AllOriginal91

So, granted this is my first Jeep, which for the record, I really like (dead original 91 Cherokee Laredo) I normally figure these things out myself, even the tricky stuff. However, I have been outside all day in -5 degree weather trying to fix this. I am now enjoying a strong cocktail, seeing if I can pick the brains of other Jeep owners:
Yesterday, I warmed up the Jeep before heading to work. I had no heat, and the Jeep was running fine. I stopped at the corner store down the street, grabbed my caffeine, got in the Jeep and everything was working fine. I chalked it up to the fact that it's in the negative degrees over here in Montana. No problems for the rest of the day.
Today after running the Jeep for a few minutes in the morning, I had no heat again, and the temp gauge kept climbing. It hit max temp all of a sudden and stopped running. It wasn't overheating, nor had any symptoms of overheating. I started it right back up and went back home. I had realized that the last time I added antifreeze, was over a month ago when I replaced the water pump and serpentine belt. I used 50/50, therefore, in the negative temps, I'm going to have frozen coolant. I yanked the thermostat out, and also found a frozen chunk in the upper radiator hose. Started it back up, and the temp gauge never left 1/4, so much less than normal operating temperature, but safe. The heater was also working fine and consistent, just obviously not as hot as it could be. I headed down to NAPA and got some straight antifreeze and a new thermostat - just because. Drove just fine on the way back home with the same consistency.
I did a complete radiator flush once home, replaced the thermostat, put it all back together and went for a test drive: The temp gauge is all over the place, and the heater is cutting in and out. Cold-warm, cold-warm, it couldn't figure out what it wanted to do. Motor was running just as it always has, the whole time. Nothing wrong there. Some points:
*No water in the oil
*New thermostat
*New water pump
*No leaks at all
I would love some opinions. I also thought about the heater core being plugged/blocked, but why would that affect the temp gauge? I thought the temp sending sensor being out would be a possibility, but why would that affect the heater? There has to be some kind of correlation between the heater cutting in and out, and the temp gauge floating around between normal operating temp and max. Thoughts?
Thanks in advance!

skunkwork xj

Man I feel your pain, its the same temp
In Chicago.

I had the same problem with a '93 zj I bought years ago, it drove me insane and
I even changed the water pump thinking that
Was the problem.

Assuming you have the 4.0, I have learned the hard way these engines have cavitation issues.

The coolant isn't full even when you think it is. The system needs to be burped some how like parking the front of the Jeep on a curb and then fill it.

I had to keep pouring in coolant little by little and the air keeps bubbling out, (took forever).

I really hope this helps man, its no fun in the cold.

Nate5067

Heater not working? I just had the same problem where all of a sudden the heat stopped. I don't know if this is the same issue, but my issued ended up being the blower resister. After figuring out where it was it was very easy to fix.

cruiser54

If the radiator cap is bad, it may not be purging air from the cooling system. Air in heater core=no heat.

Natebub

do you have a closed cooling system? My friend had one and he converted it to and open system seem to help......its hard to get all that air out of those if they have that closed system.

cruiser54

It's not hard to get the air out of a Renix system if your cap is good. From the service manual:



Check this out from the FSM

This system does not have a radiator pressure cap. Instead the pressure cap is mounted on the coolant pressure bottle. System coolant flows directly through a fully pressurized Hot-Type expansion bottle. Coolant flows through the pressure bottle at all times during engine operation whether the engine is cold or at normal operating temperatures.

Larger coolant volume caused by thermal expansion during engine operation is absorbed by the expansion chamber in the bottle. Air trapped in the system is purged through the pressure cap vent valve during maximum coolant expansion..