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Old 04-06-2012, 11:24 AM   #1
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Need help replacing temp/coolant sensor

Hey guys I just ordered a new coolant sensor to (hopefully) fix a problem I am having with my temp gauge in my 02 TJ (4.0L). I know this thing is supposed to be relatively easy to replace, but I am not very engine repair savvy and was wondering if someone could help me out here. Could someone take a picture of where this thing is located so I know where and what to look for? I know it's a lot to ask but I'd like to know what I'm looking for beforehand so it could possibly be the first do-it-myself repair that goes off without a hitch (if that's even possible ). Thanks in advance to anyone willing to help me out!

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Old 04-06-2012, 03:45 PM   #2
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Here you go.. It's located on the left (driver) side of the t-stat housing. Wait for the engine to cool before you start removal. Keep in mind you will lose some coolant when you remove the old one and be sure to use some pipe sealant or Teflon tape to seal the threads on the new one.

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Old 04-06-2012, 05:50 PM   #3
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That one reads for the ECM, not the gauge. The one for the gauge is in the block, passenger side I believe. IIRC that is.... And don't put anything on the threads, those sensors ground through the threads and anything on them may cause trouble....
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Old 04-06-2012, 07:50 PM   #4
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That one reads for the ECM, not the gauge. The one for the gauge is in the block, passenger side I believe. IIRC that is.... And don't put anything on the threads, those sensors ground through the threads and anything on them may cause trouble....
As I recall the older 4.0's had two sensors however I'm pretty certain TJ's only have the one on the t-stat housing. As you mentioned it is for the PCM however the PCM sends a signal to the temp gauge on the dash. As for the sealant you may be right however I have had them leak in the past without some type of sealant. The photo is of my Heep which ironically I just replaced the Temp sensor 2 weeks ago. I had to replace the housing and I could not remove the old sensor so I just replaced it.
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Old 04-06-2012, 08:07 PM   #5
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As I recall the older 4.0's had two sensors however I'm pretty certain TJ's only have the one on the t-stat housing. As you mentioned it is for the PCM however the PCM sends a signal to the temp gauge on the dash. As for the sealant you may be right however I have had them leak in the past without some type of sealant. The photo is of my Heep which ironically I just replaced the Temp sensor 2 weeks ago. I had to replace the housing and I could not remove the old sensor so I just replaced it.
Yes, CORRECT.

I just went thru this yesterday...I flushed system and heater, installed proper tstat and temp gage still read 150 degrees, so replaced sensor and all is well.

Original Poster, follow top raditor hose to engine where it meets metal cast housing. just to left on housing is wire connector for sensor. Unplug wire and USING PROPER SIZE TUBING WRENCH or box end if wire connector will go thru wrench opening, if not and if you don't have tubing wrench just cut off old wire connector from sensor and remove sensor...it is brass and prolly siezed into housing so regular open end wrench might round it off.

Teflon tape works very well to seal new sensor, but only wrap it with two wraps....that way threads will cut thru tape and you will be grounded and sealed

And P.S. mine too is a 2002 4.0
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Old 04-06-2012, 08:17 PM   #6
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As I recall the older 4.0's had two sensors however I'm pretty certain TJ's only have the one on the t-stat housing. As you mentioned it is for the PCM however the PCM sends a signal to the temp gauge on the dash. As for the sealant you may be right however I have had them leak in the past without some type of sealant. The photo is of my Heep which ironically I just replaced the Temp sensor 2 weeks ago. I had to replace the housing and I could not remove the old sensor so I just replaced it.
Maybe I am thinking of the 258's..... or the early 242's... I'll check my book when I get home..
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Old 04-06-2012, 08:19 PM   #7
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Yep the TJ's sole coolant temperature sensor is mounted next to the thermostat housing. I was curious and checked both of my FSMs to verify my thought there was only one sensor.
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Old 04-06-2012, 08:35 PM   #8
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Yep the TJ's sole coolant temperature sensor is mounted next to the thermostat housing. I was curious and checked both of my FSMs to verify my thought there was only one sensor.
Yup, that's because they went cheap and combined them...if the truth is known some geek prolly got bonus for saving bucks with it

I like the idea, cause if ECU temp sensor was starting to go bad, there would be no indication until it popped a code if ever...With just one sensor you can see it going bad like mine when gage refused to register actual higher temp.....I shot my tstat housing with infred gun and it was reading 200, but gage still reading 150, so I knew it was sending bad info to ECU...I'm sure my MPG's will increase a little above 13.2 now that ECU knows engine is at proper temp
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Old 04-06-2012, 11:09 PM   #9
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I want to thank all of you guys for the input and for taking the time to take pictures. This is all going to help a lot when I get the part and am ready to install it. It always helps to get advice from people who have done it before and know what to look out for.
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Old 04-06-2012, 11:49 PM   #10
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I want to thank all of you guys for the input and for taking the time to take pictures. This is all going to help a lot when I get the part and am ready to install it. It always helps to get advice from people who have done it before and know what to look out for.
Your'e fersure welcome...Hey consider this while you are in there pissin coolant on the shop floor replacing the sensor; Odds are the tstat is original too.

It's good practice to replace thermostat, flush cooling system and also replace radiator cap every 3-5 years...and examine hoses and such..just really good time to do it all cause it all wears out together and it helps prevent unexpected failures.

If you decide to get that deep into it right now, then make sure to bust old sensor loose before you remove thermostat housing from head...it's lots easier to do while still bolted in place.

But even if all you do it replace the sensor, you gotta be super careful to burp all air out of head and top off radiator before you start the engine...It's easy to leave air pockets trapped in head and cause head to warp/crack or overheat after opening up the cooling system at any point lower than rad cap and behind closed cold tstat....Don't be skeered, just be aware.

One trick you can use is to have sensor ready to install and pour water in radiator cap opening as you hold sensor just above it's hole and once you get solid water overflowing out of sensor hole, screw it in place finger tight, then wrench it.
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Old 04-07-2012, 12:06 AM   #11
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Not to be a blabber mouth, but when I replaced my thermostat last night, I noticed my brand new Made in USA top price NAPA stat didn't have an air bleed hole or flap valve....Now that really sux when you consider how stat mounts horizontally on front of head and once installed it will trap huge air pocket behind sensing bulb on stat.....with no air bleed hole in stat flange and no way for stat to sense temps to open then head would quickly over heat.

I drilled tiny bleed hole in stat flange, but for you guys who decide not to, make sure that once stat is installed and housing bolted tight that you vent all air behind sensing bulb via heater hose or sensor hole.

In my younger days I destroyed a very expensive Toyota 22R performance alum head right out of the box when I left just a few cubic inches of air trapped behind tstat....The sensing bulb on tstat can't sense thru air and will remain shut...It has to be submerged in water same as gage sensor.
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Old 04-07-2012, 08:02 AM   #12
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Your'e fersure welcome...Hey consider this while you are in there pissin coolant on the shop floor replacing the sensor; Odds are the tstat is original too.

It's good practice to replace thermostat, flush cooling system and also replace radiator cap every 3-5 years...and examine hoses and such..just really good time to do it all cause it all wears out together and it helps prevent unexpected failures.

If you decide to get that deep into it right now, then make sure to bust old sensor loose before you remove thermostat housing from head...it's lots easier to do while still bolted in place.

But even if all you do it replace the sensor, you gotta be super careful to burp all air out of head and top off radiator before you start the engine...It's easy to leave air pockets trapped in head and cause head to warp/crack or overheat after opening up the cooling system at any point lower than rad cap and behind closed cold tstat....Don't be skeered, just be aware.

One trick you can use is to have sensor ready to install and pour water in radiator cap opening as you hold sensor just above it's hole and once you get solid water overflowing out of sensor hole, screw it in place finger tight, then wrench it.
All great advice. I actually replaced the thermostat already a couple of months ago thinking that it would solve my temp gauge problems to no avail. The radiator was also replaced with a new one just under 2 years ago. Hopefully this sensor will fix everything and the world will be right again afterwards, but we all know that something is bound to go wrong.

One question about burping the system. You said to pour water into the radiator until it starts flowing out from the sensor, and then screw it in place. Am I good at that point? Or do I still need to start the Jeep with the radiator cap off and wait until it burps?
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Old 04-07-2012, 09:08 AM   #13
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All great advice. I actually replaced the thermostat already a couple of months ago thinking that it would solve my temp gauge problems to no avail. The radiator was also replaced with a new one just under 2 years ago. Hopefully this sensor will fix everything and the world will be right again afterwards, but we all know that something is bound to go wrong.

One question about burping the system. You said to pour water into the radiator until it starts flowing out from the sensor, and then screw it in place. Am I good at that point? Or do I still need to start the Jeep with the radiator cap off and wait until it burps?
Yeah, fersure leave cap off until you are certain all air is out. This gives you visual indication and allows you to add water quickly if level drops, and provides place for entrapped air to vent PROVIDING your thermostat has air bleed hole in flange.Once you are certain all air is out and you feel upper hose getting warm as tstat opens, fill radiator to very top and install cap, then make sure coolant recovery bottle has water to at least cold mark.
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Old 04-07-2012, 10:27 AM   #14
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2002TJ is providing very sound advice in this thread! Especially look at what he says in his post #11.

The first pic shows the air bleed hole from a manufacturer, the second shows one that has been drilled through the flange of a thermostat that did not originally come with the air bleed hole.
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Old 04-07-2012, 10:45 AM   #15
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Yeah, fersure leave cap off until you are certain all air is out. This gives you visual indication and allows you to add water quickly if level drops, and provides place for entrapped air to vent PROVIDING your thermostat has air bleed hole in flange.Once you are certain all air is out and you feel upper hose getting warm as tstat opens, fill radiator to very top and install cap, then make sure coolant recovery bottle has water to at least cold mark.
Thanks. My thermostat definitely had a air bleed hole in it because I remember that before I installed it. I will let you know how this goes. The part probably won't be here until Monday or Tuesday. Thanks again and have a great Easter!
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Old 04-07-2012, 10:49 AM   #16
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Make sure to always position the thermostat so its air bleed hole is at the very top.
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Old 04-07-2012, 12:20 PM   #17
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Not to be a blabber mouth, but when I replaced my thermostat last night, I noticed my brand new Made in USA top price NAPA stat didn't have an air bleed hole or flap valve....Now that really sux when you consider how stat mounts horizontally on front of head and once installed it will trap huge air pocket behind sensing bulb on stat.....with no air bleed hole in stat flange and no way for stat to sense temps to open then head would quickly over heat.
Is it possible that the heater bypass in the thermostat housing completely eliminates the need for a bleed hole in the thermostat?

Just disconnect the heater hose when you refill with coolant; the air will come right out. I have been doing just that on 4.0L engines for over 25 years; haven't melted a cylinder head yet.
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Old 04-07-2012, 12:39 PM   #18
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Not to my way of thinking. The bleed hole lets air bubbles directly out into the top of the radiator via the top radiator hose.
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Old 04-07-2012, 03:42 PM   #19
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Is it possible that the heater bypass in the thermostat housing completely eliminates the need for a bleed hole in the thermostat?

Just disconnect the heater hose when you refill with coolant; the air will come right out. I have been doing just that on 4.0L engines for over 25 years; haven't melted a cylinder head yet.
Well yeah, pull a heater hose or what ever to vent trapped air from behind stat sensing bulb( as I stated already in post #11 above, para 2)...Seeing as how all he needed to do was replace sending unit, he can vent it there...The idea of my advice isn't so much for him, it's for the next guy researching same problem.

I'm just givin back what I get from this forum

As it turned out original poster and I were chasin the same problem; We noticed our engine temp was running below specs, so we first changed tstats, then discovered it was coolant temp sending unit bad....Mine came right out, his froze in housing so he asked best way to get it out....I just threw in some precautions all of us sometimes eventually learn the hard way.

I have to vent my buddys '89 Cherokee cooling system by removing heater hose from stat housing, attaching 12" hose to it, then standing both hoses up and fill one with funnel as engine slowly burps air...His radiator is sealed and has pressurized recovery bottle, so it's quicker to use hoses as stand pipes and keep them well above top of radiator as we force water thru it to expell air.
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Old 04-07-2012, 03:55 PM   #20
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Hey, while we are talking about thermostats and temp sensors, I'll toss in yet another hard learned lesson.

Years ago, my first wife was scootin along and noticed temp gage was in the red...
So she decides to find gas station that looked really clean and safe, and while driving she saw temp gage return to normal-she she figured it fixed itself and kept driving until it locked up.

What lots of folks don't realize is once it goes into the red the cap starts relieving water and pressure into recovery tank or straight to underside on ground....and once enuff water has escaped to expose temp sensor, the gage will drop even tho it's boiling over and running dry.

Like I told her once she got towed home, "Babe, they just don't fix themself"
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Old 04-07-2012, 05:00 PM   #21
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I have to vent my buddys '89 Cherokee cooling system by removing heater hose from stat housing, attaching 12" hose to it, then standing both hoses up and fill one with funnel as engine slowly burps air...His radiator is sealed and has pressurized recovery bottle, so it's quicker to use hoses as stand pipes and keep them well above top of radiator as we force water thru it to expell air.
Since the heater hoses on an '89 Cherokee are already plumbed into the pressure bottle, the cooling system will vent quite nicely with the heater valve open. No need to disconnect anything. The primary reason that people have so much trouble venting that system is that they fail to turn the heater on, even though the owner's manual specifies just that.
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Old 04-07-2012, 05:48 PM   #22
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True on an XJ but there is no flow control valve to control the coolant flowing through the heater core on any model year Wrangler TJ. The coolant continually runs through the heater core at the same rate no matter what the temperature is set to or if the heater is even on. Heat/temperature is controlled solely by a flapper door.
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Old 04-07-2012, 06:18 PM   #23
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Since the heater hoses on an '89 Cherokee are already plumbed into the pressure bottle, the cooling system will vent quite nicely with the heater valve open. No need to disconnect anything. The primary reason that people have so much trouble venting that system is that they fail to turn the heater on, even though the owner's manual specifies just that.
Maybe on all factory stock /'89 Cherokees, but if you saw the contraptions he's go going on under his hood, you would laff.

Yeah, he's got a pressurized bottle, but where he got I don't know. It's taped to the fire wall, and none of his hose rigging resembles anything that makes sense. He's got an electric fan he dug out of junk pile and tie wrapped it to front of radiator causefan clutch went out. He installed a copper U tube fitting to redirect water from heater cause core rotted out years ago....Really, it's a mess, so I just do it the easy way when he needs help
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Old 04-07-2012, 07:41 PM   #24
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Hey, while we are talking about thermostats and temp sensors, I'll toss in yet another hard learned lesson.

Years ago, my first wife was scootin along and noticed temp gage was in the red...
So she decides to find gas station that looked really clean and safe, and while driving she saw temp gage return to normal-she she figured it fixed itself and kept driving until it locked up.

What lots of folks don't realize is once it goes into the red the cap starts relieving water and pressure into recovery tank or straight to underside on ground....and once enuff water has escaped to expose temp sensor, the gage will drop even tho it's boiling over and running dry.

Like I told her once she got towed home, "Babe, they just don't fix themself"
When this happens, is it the thermostat sticking? This actually just happened to me yesterday and stuck thermostat was my first thought. however I did not add any coolant because I did not realize I had lost any. Guess I'll be getting some on the way home and refilling in the g-station parking lot.

If not a faulty t-stat, any other ideas?
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Old 04-07-2012, 07:46 PM   #25
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True on an XJ...
In fact, it's NOT true for the vast majority of XJ year models.
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Old 04-07-2012, 09:07 PM   #26
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When this happens, is it the thermostat sticking? This actually just happened to me yesterday and stuck thermostat was my first thought. however I did not add any coolant because I did not realize I had lost any. Guess I'll be getting some on the way home and refilling in the g-station parking lot.

If not a faulty t-stat, any other ideas?
Hang on, don't get excited...it's normal for many temp gages to first go above thermostat rating, then fall back to normal, but NOT into full red zone and stay there for any period of time.

I've had many vehicles that showed higher than normal temp after starting and running awhile..the temp would increase above stat set point and stat would open, then gage would fall below set point, then finally settle on it's normal reading with a little flux as stat opened and shut a little....

On a cold engine you have a radiator full of cool water and an engine full of heating water....Then whole measure of water in the engine has to rise to set point before stat opens and when it does lots of cooler water enters engine so gage will fall often below set point until stat stops hunting and temps equalize between radiator and engine...then stat will kinda find a happy place and not open or shut much until conditions change.....Usually our gages react quicker than stats, so consider that too.

Yeah, stats fersure stick, and usually in shut position when they lose their charge, but I've had several stick open.

Bottom line-if this is something new, and Jeep gage usually don't act this way, then investigate....Hold off on buying expensive coolant until you know if you actually have a problem. It's easy to waste coolant when water can be used to top off until you figure out if you have to open the system to fix leak or replace components.
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Old 04-07-2012, 11:31 PM   #27
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Hang on, don't get excited...it's normal for many temp gages to first go above thermostat rating, then fall back to normal, but NOT into full red zone and stay there for any period of time.

I've had many vehicles that showed higher than normal temp after starting and running awhile..the temp would increase above stat set point and stat would open, then gage would fall below set point, then finally settle on it's normal reading with a little flux as stat opened and shut a little....

On a cold engine you have a radiator full of cool water and an engine full of heating water....Then whole measure of water in the engine has to rise to set point before stat opens and when it does lots of cooler water enters engine so gage will fall often below set point until stat stops hunting and temps equalize between radiator and engine...then stat will kinda find a happy place and not open or shut much until conditions change.....Usually our gages react quicker than stats, so consider that too.

Yeah, stats fersure stick, and usually in shut position when they lose their charge, but I've had several stick open.

Bottom line-if this is something new, and Jeep gage usually don't act this way, then investigate....Hold off on buying expensive coolant until you know if you actually have a problem. It's easy to waste coolant when water can be used to top off until you figure out if you have to open the system to fix leak or replace components.
OK, more details then. 2004 X, 4.0 original owner. Temp gauge has been consistent since day one. Right up the middle.

Yesterday, I had driven for ten miles. As I pulled into work, I hear the check gauges light go off. So I did what I was told and checked them gauges!

Temp was in the red. I was close to my parking spot so I headed for it. When I got to it the temp was falling back down to a little above normal. I opened the hood, everything looked fine and didn't see any coolant leaking from underneath.

It was fine on the ride home and today, but I can see it fluctuating. I didn't actually buy any coolant on the way home, I was pretty sure I had some in the garage. Now I'm looking at it and its definitely low.

I was schooled a bit in the way of cooling systems from my first car, an 86 MR2. and later I had an Acura legend that liked to do weird temperature things. But honestly, since the day I bought my Jeep in 04, engine problems have been the absolute least of my worries. By that I mean, I've taken very good care of it and it's been solid as a rock.

Any way, long story short, I'm going to throw some in there. I should have looked before I typed my response. lol But when I saw that post I just had to ask because it was the reason I was watching the thread.

And the feeling you get when you look down and see a temp gauge like that, that quick shock to your system that were not expecting?

Yeah, I haven't felt that in a long time. lol

Thanks for your advice. If it turns out to be something else, I'll be back for more. This forum rules.
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Old 04-08-2012, 06:53 AM   #28
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OK, more details then. 2004 X, 4.0 original owner. Temp gauge has been consistent since day one. Right up the middle.

Yesterday, I had driven for ten miles. As I pulled into work, I hear the check gauges light go off. So I did what I was told and checked them gauges!

Temp was in the red. I was close to my parking spot so I headed for it. When I got to it the temp was falling back down to a little above normal. I opened the hood, everything looked fine and didn't see any coolant leaking from underneath.

It was fine on the ride home and today, but I can see it fluctuating. I didn't actually buy any coolant on the way home, I was pretty sure I had some in the garage. Now I'm looking at it and its definitely low.

I was schooled a bit in the way of cooling systems from my first car, an 86 MR2. and later I had an Acura legend that liked to do weird temperature things. But honestly, since the day I bought my Jeep in 04, engine problems have been the absolute least of my worries. By that I mean, I've taken very good care of it and it's been solid as a rock.

Any way, long story short, I'm going to throw some in there. I should have looked before I typed my response. lol But when I saw that post I just had to ask because it was the reason I was watching the thread.

And the feeling you get when you look down and see a temp gauge like that, that quick shock to your system that were not expecting?

Yeah, I haven't felt that in a long time. lol

Thanks for your advice. If it turns out to be something else, I'll be back for more. This forum rules.
Yeah, little thangs like that make me pucker too, LOL

Our Jeep cooling systems are sized to handle extreme loads and ambient conditions when all is properly oriented and in good condition. So seeing that during a simple trip to work on a spring morning in Wis fersure "ain't not right"

If it happened to me yesterday, I'd be spending my Sunday replacing stat and radiator cap and doing pressure test..Might be time for a new water pump, too....if all is original, then you got pretty decent service life from it.

With our closed type cooling sytems, the radiator cap operates as a pressure release and recovery device every time we warm and cool the engine, which can be several times a day..In days of old, they didn't unless there was a problem because radiators were sized to have air space above coolant level to accomodate expansion of heated coolant.

So now that you've experienced this episode, let engine cool fully and see if there is any coolant in recovery tank...then pull rad cap and check there too....When sytem is working properly, water in tank means rad is totally full like it should be...If no water in tank when cold, then don't simply add water to tank and not also check radiator...The FSM is kinda sorta misleading in how it states rad cap should never be removed, bla, bla, bla...stuff wears out and when we notice something unusual, it's time to check it all.
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Old 04-08-2012, 07:12 AM   #29
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That one reads for the ECM, not the gauge. The one for the gauge is in the block, passenger side I believe. IIRC that is.... And don't put anything on the threads, those sensors ground through the threads and anything on them may cause trouble....
Another little note about our single sensors with two wires...Our sensors don't ground to engine...in fact when testing with meter, there should only be resistance between contacts and full open from pins to brass sensor body.

If you get open between pins, or resistance from either pin to body, then sensor is fersure bad.
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Old 04-08-2012, 09:19 AM   #30
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^^^ Hmmm.

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