Okay so here goes im gonna start from the beginning.

About 2 weeks ago my 88 cherokee 4.0 started to run hot (Hotter than usual, and its winter) but not quite over heat yet. Now 2 days ago I was going to school around 8 Am (About 30 degrees outside) and the gauge completely buried into the red and over heated. So today I replaced my thermostat (195), my resovoir(it cracked) and the cap. For some reason it climbs to about 210 and stays now. Sounds normal but it does it really quick. Its cold out and I just drove it about a mile or 2 to the store and it climbed to 210-215 (a little past the middle 210 mark) in the short time i drove it. (Maybe 5 mins) As soon as I turned into my development the temp gauge dropped to about 185-190. Then went back to 210, sat in my drive way for about 3 mins and it dropped to 190. My question is why is it climbing to 210 degrees so fast? Before all this it would stay around 185-190 right below the 210 mark and never really go over. From a cold start it only takes like 4 mins to get to 210 and that doesnt seem right to me? Before this it would take much longer to warm up. The water pump seems to be fine (replaced not too long ago) and so is the clutch fan

 

Sounds like an air pocket, those closed systems are notorious for trapping air. The sender needs, liquid not steam to read the temp.

 

How could I get rid of the air pocket? Q

 

https://www.cherokeeforum.com/f2/bur...-system-19001/ Lots of good info here

 

From cold to 210 in 4 min doesn't seem possible.....can u access a infrared temp gun to verify actual coolant temps/dash gauge readings?

 

ok maybe not 4 mins, but quick. Maybe I just never had a correctly working tstat in and i'm not used to it? But it doesn't seem right. Today I hooked my electric fan up to a switch and even with it on I'm staying in the 210 area, with it fluctuating back and forth while driving around the 210 mark. (-+ 10 degrees or so) Sometimes it drops the faster I go but sometimes it doesn't..and when it doesn't eventually it will randomly just drop then rise again

 

Note that it hasn't exceeded 215 220 or overheated since I replaced the tstat and resovoir but before all this it ran around 190-195 most of the time and only got up to 210 when i was on the trails. Could it just be air in the system or could my rad be clogged? Or am i just too worried about it and its normal?

 

 

Other than the t-stat, what other cooling system components have been replaced? May be time for some routine maintenance.

 

With the Renix, having a leak in your reservoir is bad, since it's no longer in vacuum... so replacing that probably helped.

Routine maintenance, like DJB suggested, is probably in order. If you've never done it, the fan clutch probably needs replacement. You can replace the coolant temperature sensor, it's located on the radiator, and tells the electric fan when to kick on to help keep it cool.

There's a trick to bleeding the air out of the Renix... I'll have to find the article, but basically it's something like drilling two 1/16" holes into the thermostat, one at 6 o'clock and one at 12 o'clock. The notion is that there is always some bit of fluid flowing from the lower hole, and any air bubbles should purge through the upper hole... at least, that's how it was explained to me.

 

The water pump is new, tstat is new and resovoir is new. Also I wired my electric fan to a toggle switch and even with it on and the heat on it stays at the 210. probably just air in the system

 

I suggest you convert your cooling system to a later open cooling system but when i bled my friends 90 XJ i left the heat on full let it idle with the pressure cap off and let the thermostat open it should get all the air out

 

Is the radiator a different size on closed vs open system?

 

i took my thermostat out and its perfect, so maybe the new thermostat was faulty? Before this the hose going from the thermostat to the radiator had no pressure and didn't get warm. I could squeeze it together. Now without the thermostat it gets warm and i cant squeeze it so obviously its flowing through there now.

 

Don't know about '88 FSM, but the '98 FSM states " do not operate without a t-stat".