90 XJ Cooling System ?

Didn't see anything in FAQ's, did a brief search and didn't see a replicated post.. so..

Have a 1990 Jeep Cherokee with the 4.0 and an automatic. Question is.. If I start it, and let it warm up, it'll get to a temp of 210. Will not over heat. Then when I start driving, it will start fluctuating between 210 and 165. Whether the heat is on or off. Whether the defrost is on or off. The coolant level is fine, I just did a thermostat on it yesterday night, refilled and bled the system. And it's still doing it. If the heat is on, when the temperature drops, the heat inside the truck is noticeably cooler so that leads me to believe it's not just the temperature sensor. Any ideas as to what would cause this? Belt is tight and all, not sliding around on the water pump. No coolant in the oil, no missing coolant at all. Just looking for ideas as to where to begin before I waste a bunch of money on parts I didn't need. I live in WI, and the highest temperature we've seen since I bought the truck is about 25 but it's been low teens or colder since. Truck was fine when I first got it, then we had a few REAL cold days (-10) when I got up to go to work, and now it's doing this.

Thanks in advance,

Jer

Not a great first post, but I'm stumped..

Did you check how tight the plug is on the temp. sending unit?

xjboy said:

Did you check how tight the plug is on the temp. sending unit?

Click to expand...

That would just affect the gauge though, correct? When the temp. drops on the gauge, the heat is noticeably cooler coming out of the dash as I stated. Unless the sending unit somehow controls the coolant going through the heater core?

Did you use a standard 195 degree tstat?

winterbeater said:

Did you use a standard 195 degree tstat?

Click to expand...

Yes. I bought it a few days ago, was going to put it in but I let the truck cool down. Then started it with the cap off, when the gauge hit about 190, coolant dropped, and there was heat inside the truck. Then while poking around for an answer last night I thought I'd just change it anyway.

The temp literally bounces up and down.. It'll be at 210 when I leave for work.. 30 seconds later its at 165ish, then right back to 210, the right back to 165. As soon as it hits one point (high or low), it's on it's way back towards the other.

Could very well be air in the cooling system. These seem to need to be burped. Disconnect the upper radiator hose and fill it up with coolant. Might also help to disconnect the upper heater hose from the motor and top that off too. Some people have had it start to work right after they go up a very steep hill. Possible raise the front end and run and fill it.

It's either that or the tstat is broken or in wrong.

winterbeater said:

Could very well be air in the cooling system. These seem to need to be burped. Disconnect the upper radiator hose and fill it up with coolant. Might also help to disconnect the upper heater hose from the motor and top that off too. Some people have had it start to work right after they go up a very steep hill. Possible raise the front end and run and fill it.

It's either that or the tstat is broken or in wrong.

Click to expand...

Tstat was put in, spring inside the engine as per the manual. Could be broken I guess but it opens and pulls coolant down right at 190ish.

Gonna try burping it again with the front end jacked up.

take the heater hose off from the t-stat housing and fill the system, that should push out any air you may have

The proper way to burp a Renix era Jeep is to put the rear wheels on a set of ramps and remove the sender in the rear of the drivers side of the head. Fill with coolant until it flows out of the sender orifice. replace the sender and put the Jeep back on level ground and fill the pressure tank until it is to the post in the tank. DONE!!!

Plan as it sits, considering the tools I have (everything worthwhile is at work). Is to install a Flush n Fill Tee in the upper heater hose. Fill it through the upper radiator hose. Use the Tee to burp the system, being as sitting level it will be the highest point.

Hoping this works/helps. Actually saw this on CF. Someone else had a very similar issue just recently.

i use the prestone plastic flush n fill tee's, they work perfect for this...

at first, i used to buy the entire flush system just for that little tee... then i noticed a local autozone has a BIN full of em! in any size you'd need! awesome!

what about your heater valve? sounds like its opening and closing erratically....
its $15, so you might want to throw that at it... just dont buy an autozone one.

Looking at mine, it looks like the radiator hose is the high spot.

don't let the t in there though - they get brittle and crack ... and the cap on them sometimes stops sealing after a while

xcm said:

what about your heater valve? sounds like its opening and closing erratically....
its $15, so you might want to throw that at it... just dont buy an autozone one.

Click to expand...

After thinking about it that makes a lot of sense... faulty valve thats opening and closing as it wants. Tried to burp it with the Tee.. no bubbles, just instant coolant.

How much coolant should that valve stop when the heat is turned off? With the heat off, and all the coolant that seemed to be rushing through the upper heater hose.. none of it was going back up to the reservoir or anything. Had to all be going through that valve. If I was in fact looking at it right.. The upper heater hose leads to kind of a junction that it can either go through the valve, or up another hose to the reservoir..

My 90 coolant system is over-efficient as well. Even in the mild winters here in VA I have the grill blocked off and I'm running the lower cover under neath the engine. I barely stay in closed loop on the interstate in 30 degree weather. It never overheats in the summer, the electric fan rarely turns on.

I have a 3 row all metal radiator from radiatorbarn, my engine is new and so is the rest of my coolant system.

Try blocking the grill and see if that helps.

Lock the valve open and test your system temperature. Don't just replace stuff when you can test it. Even if it's only $15. Those valves are weird. Later models like mine don't have them. Someone else had theirs in wrong and it screwed up their heat, but I don't think it messed up their engine temp.

winterbeater said:

Lock the valve open and test your system temperature.

Click to expand...

now we're brainstormin! good idea.

I'd check the vacuum to the heater valve...see if it's even attached. If so I'd be checking my "small line" vac system (the lines to the HVAC from the cannister behind the bumper). Still sounds like you have an air bubble floating around in there.

Have you replaced (and/or pressure checked) the bottle and cap? In the winter, especially nearer to the Arctic Circle it may cause a fluctuation in temp. Out here in Northern CA (it was a balmy 38F today) the fluctuation might be from 190 to above 220, which would indicate air or a perforated pressure bottle or a bad cap.

I've forgotten alot of this since I converted to an open system a year or so ago. Trundle on, let's troubleshoot.

best way to burp is to take the radiator intake hose off and blow with the reservoir open. and when you first start keep the cap off and watch for coolent flow.

Ok, I say air bubble because of the temp gauge fluctuation--been there, done that.

Lack of heat in cabin, then heat comes back--vacuum problem. Engine at idle, no load, plenty of manifold vacuum. Engine at WOT, load, low manifold vacuum. Vacuum reservoir system is meant to compensate for this.

Test the reservoir system first, because the heater/defroster/instrument panel outlet door vacuum motor is a pain.

BTW--needed to change the low-temp thermostat in my 90 4.0 so I did it today, put in a late model thermostat housing with the sensor outlet. Re-tapped the sensor outlet to 3/8 pipe, then put in a 3/8-1/4 pipe adapter and a petcock. After installing the new thermostat and modified housing, with new SS bolts, topped up the cooling system and warmed the engine to operating temp (210 indicated on gauge). Shut off the engine, jammed a 12" piece of 3/8" fuel line over the end of the petcock, held the fuel line up in the air and opened the petcock--air rushed out. Topped up the reservoir again and closed petcock--no air in system. Total cost of the changes--less than $20!

For the rest of the life of this 4.0 I can burp the system in under a minute, not including warm up time.

This modification was not my idea, I read it somewhere.

:eyes: