Temperature gauge is going crazy

jeeperguy21

jeeperguy21

NAXJA Forum User
Location
Syracuse, UT
Ever since I got a new radiator over a year ago, my temperature gauge has had moments when it has worked just fine, but then some days it just falls suddenly to about 120 degrees. Is it just a bad thermostat or what? Just in the past couple of days, it seems like it has gotten much worse, never showing the temperature reaching above 120 degrees, which has me concerned because the cooling fan will not come on with the temperature reading only that warm, when the engine is extremely hot, seeing how it is summer and everything. Anyone else have this problem? I have a 99 XJ Sport.
 
Temp sending unit....try swapping that out with a new one....
 
But is the vehicle really overheating????

I'm an oldskooler (back when Naxja had the other kind of forums), but got out of the internet scene. I kept my 98 XJ (highly modified), mostly up at the ranch in Colorado, so it's not a daily driver. I am having serious overheating problems (so the gauge says), congruent with the other posts in this thread.

Basically, I have no issues at idle or slow speeds. Anything over a constant 60 MPH and the engine overheats, with the problem getting worse. It used to be 80, then 70, now 60. Driving along, the engine has good power with the temp gauge indicating about 220. The gauge goes all the way up to max temp immediatly, from about 220 indicated. This especially happens on long climbs uphill at freeway speeds. Anything over about 25% throttle at freeway speed for more than a few seconds will produce a factory temp gauge spike. Letting off the throttle for a few seconds (sometimes 10-15) and the gauge will instantly drop back to 220, where I can resume my 25% throttle position. Flooring it up a grade will produce a spike within about 25 seconds.

I'm not sure it's just indicated though. The gauge works in all other conditions, for cold starts, and specifically when using the engine as a brake down long descents, I've seen it get down to about 160. I really think the engine may actually be overheating, because a couple of times I have pulled over and stopped, and I can hear boiling coolant in the engine. Even the overflow is boiling.

At first, I thought it might be the winch and lights on the front bumper occluding airflow, but the problem is getting worse and worse, which is contraindicitive. Then I thought it might be a clogged radiator, so I replaced that with a 4-core, the thermostat and the water pump. Now my theory is a clogged cat/exhaust (factory system, 120,000 miles), which is starting to show its age from the constant beatings I give it. Before I went and bought a whole new exhaust system, I turned to my old friend NAXJA, where doing research leads me to question that it may just be indicated, as all XJ's have a tendency to run hot.


Opinions?????? Tests?????
 
Post #1, I would look for a loose wire/connection at the Temp sensor.

Post #4, I would see if the torque converter is locking up, if not the heavy uphill load could be the overheat tipping factor.

How is the Fan clutch? Is it locking up when it gets hot or not?

Could have air trapped in the block. 4.0's are hard to purge according to many others here.

Plugged Cat or muffler or both usually causes a noticable performace loss at higher RPMs.
 
My 98 would be running fine, 210, then suddenly the temp guage would max out. Logically, that can't happen. Cleaned the sensor since it's self grounding, no help. Followed the wiring back, the fuel rail pinched the wires between itself and the firewall, wearing through the CTS and another wire. Bit of electrical tape, and some wire ties and all is well. If the gauge is moving fast, it's probably a sensor/wiring issue.
 
I thought 97+ Jeep had the gauge sender / PCM sensor / and fan sensor all-in-one on the thermostat housing?

Am I wrong or was 96 the last year to have the gauge sender seperate at the rear of the head?
 
Blaine B. said:
I thought 97+ Jeep had the gauge sender / PCM sensor / and fan sensor all-in-one on the thermostat housing?

Am I wrong or was 96 the last year to have the gauge sender seperate at the rear of the head?
Click to expand...

You are correct, it's all done by a 2 wire sensor. Coolant Temperature Sensor. Basically a thermistor or thermocouple, I assume. The wiring from there goes to the left of the fuel rail under a covered wireway. At the firewall it merges with the wiring harness and passes between the fuel rail and the firewall en route to the PCM. Right near the big ground.
 
I replaced the t-stat, temp sensor, fan, then relay. Once I replaced the relay the problem went away. While I was at it I put a nice new aluminium radiator in. Now temp never goes above 210 even on 100deg days, and electric fan rarely kicks in.
 
Ecomike said:
Post #4, I would see if the torque converter is locking up, if not the heavy uphill load could be the overheat tipping factor.

How is the Fan clutch? Is it locking up when it gets hot or not?

Could have air trapped in the block. 4.0's are hard to purge according to many others here.

Plugged Cat or muffler or both usually causes a noticable performace loss at higher RPMs.
Click to expand...


The transmission position sensor is physically damaged and defunct, which among other things acts as a pretty good kill switch when I manually move the dial so the engine won't start (Remember, the truck sits alone 350 days a year). The lack of a functioning sensor has downhill decelleration in the 1-2 postion nearly impossible, as the transmission won't pick between first or second. It just cycles back and forth between the two about every three seconds, which makes for a rough ride. I just ride the brakes for now, or drop it into 4L with the tranny in third.

The Torque converter is a possibility, but it still overheats on flat land, and seems to be specific to throttle position. If I set the throttle at about 25%, the truck is fine. If I set it at 40%, it overheats (indicated). I haven't noticed any other torque converter related issues. It seems to lock and unlock appropriately.

The fan clutch is fine, and the electric fan is operating normally, which leads me to again believe that it may not be just indicated, as both are at full throttle when the gauge indicates overheating. I should have gone with dual electrics when I changed the radiator.

I have burped and burped the thing. I don't think it's air trapped, unless it is creating an air bubble from boiling the coolant, which is not an "indicated overheat" but rather an "actual overheat".

The plugged cat could be a possiblity, something I noticed last time I was up there (the truck is 800 miles away), was that I had the hood up and was revving the engine to charge the battery from an almost-complete winching session (I was having to winch out of the garage). The headers (factory) were literally glowing red hot, from an idle at 2000-2500 RPM's, They were glowing for a good 10-12 inches down the pipes. I've never really looked before, and it could be that it was just a lack of light, but that didn't seem normal.

My next planned step: Replace the temp sensor and check the wires. I'll probably also wire up a manual pro-comp temp gauge, to ensure that it is not a gauge error.

If the truck is truly overheating: to bypass the exhaust via straight pipe and a hole saw and see if that fixes the overheating on the highway. If it does, I'll order a new cat, and muffler, with SS stuff in the middle. If it doesn't, I'm at a total loss as to what it could be. The A/C has no charge, so that's not it.

The real problem is that the truck is also 800 miles away from all my tools, which makes diagnosis and correction difficult at best. The last thing I want to do on my vacation is lug a welder 800 miles to cuss at a truck.

Thanks,

Robert
 
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Barger said:
I replaced the t-stat, temp sensor, fan, then relay. Once I replaced the relay the problem went away. While I was at it I put a nice new aluminium radiator in. Now temp never goes above 210 even on 100deg days, and electric fan rarely kicks in.
Click to expand...

Where is the relay?? In the engine fuse box??
 
The headers should not be :flame:glowing, that is a definate, meaning, as in DEFINATE sign of either an engine running WAY too hot (extreemly late ignition, not necessarily running lean, but could be too lean, but my guess is very late igntion so that fuel is still burning when the exhaust valve opens!), or a backed up, partly restricted exhaust system. Forget the relay and cooling system, cut the exhaust and run your test, including inspection of the headers at night to see if the glowing stopped. IF yes, it's the exhaust plugged up, if not there is something way wrong with the timing. Exhaust plugging can come and go (broken loose, rattling catalyst inside the cat housing), or only show up at peak flows from a delaminated piece of pipe where a layer of metal comes loose inside the pipe and acts like a damper valve as the flow increases.

What year is it? What is the history of the engine and computer controls? Modifications by you or POs?

I had a similar problem with a carburated 351 V-8 Ford LTD (73, I still have it!). Kept blowing exhaust manifold gaskets every 100 miles and smelled like something was burning (leaking valve cover gasket oil as it hit the manifold flashed off, but it never acculated long enough to show up or see the leak).

I poped the hood one night after 5 minutes of idling and the manifolds were cherry red! Turned out the centrifigal and vacuum advance on the distributor was locked up at a fixed setting. Ran great except for those problems. Anyway, they should not be glowing at all! If it is timing this is going to turn into an interesting thread since the timing is computer controlled (yes?).



WildernessJeep said:
The transmission position sensor is physically damaged and defunct, which among other things acts as a pretty good kill switch when I manually move the dial so the engine won't start (Remember, the truck sits alone 350 days a year). The lack of a functioning sensor has downhill decelleration in the 1-2 postion nearly impossible, as the transmission won't pick between first or second. It just cycles back and forth between the two about every three seconds, which makes for a rough ride. I just ride the brakes for now, or drop it into 4L with the tranny in third.

The Torque converter is a possibility, but it still overheats on flat land, and seems to be specific to throttle position. If I set the throttle at about 25%, the truck is fine. If I set it at 40%, it overheats (indicated). I haven't noticed any other torque converter related issues. It seems to lock and unlock appropriately.

The fan clutch is fine, and the electric fan is operating normally, which leads me to again believe that it may not be just indicated, as both are at full throttle when the gauge indicates overheating. I should have gone with dual electrics when I changed the radiator.

I have burped and burped the thing. I don't think it's air trapped, unless it is creating an air bubble from boiling the coolant, which is not an "indicated overheat" but rather an "actual overheat".

The plugged cat could be a possiblity, something I noticed last time I was up there (the truck is 800 miles away), was that I had the hood up and was revving the engine to charge the battery from an almost-complete winching session (I was having to winch out of the garage). The headers (factory) were literally glowing red hot, from an idle at 2000-2500 RPM's, They were glowing for a good 10-12 inches down the pipes. I've never really looked before, and it could be that it was just a lack of light, but that didn't seem normal.

My next planned step: Replace the temp sensor and check the wires. I'll probably also wire up a manual pro-comp temp gauge, to ensure that it is not a gauge error.

If the truck is truly overheating: to bypass the exhaust via straight pipe and a hole saw and see if that fixes the overheating on the highway. If it does, I'll order a new cat, and muffler, with SS stuff in the middle. If it doesn't, I'm at a total loss as to what it could be. The A/C has no charge, so that's not it.

The real problem is that the truck is also 800 miles away from all my tools, which makes diagnosis and correction difficult at best. The last thing I want to do on my vacation is lug a welder 800 miles to cuss at a truck.

Thanks,

Robert
Click to expand...
 
Ecomike said:
What year is it? What is the history of the engine and computer controls? Modifications by you or POs?

If it is timing this is going to turn into an interesting thread since the timing is computer controlled (yes?).
Click to expand...


The engine is strong, really strong, all factory EFI (1998). I have a snorkel (ARB), which helped it run cooler too. It has 120,000 miles on the rig, but the first 45,000 of that was being towed behind a RV, so there was no wear on anything other than the axles. I've never had any problems with the engine/tranny, and it's been underwater 8 times, flooded 3 times, and upsided down twice. Some of the other sensors have gone bad from the flooding, and with the lower end of my property at 8900 feet and the Jeep routinely at full throttle over 14,000 feet, I go through a MAP sensor about once a year. I have a Snap-on MODIS, and I'm not getting any codes at all, other than the previously mentioned transmission sensor. The timing looks and feels good, considering the altitude. I can't ever get the computer through a global cycle, because the Jeep sits at a vacation home, with the battery disconnected 350 days a year. Even if I do get the computer dialed in, it won't last because I disconnect the battery two days later when I leave.

I'm gonna take a hole saw and a new temp sensor with me when I go in a couple of weeks, and will report back. Also, I have to drop the T-case because my output shaft on my Tom Woods SYE has a leaking (or missing) seal. Because it's not very environmentally friendly to dump a bottle of gear oil every twenty miles, I may just bring the entire rig back to Houston for full diagnosis and to drop the X-fer case. That would give me the time to fab up a nice exhaust and accompanying belly pan. Dammit!! I thought I was done pumping money into this thing!!


Thanks,

Robert
 
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Why not start another thread with a new title about the 98 having glowing red exhaust manifolds and lets get some experts together to discuss just that. I am really curious how the timing could get that far off on a 98 with out throwing a code on a 98! Also sounds like a fire hazard

If you do start a new thread be sure and post the link to the new thread here.

Has the distributor ever been pulled?

WildernessJeep said:
The engine is strong, really strong, all factory EFI (1998). I have a snorkel (ARB), which helped it run cooler too. It has 120,000 miles on the rig, but the first 45,000 of that was being towed behind a RV, so there was no wear on anything other than the axles. I've never had any problems with the engine/tranny, and it's been underwater 8 times, flooded 3 times, and upsided down twice. Some of the other sensors have gone bad from the flooding, and with the lower end of my property at 8900 feet and the Jeep routinely at full throttle over 14,000 feet, I go through a MAP sensor about once a year. I have a Snap-on MODIS, and I'm not getting any codes at all, other than the previously mentioned transmission sensor. The timing looks and feels good, considering the altitude. I can't ever get the computer through a global cycle, because the Jeep sits at a vacation home, with the battery disconnected 350 days a year. Even if I do get the computer dialed in, it won't last because I disconnect the battery two days later when I leave.

I'm gonna take a hole saw and a new temp sensor with me when I go in a couple of weeks, and will report back. Also, I have to drop the T-case because my output shaft on my Tom Woods SYE has a leaking (or missing) seal. Because it's not very environmentally friendly to dump a bottle of gear oil every twenty miles, I may just bring the entire rig back to Houston for full diagnosis and to drop the X-fer case. That would give me the time to fab up a nice exhaust and accompanying belly pan. Dammit!! I thought I was done pumping money into this thing!!


Thanks,

Robert
Click to expand...
 
Okay, assuming I cut the pipe just in front of the cat, two things:

1) That still leaves open the option that something upstream is clogging the exhaust. Specifically, this exhaust has the "factory error" in it with the pinch just above the flange. What are the chances of something up there clogging the system around the flange?

2) Isn't the lack of the second 02 sensor input really going to foul up the engine computer?? I know it would over time, but would I be able to diagnose an overheating engine with the computer in default? How long would it take for the lack of a second sensor to disrupt the computer to the point of non-performance?

3) What other sensors are prone to fluctuations in air pressure? The MAP sensor I know, but what others?? It could be the engine is running overly rich due to another mis-reading sensor (which also could be destroying the MAP sensor).

Opinions???
 
jeeperguy21 said:
Ever since I got a new radiator over a year ago, my temperature gauge has had moments when it has worked just fine, but then some days it just falls suddenly to about 120 degrees. Is it just a bad thermostat or what? Just in the past couple of days, it seems like it has gotten much worse, never showing the temperature reaching above 120 degrees, which has me concerned because the cooling fan will not come on with the temperature reading only that warm, when the engine is extremely hot, seeing how it is summer and everything. Anyone else have this problem? I have a 99 XJ Sport.
Click to expand...

Thanks for all your help! It turns out that all it was was a loose wire on my thermostat housing! Once I got it secured nicely, I haven't had a single problem since, and that was over a week ago. :laugh3:
 
Wilderness - I believe that the 2nd 02 sensor is only there to tell if the cat is functioning properly, not to adjust AFR. You'll get a code, but IIRC, the computer will still run normally. That's how it worked on my 96 Chevy, anyways.....

If that doesn't work...there's always the MIL eliminators....or O2 delete kits. Check eBay....I had to use one on the Chevy. Extremly simple to wire up, and easy to hide if you need to =P
 
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