# 99 Jeep Grand Cherokee running hot at idle



## jl840 (Jun 2, 2010)

Had motor rebuilt in Feb. replaced thermostat and radiator. Now that summer has arrived, the vehicle starts running hot at idle. Once traveling it comes back to normal temp. Any suggestions?


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## NOYDB (Jun 2, 2010)

One possibility is that the wrong thermostat was used. They make ones hotter for cold climates. Don't discount the possbility that it was bad out of the box or failed early also.


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## Wycliff (Jun 2, 2010)

Check to see if the electric fan is coming on it may have a bad relay.


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## hammerz71 (Jun 2, 2010)

Depending on model it could have an electric fan or one that runs off hydraulic fluid via the power steering pump.  But the fact you are cooling off when moving dictates it's a fan not spinning.

If electric, it could be as simple as a relay, if it's hydraulic, you may have a fluid leak.

I was always under the assumption that I had an e-fan on my '02 Overland, but while changing the oil I noticed the lines running to and from it and was like "what the heck?", had to do some research on that one, never heard of a hydraulic radiator fan before...


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## Ed in North Ga. (Jun 2, 2010)

does it have the electrical fan, and the standard fan- or just one or the other?

 fans do go bad- let the engine get hot, shut it off, and try to spin the main fan- if it spins easily, it could be bad-- but they made two kinds. One has fluid in it, one a spring. If fluid, it shouldnt spin  when hot/engine off- not easily anyway. Mechanical ones have a spring that tightens when warm- same deal, it shouldnt spin easily when warm-
 For sanity, compare cold first, then hot- there should be a difference.

 Electric fans (and Im using 88-91s here) run off a relay, with the sensor in the side of the radiator- if the sensor gets gunk on it(or its wednesday-they were super finiky) the fan wont come on- most of us just bypass`d the relay to run off a switch on the dash-

 If I remember correctly, the 92s+ were designed to run around 210- I never let mine go above 200, and used a robert shaw brass thermostat at 180- (and it worked fine for years)- if I went to idle too long, I threw the switch and she cooled off pronto.

only other things that come to my mind are incorrect thermostats (Jeep/chrysler had reverse flow for a few years-) incorrect water pumps(reverse flow vs forward)- bad impeller in the pump- crud on the radiator- ect,ect- But the one thing that ALWAYS got us, was incorrect bleeding of air after a thermostat install-- probably not in your case though-


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## EuroTech (Jun 3, 2010)

its probably the fan relay located under the passenger front side of bumper behind headlamp assy,you have to remove light assy and drill two inch hole in plastic to get it out.
I own a auto repair shop and replace about four a year.


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## gtparts (Jun 3, 2010)

Things to check:

1. Coolant level
2. Thermostat (not likely since it cools with adequate air movement through the
                       radiator core)
3. Circulation (water pump)
4. Fan clutch (if equipped)
5. Electric fan operation (if equipped) This includes wiring, relays,  and sensors
6. ECM in most late models controls switching elect. fan(s) on and off.

Also, if the t/stat is left out, the coolant may move too rapidly to absorb the heat from the engine AND transfer it to the radiator core. The core / radiator may not be the proper one for the vehicle. There usually is a std. and a hd core. Yours may need the hd, but got the std. If it is the orig. radiator, it may need cleaning and repair and if repairs have been done, they may not have been done properly.


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## Grub Master (Jun 3, 2010)

If the AC  and the fan does not work  it is probably the relay.


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## jl840 (Jun 8, 2010)

The vehicle has both an electric fan and mechanical. I replaced the fan clutch (still overheating at idle). The upper hose is hot to the touch and hard when the vehicle is at temp. I did not have this problem until I got the AC repaired and now it runs hot at idle.


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## Wycliff (Jun 8, 2010)

When the air is on does the electric fan come on?


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## jl840 (Jun 8, 2010)

Wycliff said:


> When the air is on does the electric fan come on?



I cant see it because of the electric one. How do I tell?


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## Wycliff (Jun 8, 2010)

You should be able to hear it when it comes on. If its not comming on check the fan relay it should be located on the drivers side. The relay only cost about $10.


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## jl840 (Jun 9, 2010)

I let the truck run without AC on for about an hour. Did not overheat. Fan relay?


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## Wycliff (Jun 9, 2010)

More than likely the relay or something to do with the fan circuit


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## MikeEast (Jun 9, 2010)

Most stuff enables at least one fan when the AC kicks in, so if you start it without AC and then enable it, not only should there be an audible click under the hood as the compressor spins, but a fan -should- also kick in.

Best of luck in your quest!

Mike


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## Fat Daddy (Jun 9, 2010)

My 99 grand cherokee is on it's 3rd electric fan.  4.0 6cyl.
You should be hearing the electric fan kicking on.


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## fishybzness (Jun 12, 2010)

Definitely the Electric (Auxiliary) fan. It is the big one further forward...
Replace the relay first, it is cheap.

I had a 99 GC 4.0 for 10 yrs that went through 3 fans (1 factory, one under replacement warranty, sold with the 3rd) and who knows how many relays. I did some of the work and a mechanic friend did the others. 

My mechanic said it was a design flaw. The area the relay is in will hold water and short out the relay or fan or both. Last relay he put in, he drilled a drain hole somewhere and that was the last time I had an issue.


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## Fat Daddy (Jun 22, 2010)

fishybzness said:


> .....
> 
> My mechanic said it was a design flaw. ...



 Several of those on the Grand Cherokee,  anybody else have their rear brake lights melt from the heat of the lamps?


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