Overheating or Bad Temperature Sensor?

1998 Cherokee 4dr
4x4 Auto 4.0L w/ 160k miles

Alright, so first a little background:

On memorial day weekend I was out in the mountains camping, and it starting overheating. I believe what happened was that the thermostat stuck, pressure built up and ruptured the lower radiator hose, because there was no coolant the temperature sensor wasn't reading high. I continued to drive it to get out of woods where a tow truck could get me, and in the process ended up blowing the head gasket and warping the head.

So, yesterday I picked it up from the shop. They replaced the head gasket, machined the head, replaced the thermostat, and replaced the hose. On the way home (about and hour and half drive) it started acting up.

About a half an hour after leaving it was running normally, with the temp at about 225° (it was in the mid 90s outside), then the needle suddenly jumped up to about 250° (the first red dash) and the "check gauges" light came on. Almost immediately it dropped back down to 225°. It did that a couple more times before I pulled over to check it. Everything seemed fine, there was pressure on the upper radiator hose and it felt hot. The coolant overflow bottle was completely full and overflowing though.

After that I kept driving it home, and the temp kept jumping. Sometimes it would stay at 250° for a few minutes at a time. It also seemed to be related to the revs. If I gave it gas it would jump up, but if I let off it would drop back down. Eventually it ended up sticking at 250° for most of the rest of the way home (maybe 15 minutes).

This morning on the way to work it seemed fine. It warmed up steadily and normally, and didn't get above about 215°.

Any ideas what could be going on? It seems like maybe the temperature sensor could be bad, given how sporadically the needle jumped around (obviously it's not really possible for it to jump up or down 30° in one second). But, maybe it is overheating?

Sounds sort of like you had air in the system, that can cause erratic/sudden overheating according to the sensor. The air should work itself out of the system within maybe 4 or 5 start-stop cycles of the engine.

There are tow ways to verify, though: 1- get an infrared head gun and measure the temperature of coolant leaving the thermostat housing. See if it matches well with your gauge. 2- remove the gauge sender and do a resistance test. Basically you put the sender in a pot of water on the stove, and heat the water up while you measure the resistance in the connections. The resistance should change in proportion to temperature. You'll have to look up the actual resistance:temperature specs for your model Jeep. That will tell you if your gauge sender is working properly.

Also, make sure the electrical connectors are clean and tight.

It may sound odd but...
A few of things come to mind.
Firstly, what was the voltmeter reading during the "check gauges" time? If the alternator goes flakey, the PCM will give this indication.
Secondly, when was the last time you reseated the cluster? There are a multitude of faults that get generated by a bad connection to the cluster.
Thirdly, 215 is, in my opinion, mighty toasty. I do believe the needle should stay to the left of center on the temp gauge.
Fourthly, the overflow bottle overflowing sounds like it was overfilled. Coolant expands as it gets hot and this will cause the overflow if the bottle is too full. There is a line on the bottle that shows the fill point. Remove the excess and monitor.

If you have access to a OBDII scanner, it can read what the PCM thinks the temp is. Add a reading with infrared or laser thermometer and you will be able to figure out is the sender is faulty. Shoot the T-Stat housing for you reading. As the sender is a thermister and is, by nature, slow to respond, temps jumping around are not the sender. I personally suspect the cluster as it receives data over the CCD and figures out what to display. If I'm wrong, someone will chime in. It is what makes this forum great, we all can learn something.

Thanks for the tips guys. o-gauge-steamer, the voltmeter was reading normally when this was happening.

I do in fact have access to an OBDII scanner which connects to my laptop.

There are two temp sensors right? One in the thermostat housing and one on the engine block? Which one feeds the gauge and which one feeds the PCM? I should be able to monitor the temp with the scanner and compare that to the gauge to see if it's accurate or faulty.

I also might be able to find an infrared thermometer at work that I can borrow.

zerogspacecow said:

....I picked it up from the shop. They replaced the head gasket, machined the head, replaced the thermostat, and replaced the hose.

Click to expand...

What about the other hose, the water pump, and the radiator ? Are they still old, partially functional, and ready to fail ?

Cooling system issues are rarely one single part, its seems like your cooling system has been neglected and needs more than a little TLC. Neglected coolant can actually eat the pumping fins right off the water pump.

You didn't learn your lesson on the first go around ? Overheating means stop driving before you wreck something expensive.

Tim_MN said:

What about the other hose, the water pump, and the radiator ? Are they still old, partially functional, and ready to fail ?

Cooling system issues are rarely one single part, its seems like your cooling system has been neglected and needs more than a little TLC. Neglected coolant can actually eat the pumping fins right off the water pump.

You didn't learn your lesson on the first go around ? Overheating means stop driving before you wreck something expensive.

Click to expand...

Ha, you'd think so, but when I stopped and checked it out, it didn't actually feel hotter than usual, which is why I was thinking it was the sensor.

But yes, the radiator, pump, and upper radiator hose are still the original (or at least the ones that were there when I bought it). I flushed the radiator a month or two ago, and what came out resembled a chocolate milkshake. So, I'd be inclined to agree that it all needs replacing. But, that doesn't really explain the needle jumping around.

zerogspacecow said:

Thanks for the tips guys. o-gauge-steamer, the voltmeter was reading normally when this was happening.

I do in fact have access to an OBDII scanner which connects to my laptop.

There are two temp sensors right? One in the thermostat housing and one on the engine block? Which one feeds the gauge and which one feeds the PCM? I should be able to monitor the temp with the scanner and compare that to the gauge to see if it's accurate or faulty.

I also might be able to find an infrared thermometer at work that I can borrow.

Click to expand...

Starting in '97, only one sensor.

o-gauge-steamer said:

Starting in '97, only one sensor.

Click to expand...

Oh, so the same sensor is used for both the gauge and the PCM? The sensor is in the thermostat housing, correct?

^^Yup...

a half hour after you picked it up from the shop it overheated? I wouldve headed right back to the shop. I have experienced a warped head situation before. Thats not cheap. I agree that they probably didnt get all the air from the system but it seems you could get them to fix it for you.
I rarely take my vehicles to a shop because i am self abusive and love a challenge. But when i do, i expect it to roll out working as it should.
Not that it matters now but i second the fact that the block wasnt properly purged of air.

I'd also be willing to bet you had a bad head gasket from the get go which may have caused all the problems. hence the erratic temps and pressurized overflow bottle.

you mentioned "chocholate milkshake and blown lower hose " and no one noticed ? had the same in my 98, superflushed mine twice , then lost heater , endid up replacing : radiator , hoses , pump , thermostat , heater core , that was fun been runnin clean and cool ever since . dont waste time , replace it all , mine became so plugged with chocolate milkshake mixed with grit it plugged everything.

Wallyman said:

a half hour after you picked it up from the shop it overheated? I wouldve headed right back to the shop. I have experienced a warped head situation before. Thats not cheap. I agree that they probably didnt get all the air from the system but it seems you could get them to fix it for you.
I rarely take my vehicles to a shop because i am self abusive and love a challenge. But when i do, i expect it to roll out working as it should.
Not that it matters now but i second the fact that the block wasnt properly purged of air.

I'd also be willing to bet you had a bad head gasket from the get go which may have caused all the problems. hence the erratic temps and pressurized overflow bottle.

Click to expand...

Shop was closed when I picked it up, and is an hour and half away from my house. Otherwise I would have taken it right back. I'm going to get a laser thermometer from Harbor Freight tonight to see if it's actually overheating.

Nah - Get a hold of the shop and do what you need to do to get it back there. The more days / miles you have it, the less likely they'll be to fix what should've been fixed the 1st time..

It seems that the problem may have been air bubbles or something. The gauge hasn't been jumping anymore, and when I measured the temp with a laser thermometer it seemed normal.

The temp gauge was reading just under 210°, and the thermometer was reading between 200°-210° depending on where on the thermostat cover I pointed the laser.

I obviously need to keep monitoring it, but hopefully that was all it was.

I did call the shop and let them know what's going, so that they're aware in case I need to bring it back in.

Just a quick question, does the T-Stat have the air bleed hole in it? If not, it needs one and the T-Stat needs to be installed with the hole at the top.

I'm not sure, they installed it at the shop. I can pull it off and take a look if necessary.

Alright, so it is actually overheating. On my way home yesterday I saw it jump up to the first red line. So, I pulled over and measured it with the laser thermometer and it was at about 245°.

It may indeed be that I need to replace the cooling system components, so I'll look into that.

fulltimer said:

you mentioned "chocholate milkshake and blown lower hose " and no one noticed ? had the same in my 98, superflushed mine twice , then lost heater , endid up replacing : radiator , hoses , pump , thermostat , heater core , that was fun been runnin clean and cool ever since . dont waste time , replace it all , mine became so plugged with chocolate milkshake mixed with grit it plugged everything.

Click to expand...

Did you flush it first, then replace the parts? Or did you wait until after replacing everything to flush it?

I'd be worried about leaving junk in the engine if you did it the first way. But, then I'd be worried about getting junk into the new parts the second way.

Is it just the radiator, water pump, heater core, and hoses that you replaced?

I know this is a old thread, but I had my temp gauge jump today as well.

It was 107 here today and I had the AC running. The xj was holding steady at 225 and then shot up to the redline. I immediately pulled off the interstate, rolled down the windows, turned the heat on high (to pull heat from the engine) and sat for a second and it came right back to 225. I then drove home with the heat on and the windows down and that helped pull the temp down to around 215 till I got home. It was brutally hot here in SC today, so I am not worried about the long term driveability
of the Jeep. I am thinking that an oversized radiator might be nice though to serve as a bigger heat sink and heat transfer unit.

If your cooling system is properly maintained, and functioning to original specifications. you do not need an over-sized radiator. Holding 210* is normal and well within your Cherokees capabilities.

You should be fixing the source of the problem, not putting over-sized Band-Aids on it.

Overheating can be caused by anything that decreases the cooling system’s ability to absorb, transport, and dissipate heat, such as a low coolant level, loss of coolant (through internal or external leaks), poor heat conductivity inside the engine because of accumulated mineral deposits in the water jackets or radiator, a defective thermostat that doesn’t open, poor airflow through the radiator, a slipping fan clutch, an inoperative electric cooling fan, a collapsed lower radiator hose, an eroded or loose water pump impeller or even a defective radiator cap.

The cooling system is a group of related parts that depend on proper function from each of its component parts to keep the engine cool. Service the cooling system and replace any under-performing or suspected weak parts. Any component part of the cooling system that is not fully doing its job will stress the others and your cooling system will overheat. The most important maintenance item is to flush and refill the coolant periodically. Coolant should be replaced every 36,000 miles or every three years. Anti-freeze has a number of additives that are designed to prevent corrosion in the cooling system, but they have a limited life span. The corrosion causes scale that eventually builds up and begins to clog the thin flat tubes in the radiator and heater core, causing the engine to eventually overheat.