Overheating issue - Can't understand symptoms

So I've done a search and can't pin down the issue.

This is a 2001 Cherokee. Automatic with the 4.0. That said, previous owner installed a 97 motor due to head issue and swapped water pump, radiator, hoses. About 180,000 on it, maybe 100,000 on motor. I don't know about the fan clutch(???).

That said, my daughter is driving the vehicle from Augusta GA to Dallas tonight. ON the way here to the Augusta area, the truck was overheating but she would fill up the overflow bottle which was emptying itself. When she got here, I drove it after putting on a new radiator cap and topping off coolant. No issues. She's been driving all day now and it just overheated outside Dallas. Overflow empty. So it went about 950 miles. She not much pressure on the upper hose so she (with towel) opened cap to first dedent and said that there was fluid coming out so she secured it. Car has cooled down so on the road again.

OK, so when I had it here, putting coolant in the radiator was kind of pain, it wouldn't really fill fast. Put a little in, it comes up to the neck and then flows down. Repeat. My thought was that there was air in the system and it was drawing the coolant out of the overflow over time. So I was told to take cap off, had it running and squeeze bottom hose to burp it. Did that, couple of air bubbles, but nothing major. Did this for about 15 min or so. It ran coolant stayed level in radiator neck. Maybe thermostat not opening? But if that's the case, it wouldn't have made 950 miles...

No leaks, oil dipstick is clean, new oil and the oil refill cap had no milk shake in it.
I'm at a loss why the overflow keeps emptying, no leaks evident, no smell of coolant inside the cabin or in the engine.

BTW, before she left, I had it up to temp and the gauge was a tick below center. Now it seems to be a bit above center but of course when it heats up, it moves to the right.

Thoughts greatly appreciated.

Heater core? I'm not sure if you'd be able to smell it leaking.

It doesn't have to be a big leak with a day of driving.
Had Hose clamps seeping minuscule amounts, never able to pin point it , every two weeks we would add fluid to reservoir, ended up redoing the water pump and hoses (because a pulley broke) and it's gone now. When the idle pulley broke it broke the metal pipe that goes from the water pump to heater hose . (Btw where that screws in to the pump is a good spot for leaks)
When that pipe broke the temperature shot up and pushing a lot of fluid into the over flow.


Head gasket can go just enough to seep into the combustion chamber but not smoke or get in the oil.

I kind of feel it's a leak in cooling system. But I'd tell her to top off the reservoir tank every pit stop.
Hopeful she'll get where she's going without further trouble.

Maybe after a night of cool down and burping out air it'll run closer to temp.
If the blocks hotter than normal when it ran out of coolent maybe even with the t-stat wide open it not going to cool down past a certain point.

Hope she gets where she is going safely.

Some points of consideration.

The cooling system is normally under about 15 psi of pressure. If the pressure is there, coolant would not be sucking out of the reservoir while running. Its supposed to expand into it, and then when it cools, vacuum sucks it back in. The pressure raises the boiling point of the coolant. So, if there's no pressure, then it would seem like you have a leak. A common leak point is where the plastic caps are crimped onto the aluminum radiator core. But if it was sufficient to cause a problem, I'd think you'd see coolant. And if the heater core is leaking, I think you'd smell it and your floorboards would be wet.

If the head gasket or head blew direct from exhaust to a cooling passage, you'd often still get a pressurized cooling system because it would be filling with exhaust gasses.

If the air bubble in the engine coolant was significant it would overheat just idling.

I don't really understand how you have no pressure in the cooling system, but seem to have a vacuum effect, without an external leak unless the radiator cap is bad. I would start by changing the radiator cap. Then maybe pressure test the cooling system.

One thing is for certain, repeated overheats will surely blow the head gasket and or the head so immediate attention will reduce the overall repair bill.

You could also try burping the system by raising the front driver side and running with the radiator cap removed.

I much appreciate the insight. Thank you.

As for the problem, she filled up the overflow tank and let the vehicle cool down as it started to overheat again after immediate filling of the overflow. She released the new cap to the first detent and fluid came out. So I told her to reseal it. The upper radiator hose was not really pressurized she said.

Once the temp came down a bit, they drove another 100 miles or so to their hotel. She drove a little less than 75mph (speed limit in TX) and the gauge pretty much did not move from the middle of the gauge.

So, question is is seems to go ok with coolant in the overflow tank. It sucks it out of there and then overheats. I don't think its normal but can't put my finger on the cause. If she ran it with the radiator cap removed, wouldn't all the fluid come out?

Further, when filling, as it warmed I never got a real good fill meaning I'd fill the radiator a bit, wait until it went down then kept filling. Took a while. I thought with the thermostat opening, you generally watch the coolant go down and then fill quite easily. Would these symptoms occur with a bad thermostat...or forbid...a thermostat put in wrong? But then the motor would overheat immediately, right?


New cap by the way and its a Mopar piece.

I think its going to be kind of hard to resolve this when the Jeep is 5 states away. I suggest your daughter take it somewhere to have it looked at. Failing any external leaks, and a good working radiator cap, the problem is usually the head or head gasket. In some cases, the radiator cap just doesn't seal on the radiator correctly.

My take was the radiator cap as well, so that's why I put on the new one. She'll be in Lubbock, TX and I don't know any shops there. She's had it checked once due to the symptom of smell and heat and they tightened a hose after doing a pressure check.
I hate to throw parts at it, but I feel like I need to start from the beginning with parts again. Including fan clutch and maybe figure out why electric fan isn't coming on except when I put on AC and then only for a short bit. Frustrating! (especially from 5 states away!)

On 01 XJ PCM's the e-fan is triggered by the coolant temperature sensor at 226F or when A/C high pressure combination switch/sensor reads high enough. If it came on for the A/C high pressure switch, it will cut out when the pressure drops.

On 97 XJ PCM's the e-fan is triggered by the coolant temperature sensor and whenever the A/C clutch is activated.

Probably a lot of air trapped, overflow won't automatically refill the radiator especially after the it lost enough fluid to over heat.

If the thermostats weren't working it's never made it out of your state.

How much farther does she have to go?
Maybe, just maybe keeping the tank topped off will forestall the overheating but.... As much as this is losing with no signs, I mean it's practically going dry when it finally spikes temp. I'd be very worried something big is going to fail very soon.

It needs to be checked out, pressure tests such. If lucky it's a matter of refreshing the cooling system parts.

I'm wondering what else the previous owner did. 97 motor in a 2001 doesn't work with the factory exhaust headers. At least as I understand it the 2000-2001 have different exhausts than 91-99. There are work arounds so it very possible it's exactly was he said.

There are some leaks say in a bypass hose that will only appear under great pressure. Got a Toyota that has very high pressure water pump, couldn't find the leak because it'd never except under extremely pressure. Also freeze plugs can leak intermittently.

Sorry I know this is disjointed stream of thought, tapping this out on my phone so it's a mess.

What was the symptom of smell? Lack of pressure and smell tells me heater core. And that's not uncommon, but it's not cheap because of the labor to pull the dash and heater box.

md21722 said:

What was the symptom of smell? Lack of pressure and smell tells me heater core. And that's not uncommon, but it's not cheap because of the labor to pull the dash and heater box.

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If it is heater core that's the best news they can get. Bypass it and loop the system. Fix the core when there is time and money.


But in my experience there are so many pin hole leaks which can evaporate it's hard to trace. IMO Heater core leaking at this point is good news.

They made it to hotel with no issues, keeping overflow full. She topped off this morning and temp is below 210.

But, here is a symptom of future things to come as was mentioned: when she stops, oil pressure drops to 0, and when running it sits at about 20. "Check gauges" light also comes on when she stops. Is that a sign of head gasket trouble? Idles ok and oil was changed before they left with Mobile 1.

Before hose was tightened at shop and pressure check done, there was a coolant smell only at startup in cabin, very short and a whiff of coolant under hood, probably from that hose. This in answer to the heater core question.

Appreciate all the thoughts and twists and turns this is taking. Does not sound like it is aux fan issue then.

Just came off a stop, filled coolant reservoir and oil is full...temp gauge below 210 now and oil pressure, with 40 in the center, needle is reading between 2nd and third mark up or 1.5 marks below the 40psi mark.

It's possible that the shop knocked into the sending unit by the oil filter and broke it. Should be checked with mechanical gauge.

In my experience, hwy overheating is usually a radiator problem or head gasket, since the fans really have to effect at 75 mph. If you see it again, take a temp gun and move back and forth and up and down across radiator, looking for sharp changes in temp. Any knowledgeable mechanic should be able to do this if she takes it somewhere. Doesn't exactly help with the losing coolant, but I would check this, especially with the mileage.

Thank you. Sounds like the oil pressure gauge isn't indicative of something right now.
She's going to have to keep the radiator overflow full.

vertrx said:

Thank you. Sounds like the oil pressure gauge isn't indicative of something right now.
She's going to have to keep the radiator overflow full.

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Um. I'm not so sure.
While it is true everything needs to be done with proper tests to verify actual oil pressure, compression, leak down, etc. That is sound advice and will save money and trouble later one.

My experience with XJ oil sender is they are pretty accurate, either work or don't. I've had my older 80s xj actual clog up the sender. Our 92 4.0 has always been accurate and I could judge when it need oil changes by the gauge.

The reading near 40 is about right for on the highway at operating temp. Should drop to no lower than 13 at idle.
If the pressure is low enough the engine should start making low oil sounds, like lifters. If it does that then the sender is giving accurate readings.

Low reading can be caused by bad sensor, short in the wiring, bad Dash cluster.
Low pressure (meaning accurate gauge in this case) can be bad oil filter (fram), clogged pick up screen (usually gunk, sometimes paper from a bad filter), bad pump or pump related gears.
Maybe oil was so hot viscosity dropped too much

Possibility its over filled too, long time ago a place it work had a rollback making a 400 mile round trip, about half way home the driver called to say he parked it and need someone to come get him because as the tripped possessed the oil pressure kept dropping. turned out the I think it was a fuel pump on the old diesel engine broke and was slowly leaking fuel into the crank case.

I'm not sure if head gasket can do the same.

Ask if the engine made any sounds when it did the low pressure warning. Seems like it is working close to right now, but if it did clatter with low pressure the gauge is telling truly and there's trouble somewhere in that motor.

Glad it seems to be staying at operating temp with keeping the res tank full, just to be sure she should check the oil level/condition too next time she stops.

She says its been making same noises as always. Could be oil got hot, but with Mobil 1, I'm not sure it will thin out to that degree.
It drops to close to zero she says when she stops.

This remote diagnosis is a pain. She made it back, no fluids underneath. Weird.

Glad she made it safe.

The 4.0 is a good motor if she had a Newer jeep with a 3.7 I don't think she'd have made it.

Agreed.
Interestingly, she has the 4.0 in the Rubicon as well. Its an 04. She's just driving the Cherokee since its bigger and was carrying two dogs. That vehicle was also experiencing overheating issues on this trip. That may truly be an air bubble issue as we changed radiator, hoses and water pump. I haven't seen the issue manifest itself locally though.

But, when it rains it pours and so she collected herself a speeding ticket in a speed trap. I thought she had slowed down with the issues...but no. So, she's got to deal with that.