This has been driving me insane since I first got this XJ. Gauge will show high engine temps. 99% of the time the needle will sit right on the line before the red but never cross into it. Lately the needles been dipping over into red, but other than the gauge I don't see any signs of over heating. No leaks, no steam, ect. I'm constantly driving around with the heat on full blast to keep it below the redline. If I'm moving it sits around 220, but if I'm moving slow or it's idling it will sit right at the line before the red most the time.
I had a leaky heater core hose I fixed today and refilled the coolant that was lost, but it's still the same.
In the last year or so since I've owned the Jeep I've replaced:
cylinder head (unrelated to heating issues)
coolant temp sending unit (three times. first two simply broke. made sure I've replaced with the part for the gauge, not a dummy light)
Haven't replaced the aux fan but it works.
I've also backflushed the heater core and backflushed the rest of the cooling system with no changes.
Is there any chance that the head gasket is leaking compression into the cooling system? I had a cracked head on a Mazda do that. The cooling system pressurized within minutes of starting and heated very quickly. Another possibility is the water pump spinning in the wrong direction. I don't know if Jeep did this with their engines, but Chevrolet small block V-8's have two different water pump rotations, depending on the year, application, etc. The pumps are physically the same body, however the impeller pitch is different. Again, personal experience. Everything else is normal and looks good, but the engien overheats with a new water pump.
Has the t-sat been replaced since it last overheated? Was it the stock 195 degree rating? Any chance it was put in backwards or has an obstrction? Just tossing out possibilities.
I don't think so. It runs great other than the gauge.
As for the water pump, I don't think so but I doubt it. Belt is on correctly.
I don't think anything is mechanically wrong since it's not actually overheating, or doesn't appear to be anyway. I'm leaning more towards an issue with the gauge itself or something. I might just bite the bullet and buy and OBD I tool to see what the ECU is picking up since it's a different sensor.
If you suspect the gauge, get an inexpensive mechanical temp gauge and install it where the stock temp sensor mounts on the t-stat housing. I had to do this with an idiot light-equipped Jeep, to verify nothing was wrong when the light began to flicker. $20 for piece of mind, even if it is a temporary mount.
I'm with Purell69. There's every possibility the previous owner put the wrong sending unit in, and it's been mis-reporting ever since. If there are no signs that the engine's running hot, I'm betting it's just a gauge issue. You might even just break down and buy a new sending unit (or find one at the local junkyard) and put it in... quicker and easier than mounting, then un-mounting a second meter to check the actual temperature). Sure, that doesn't really cover the possibility that your gauge itself is bad, but there aren't many things that are going to cause a temperature gauge to read high other than the sending unit, IMHO.
Oops - chalk that up to me not reading your list of replaced parts well enough. It might just be that gauge after all. I suppose it's worth hooking up a generic (or junkyard) gauge to see what happens. I guess I wouldn't bother replacing the sending unit again though... after three replacements, I guess you can cross that one off!
I have to agree with the manual guage idea, or else an infrared temp scanner. As I was reading through the thread, I was thinking about if you may have replaced the wrong temp sensor, but then you covered that. I'd have to say that three bad temp sensors in a row, while possible, isn't very likely, and have to lean toward either the guage being bad, or else its actually getting that hot. Those 4.0's are pretty tough. I've ran mine pretty hot before, because the fan output from the computer is bad and the aux. fan wasn't coming on, and it didn't really seem to mind. I was idling along pulling a pretty large trailer, while guys were piling 5" fire hose (about 4000') that we had just tested onto the trailer. Aside from a slightly rough(er) idle, it didn't really seem to mind the higher temp.
4-wheeling: because football, basketball, baseball, and golf only require one ball!
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Not sure how old the radiator is. I've had this XJ about a year and it was in it when I got it. I've backflushed it both during backflushing the whole cooling system and just on it's own and it appears to be fine. The fins aren't in bad shape, there's no leaks, and it's not caked in mud.
I have access to an IR temp gun, just hadn't bothered to check with one yet. If I pull over and pop the hood when it says it's hot the hoods not hot, the hoses are warm but not burning hot and the motor and engine bay in general aren't all that hot.
Need to verify the dash gauge with the infrared gun 1st. The only way to tell internal condition of a rad is to have the tanks removed from the core (radiator shop). Rads tend to fill with solids from the bottom up. Upper tubes may flow freely while lower tubes are blocked. The XJ rad is very dinky (surface area size definitely matters) so any blockage in the bottom portion reduces efficiency.
Man Law - hoods should be raised at least once a week.....have a look see.
I've since replaced the radiator and aux fan. Backflushed the system prior to installing the new radiator and backflushed the heater core again on it's own. New rad is in, aux fan works. Same situation. Put an IR gun on it today, it's definitely not running hot or anywhere close to what the gauge is reading.