I have seen a whole lot of posts about jeep coolant system problems, but haven't seen the answer to my problem. Cherokee over-heated a couple days ago & boiled out the over-flow bottle. I replaced the t-stat yesterday & when I first warmed up the jeep it was staying steady at 200 degrees. The heater inside was blowing cold air. Thought I would drive around the block & temp started to climb so headed home & shut it off. I saw a lot of talk about burping the coolant system. Do 1999 Grand Cherokee's need the system burped? First time encounter with such a thing.

 

No matter what you do anytime the cooling system is opened if you opened any hoses and the thermostat there could be air trapped in the system, was the heat working before you replace the thermostat? Also have you ever bled a cooling system?

 

I am 60 & I have never had to bleed a cooling system. Sounds simple enough, but can not see the sender unit on the rear of the head.

 

Hadn't had any problems with the heater, but the day it boiled over it was sunny & maybe 40 degrees. Didn't notice if heater was blowing hot air. I've been boiling the old t-stat for the last 10 minutes & it doesn't seem to openeing up much.

 

If this is a 4.0 straight 6, They are notorious for air pockets. The best way to bleed the system I'VE found is to (with the engine cold) take out the guage sensor on the thermostat housing and fill the radiator until coolant comes out the sensor hole. put the sensor back in and top off the radiator. For an air blockage in the heater core, take off the return hose and put it in a bucket, crank the engine over without starting it until coolant comes out of the hose. This should eliminate all air in the system.

Both of these things should work with a V8 although I've never done one.
I've never had a problem with a V8. Most of my experience is with GM cars.

I'd say your overheating problem was a stuck thermostat.

 

should you have the jeep facing up hill? Also, it is the 4.0.
Would the return heater core hose be the one connected to the t-stat housing?

 

No, that's the supply hose. A slight uphill incline would help, but not really nesessary.

I'm 68, by the way. I also have never had to bleed a cooling system until I bought a jeep.

 

Thanks for the advice. At work now, but will do that tomorrow morning when I'm home.

 

Theres even a simpler way without removing any hoses,clamps, at all just take off the radiator cap and coolant resivor cap, next turn your truck on with the heat on full blast and "burp" the upper and lower radiator hoses, when i say burp or purge the hoses squeeze each hose a couple of times allowing air to exit each one. Remember coolant will be spilling out of the radiator cap so dont fill that to much, and be careful for hot coolant dont want to get burned. hope this helps.

 

No luck with the overheating. Pulled the t-stat & ran jeep like that after bleeding the system. Goes to 3/4 on the gauge when idling. another 1/8 when driving in the neighbor hood, but it has all water in it right now. This is my DD so I have to drve it to work tonight. It looks like I'm going to have to change the watrer pump &/or radiator. I boiled the new t-stat & it is openeing.

 

With the 4.0, it's quite possible to have shattered the water pump impeller. Some of them are plastic and if there is any wear in the shaft bearings, the blade can strike the #1 cylinder barrel. Buy the premium pump with a metal blade.

 

I'm going to start taking it apart tomorrow. I'll check that before I go buy a new radiator. Thanks for the heads-up.

 

I'm back on the road. Thank you Dave & Steve for sharing your Jeep knowledge. Turned out to be the water pump with plastic fins. They were all completeley broken off. Put a new water pump (with metal fins), used both of your bleeding technics & back to running great. Not a moment too soon, it's supposed to start snowing tonight here in W PA.

 

Glad to here your back on the road

 

Now that's funny! My WJ came from Washington, PA. It was bought by a local dealer here in Syracuse where I bought it. It was pristine and well maintained.