1990 Pioneer OVERHEATING?? CLOSED COOLING SYSTEM!

Hello all, I have a 1990 Cherokee Pioneer. Started overheating about 1 month ago, I have since repaired the auxiliary fan so I can operate it with a toggle switch from the dash, installed a new thermostat and thermostat housing. I was driving up into the hills yesterday and once again about 15 minutes into the drive It started running very hot again. I pulled over and the coolant resevoir had a crack/hole in it and was spraying out/over heating. I went to Nappa and got a new reservoir with lid. I installed it, now the battery is dead, anyways, I replaced the battrery and it fired right up. I was driving it home and it started running hot again!? I turned on the aux fan and that seemed to help, a little??, but not for long. I'm still not 100% confident that the cooling system is operating properly. It is a closed system. How do I make sure that there is no air in the system?? I have to "Burp" it somehow don't I?? Im also concerned that the alternator is not charging the battery, I forget how to check that??? Any input would be apriciated! Thanks again!

Loosen(almost to removal) the temp guage sender. Located at the rear driver's side of the head. When antifreeze starts running out, tighten up. Air will be removed from the system. Be careful, everything will be hot, duh. Obviously if you get the air out, have a new tank, thermostat, you'll have to go further, radiator been checked?

My fan works by a switch also, but will usually only need it in drive-thru's in weather like were having now.
Check your charging system with a voltmeter. You should have close to 14 volts with it running. Most of the chain parts places will check it for you.

Thanks, I checked the battery this morning with the Jeep not running, its at 12.4 VDC. I checked it while it was running and it was the same!?? Looks like the alternator might be defective dont ya think? The radiator was recently flushed at the shop, it was replaced 5 years ago or so. How can I tell if the thermostat is operating properly when the Jeep is running? How can I know if it is open or closed? Also, about how much coolant does this system hold? Ive added about a jug and a half of 50/50 since I added the new reservoir yesterday? Thanks again!!

The caps that come on the new pressure tank to be blunt, suck. Many threads on here to prove that. The fix for the crappy cap is a Volvo pressure cap. I can not remember what year but do search and it will come up.
As far as air in system, KsMJ has it right. Only thing I'll add to it is if you can get the rear of the Jeep up a tad higher it will help get all the air to the sensor. The sensor is on the drivers side rear of the head. You can back the jeep up a small incline or back up a set of ramps, etc. to get the rear just a tad higher so that that sensor is the highest point in the coolant system(besides the pressure tank that is).
What temp thermostat did you get?
Turning on the aux. fan after it starts to overheat ain't gonna help alot as you already noticed. If you have the fan to a switch only just turn it on when you start the jeep so you do not forget about it before it is too late. The stock sensor in the radiator that turns the fan on in both of my 89' jeeps has never had an issue turning the fan on when it was suppose to.
Also in your bottom radiator hose...is there still the spring inside of it that keeps it from collapsing? I have had that issue on mine once. Might be worth a check. You can check the hose by squeezing it with your hands, it'll be obvious if there is a spring in there or not.

As far as the alternator is concerned....Did you check it running at the battery or at the battery post on the alternator itself? Check both before replacing alternator. A bad main wire from the alt to the battery can cause low readings to.

The thermostat is a 195 degree. I will check the lower radiator hose when I get home this evening. I checked the VDC on the battery before it was running and while it was running., not at alternator it self. Thanks again, you guys or gals have been great!

I would do the alternator checks as suggested. In my experience, the readings would indicate a no charge situation. If you want to be safe before replacing, some places can bench test the alt. Yes, the caps suck. I'm planning to change to an open system, but I can't seem to stay ahead of problems lately as my poor MJ has become most unreliable, something broke all the time. Last time I filled my coolant system it took just shy of 2 gallons of antifreeze, IIRC. I'm dealing with cooling problems on my Deere mower at the moment, better get back at it.

The part number for the Volvo cap is 10244.

Thank you all! I had the alternator tested and they couldn't complete the test because my new battery was almost 80% drained. I knew at that point that the alternator was toast. I installed a new alternator, she's running like a champ! Still have to figure out this overheating issue, the lower radiator hose does not have a spring in it,, none of them do. Is it ok to open the cooling reservoir and add coolant while the engine is running, when its not hot of course. How exactly does this closed system work??

I tried to look up that Volvo part # for the coolant reservoir cap, it kept pulling up a regular open system radiator cap??? Any help would be appreciated! Thanks

I just gave that number to the local Napa and they crossed it right over.

It's a 703-1396 Napa number and retails for $11.68. Had to get my receipt out of the garage to find that out...

Yes, you can add coolant with the engine running but it can suddenly belch out on ya when you least expect it.

Get that cap on there. This closed system doesn't work well at all with a bad cap.

Thank you Cruiser54, I will pick up that cap today! With the closed system, how do you know when youve added enough coolant? Can you add too much? Sorry, probably sounds like a stupid question but im not familiar with the workings of a closed system. Do I need to "Burp" the system from the temperature sending unit on the head ??

About half way full. Look down in the hole and you'll see that plastic tube sticking up. Just about the level of the top of that tube.

I purchased and installed the new "Volvo" cap, fits great. I drove it around town for 30 minutes or so, it ran right at 210 the whole time. The real test is when I get on the highway and head up into the mountains, that when it has overheated in the past.

I'm looking forward to hearing back after the true acid test in the hills.

The only sure way to test the thermostat is to pull it, and throw it in boiling water. It should open quickly. If you have doubts, just replace it. It's not that hard. I had a new fancy, high flow thermostat give out on me up on Imogene Pass. Thought for sure it was the head gasket. I started disassembly, and tested the thermostat just for grins. It was bad.

Also consider replacing the radiator with a Performance Radiator Two row. It's a copy of the Modine radiator, and while aluminum and plastic has worked well in my 90.

Finally, there is nothing inherently wrong with the closed system. It is difficult to burp the air out of it. If you look under the hood of a supercharged Mustang 5.O, you will see two closed system reservoirs, one for the intercooler.

I will check the thermostat and let you guys know! Thanks again! Can I pick up the new radiator at my local auto parts store?

Also, I noticed some dark colored debris in the bottom of the new coolant reservoir, looks like very fine sand or dirt, no chunks, just a fine layer?? Does the radiator need flushed?? Whats the best way to do that?? Thanks again.

With your symptoms at highway speeds, I'm suspicious of a flow problem. Could be a clogged radiator or a water pump with rusted off impellers.

Gotta remember it's a 21 year old vehicle and everything is suspect depending on maintenance history. A real common problem after maintenance on the Renix era is an air pocket at the back of the head. Didn't see if you were pulling the temp sensor to get the air out?

The Ford bottles never had the problems these pos Jeep bottles do with holding together, not deforming the threads, far better cap.. I bet there's a model that will fit right on the stock tray.

That air pocket should be out by now after a few hearing/cooling cycles. The Volvo cap usually solves the coolant recovery bottle problem as long as the bottle isn't cracked. I'm sticking with radiator being primary problem, wate pump possible secondary problem.