1994 Jeep Cherokee (XJ) 4.0L Overheating

I have an XJ that is overheating. On the advice of a mechanic, I changed the head. I also changed subsequent gaskets, thermostat (fail safe), temp sensor, temp sender, heater valve control, all coolant hoses, had the radiator tested (after market 3 core), new trans cooler lines, tune-up, PCV and breather valves, oil filler cap, and radiator cap.

I put it all back together and I still have the same problem. It will get above 210 in about 10 or 15 minutes of just sitting and idling.

I am not sure if this is a symptom, but when I have the radiator cap off, trying to burp the system, the fluid flows out of the filler cap, when I turn on the heater.

The aux fan comes on when I unplug the temp sensor and turn on the AC, but it doesn’t come on as it is overheating. I know it used to come on in the winter, when left idling to warm up. I changed the relay, but nothing changed.

Just about the only thing you didn’t mention is the water pump. That the truck is overheating but the radiator fan isn’t coming on suggests that there isn’t good coolant flow between the engine and radiator, which would be consistent with a water pump (or a thermostat or clogged coolant hoses, but we’ve presumably already eliminated those options).

If the radiator fan won’t cycle with the temperature climbing past 210, what happens when you turn on the AC and the fan does cycle on? Does the fan keep the temperature below 210? And is there only one radiator fan? Some Cherokees have a belt operated fan and electric also.

Do you have a fan relay or a fan control module? Those are the places to look if the temp is going too high at idle and the fan isn’t going on. Does it overheat at highway speed? If it doesn’t, that also points to a fan problem.

No, I haven’t replaced the water pump. I tried to wiggle it when everything was off, but it has no play in it. I turned it by hand, and water shot out of the bottom hose connection. It also does not rattle when at operating speed and temp.

Yes, there are two fans. The auxillary fan (electric), does come on when I turn on the AC, and also when I unplug the temperature sensor on the thermostat housing.

I replaced the fan relay, before I ever messed with the head. No change.

I have a 1995 Cherokee and my auxillary fan has always come on and off at different times. Whether I’m running the AC, in the dead of winter, summer etc. I’m no mechanic but it sounds to me that you have a faulty water pump. It may not be completely broken down which causes it to rattle but that does not mean it is operating to capacity.

I replaced the water pump yesterday. While I was at it, I replaced the Water Outlet (t-stat housing), with a new one. It was pitted on the hose fixtures, and the mounting surface had a few cuts and gouges from being scraped a half dozen times during this adventure, and prior ones.

I do fine at idle, in park. As before, I am overheating in gear. I sat in front of the garage and did the following last night:
-started the Jeep and got to operating temperature (normal temp)
-turned on the heat in park (normal temp)
-put it into gear, with the brake applied (temp rose a bit)
-in gear, with brake applied, and idle to 15-1600 (temp rose towards 3/4 mark)
-with idle staying at 15-1600, put back into park (temp returned to normal)
-put back into gear, but reverse this time (temp rose again)
-put back into park, at 15-1600 rpms, and at idle (temp returned to normal)

So, what would make the Jeep overheat at the same RPMs, where the only variable is being put into gear, but does not matter if it is driven or not? Are there electronics involved, such as the PCM being fed info from transmission? I still have the aux fan not coming on when the temp rises like that.

I put in the new radiator last night. I am concerned about something, and wonder if this is logical reasoning on my part.

When I disconnected the trans cooler lines, I removed the bottom one first. When I did so, there was NO trans fluid that came out…from the line itself, nor the radiator. When I loosened the top line, I started getting drips in just a few turns of the wrench, and subsequent drainage after removal of the connection.

Would it be logical to assume that with the temperature sending unit on the BACK of the head, near the transmission, that if the trans cooler part of the radiator was plugged up, the transmission could significantly heat the rear of the engine, to raise the temperature gauge towards overheating? I ask this with the added knowledge that sitting at idle for however long, the temp stayed around 210 on the gauge, but it would get hot when I put it into gear, even if I sat at idle with my foot on the brake. I would guess, with the temp sensing coming from the engine coolant sensor on the FRONT of the engine, for the auxillary fan, they could be at two different temperatures, and hence the fan would not come on, due to the sensor on the front, while the gauge would show overheating, with the sending unit on the rear?

The reasoning would be that your trans is introducing excess thermal input to the radiator. This shouldn’t be the case at idle or anytime. It’s apparent in your case.

…but you’ve got a fundamental issue with the coolant temp sensor. Even though you replaced it, the one you put in is defective (by my take). The fan turns on when you open the circuit and the PCM detects no voltage. It enables the ground for the relay. Yet it does not enable the ground with bona fide (alleged) proper input. Your fan is not cycling because of it. It enables the ground for the relay when AC input is detected by the PCM. This tells me that the ground enable circuit is good in the PCM …hence this points to the new sensor being defective.

I don’t think the transmission could overheat the engine as you say, but if you’re not getting flow through the cooler, I’d install an aftermarket cooler. They work much better than the stock ones and will rule out this cause, however unlikely.

Will the Jeep go over 210? Have you hooked a scan tool to it to see if that’s what the sensor is really reading?

I just came back from a test run and it gets up a little above 210, but not near what it was doing prior to the radiator replacement. It may have had other issues and I guess the radiator shop may not have flushed it when they had it for the cleaning.

I have not hooked up a scanner to it though. I do not have one. Thanks for the suggestion. I will see if I can find one.

The fan came on during my test run, without the AC or heat on. The climate control was off.

very interesting, and the reasoning is sound. fluid wont always drain from a tube unless the tube is open at both ends, but a flow constiction in the cooling lines for the transmission fluid in the radiator is a possibility here for sure. Is there any damage to the transmission or head degrading of the transmission fluid from overheating? It might be prudent to change the transmission fluid at this point.

I meant the fluid came leaked from the upper fitting I was removing, never the lower.

I changed the cooling lines, fluid and filter, when I did the head. It was a muddy black color with a red tint, when I changed it, and the cooler lines were leaking, which is why I went ahead and changed them. The fluid looks find now, on the stick, and I did not have to add any after changing the radiator, which seems a little odd. I am not sure how much fluid would be housed in the trans cooler part of the radiator, though.