanic a call tomorrow and see what he says. I really appreciate your help, and i'll update this as soon as i figure it out, cause it does seem to be a fairly common problem a whole lot of jeep owners are having based on all the forums! Waiting to see if anyone else has or had the same problem, that always makes it a bunch easier. Thanks again Throttle Jockey!

 

any time friend. good luck.

 

Your heater core or t-stat housing is blocked with gunk.

Mine was doing this exact same thing. After I flushed out the heater core real good, it started behaving normally.

Remove both heater hoses from the core. Flush the core backwards, from the bottom fitting, then forwards from the top fitting. Do this several times until no more particles come out. You will have to use some pressure, but not too much pressure, otherwise you will damage the core.