Temp guage is reading HOT...but its not

Has anyone seen this before?

So I went to fire up the XJ to run down the street and top off on gas before this weekends trips...about half way there...(1 mile or so) I notice the temp guage is like pegged to the right....so i'm like WTF..:rattle: ...??? pull over...check the thing out...and the thermostat hasn't even poped open yet...so i KNEW it wasn't over heating... check the water level, it was good...the hoses (the ones leading to the thermo housing were soft, is why in know the thermostat wasn't open yet)....so i sad F-it...drove to the gas station...about another mile or 2 down the road...got there filled up and re-checked everything, now the thermostat was open all the hose were "stiff" (fluid in them) and everything still seemed to be good... I tried several times during this to stop and restart the motor, turned off the battery (trying to rest the guage).... I drove it home the 3 miles or so and it ran great...it didn't seem hot at all, even though the guage was saying it was like 350+...I've never had an over heating issue before...

Any ideas? is this a loose wire? bad sensor? (by the way where is the sensor?)... could I have bumped something under the dash? (i recently put some KC lights and mounted the switch on the left side of the steeringwheel on the dash)...

Suggestions....ideas....

Thanks...

thats weird stuff, maybe need a new temp sender thing on the thermostat housing. your gauge goes all the way to 350? that would definitely be meltdown time had it really been accurate. sounds like a bad gauge or sender. thats an interesting one there.

Have you done any work to the cooling system lately? Could have air trapped in the system.

Air trapped in the cooling system? ...hmm...well i haven't done anything to it lately...and i do nearly all the work myself.

I'll have to explore the temp gauge sender unit....problem is i don't know exactly what it is or where it is at....

Yeah 350 would be VERY bad...I don't think it is the guage itself...it still moves..but it just seems to be like 180 off... i can turn the fan on manually and it moves or play with the cooliant lines and it "bounces" according to the GF... but it always stays to the right...

Atl XJ said:

Have you done any work to the cooling system lately? Could have air trapped in the system.

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would air do this? I haven't changed the radiator in like 2 years or more when i sent the engine through it...since then its been problem free...

try a basic standard standt thermostat in there, for some reaosn or another those work well. i put one in my xj and i could not be happier, it opens and does not spike and so far has kept me at 195 with very little change unless im pullin or hauling kayaks on top. should be a simple fix , i bet its the sensor.

some info:
http://www.1aauto.com/1A/coolanttemperaturesensor

sounds more sensor related.

Mrburns said:

I'll have to explore the temp gauge sender unit....problem is i don't know exactly what it is or where it is at....

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What year is your XJ?

On my 91 the temp sensor is in the back of the head, drivers side...


On my 99, its on the t-stat housing.

Mrburns said:

would air do this? I haven't changed the radiator in like 2 years or more when i sent the engine through it...since then its been problem free...

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Bad motor mount? Been there done that, ruined a brand new Modine. :tears:

Air trapped in the system can cause both overheating and faulty temperature gauge readings. From his profile he has an 88, which would be a Renix Motor, and they're very prone to getting air in the cooling system.

Atl XJ said:

Bad motor mount? Been there done that, ruined a brand new Modine. :tears:

Air trapped in the system can cause both overheating and faulty temperature gauge readings. From his profile he has an 88, which would be a Renix Motor, and they're very prone to getting air in the cooling system.

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that sounds right, maybe there is a pocket of steam where the sender is and steam is hotter than the liquid its comin off of.

i think you hit the jackpot!

yes, its an '88 with converted system....closed loop or whatever.... so no bowl here... What could cause the air to get there? i had the jeep out about a month ago :anon: and it ran perfect for 3days... It was out of coolant in the overflow...could that do it? like a back flow type thing? its full now and runs great so i was just going to go with it this weekend and really trouble shoot when we get back....that seems safe right?

Mrburns said:

yes, its an '88 with converted system....closed loop or whatever.... so no bowl here... What could cause the air to get there? i had the jeep out about a month ago :anon: and it ran perfect for 3days... It was out of coolant in the overflow...could that do it? like a back flow type thing? its full now and runs great so i was just going to go with it this weekend and really trouble shoot when we get back....that seems safe right?

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hey mrburns, i think when your reserve ran low the enging possibly took on some air into the system . then the air bubble is in the loop. sounds a lot like its sittin where the temp sender is and just needs to be let out. maybe you can undo the highest coolant hose on your motor, see if any air comes out and fill it, quickly slap the puppy back on and run the xj with the heater on for 15 minutes, nice nad hot and the thermostat will do its thing and circulate coolant. that might help to diagnose the issue you have, i would much rather spend an hour troubleshooting for a whole weeken'd peace of mind.

i just hate not knowing whats happenin under the hood on a vehicle i am relying on. could also possibly save yourself a hit in the wallet too.

please let us know what you find if you can. were wonderin' too.

fill the overflow bucket, open the radiator cap and let the engine idle...that should suck the air out....at least it worked for me

I'm having this hot temp reading gremlin myself. I have replaced everything in the system... sensors, pump, radiator (3 core), cap, belt.... even back flushed the motor and have no air trapped in the system. Still get readings of 220ish on the highway. We even used the infrared on it and it seems right. This is really stumping me. It has not gone to red yet..but it frightens me. I'm a guage guy and like to know what my motor is doing. PLEASE HELP!!

When air gets trapped in the system, you have to burp it. The easiest way I have found to do this is to jack up the rear of the vehicle (secure it on jackstands) and let it run up to operating temp. Then, with a very good pair of gloves on, slowly loosen the temp sender (in the rear of the head). This will push the air out and then very hot coolant. If there is any other air trapped it will be in the heater core. Try to drive it around for a while and see if the problem persists. If so repeat the process, and if that does not help replace the sender.

Alot of people have had problems with the aftermarket senders not reading correctly, so go to the dealer for the sender.
Good Luck.

Thanks Jess! This is driving me nuts not to mention the $ in my account. I like knowing whats going on. I will try that. So... basically I am bleeding off vapor? That sensor is a pain to get to. Can it be bled anywhere else?

Wow, I have a high reading on the highway too. It has made me totally paranoid, even though the e-fan never kicks on (meaning the heat is only in the back of the engine I guess). I hope it is just air. I will try that tonight!

MrBurns...I would place my money on air in the system...especially if your reservoir tank went low recently. I had the same thing happen to me and my gauge bounces like a mexican spaceshuttle. Just "burp" the system, I can almost guarantee you that's what it is.

You need to check your connections. I have seen guages peg when disconnected.

go to gojeep.com? /\ /\ /\ /\

dougcurtsinger@gma said:

You need to check your connections. I have seen guages peg when disconnected.

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Yep, if the wire lead is grounding it will peg the gauge.