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Old 06-06-2012, 10:09 PM   #1
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Temperature will not go over 155*

Hello everyone,
The temperature gauge on my 1998 TJ will not go over 155*. I suspect a Thermostat stuck open but I could be wrong. Where should I start? Also, what are some other things to look at? It is not eating radiator fluid either.

Any help is greatly appreciated.

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1998 TJ 4.0

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Old 06-06-2012, 10:13 PM   #2
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You said it. Check the thermostat.

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Old 06-06-2012, 10:26 PM   #3
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155 is hard to fathom in San Diego. Maybe your gauge is bad as well? Before you pull your thermostat, feel your radiator. If it is hot then it could be your gauge. Hopefully someone near you has an IR thermometer. Aim it at your housing.
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Old 06-06-2012, 10:48 PM   #4
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Yes, make sure your gauge is accurate before you start replacing parts. If you find an IR thermometer the t-stat housing should be around 200 degrees, and the temp on the dash should match the temp on the IR thermometer.
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Old 06-06-2012, 10:49 PM   #5
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My jeep favors 260.
Let's meet in the middle.
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Old 06-06-2012, 10:53 PM   #6
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My jeep favors 260.
Are you serious?
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Old 06-06-2012, 10:54 PM   #7
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Cool thanks guys. I will definitely check the gauge. If it does come down to the thermostat, what kind of thermostat do I need?

Edit: I forgot to mention that I ran a gauge check (hold odometer button, turn key three times and the temp gauge did go all the way to the max). Would this change anything?
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Old 06-06-2012, 10:56 PM   #8
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195 degree
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Old 06-07-2012, 02:11 AM   #9
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Are you serious?
Yep that's why the blocks being rebuilt right now because it got to hot to many times.
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Old 06-07-2012, 02:19 AM   #10
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Quote:
Originally Posted by Kota7x
Cool thanks guys. I will definitely check the gauge. If it does come down to the thermostat, what kind of thermostat do I need?

Edit: I forgot to mention that I ran a gauge check (hold odometer button, turn key three times and the temp gauge did go all the way to the max). Would this change anything?
You don't have to hold the odo just FYI just turn the key three times
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Old 06-07-2012, 07:29 AM   #11
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Could also be a coolant temp sensor
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Old 06-07-2012, 07:49 AM   #12
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My jeep favors 260.
Let's meet in the middle.
Holy crap; you have a 150psi radiator cap?
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Old 06-07-2012, 08:42 AM   #13
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Kota, the 'gauge check' will show you if the gauge functions, so it sounds like the gauge is OK. What you need to find out now is if the gauge is accurate. Comparing the temp reading at the thermostat housing with an IR thermometer to what the gauge is reading should tell you that. If the thermostat housing reads ~200* and the gauge reads 155*, the problem could be the temp sending unit. If both readings are ~155*, then it is probably the thermostat (stuck open) that is the problem.
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Old 06-07-2012, 08:55 AM   #14
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Quote:
Originally Posted by 50Johnbob
Kota, the 'gauge check' will show you if the gauge functions, so it sounds like the gauge is OK. What you need to find out now is if the gauge is accurate. Comparing the temp reading at the thermostat housing with an IR thermometer to what the gauge is reading should tell you that. If the thermostat housing reads ~200* and the gauge reads 155*, the problem could be the temp sending unit. If both readings are ~155*, then it is probably the thermostat (stuck open) that is the problem.
Awesome. Im going to tackle this when I get some time. Thank you everyone.
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Old 06-08-2012, 12:32 AM   #15
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Could also be a coolant temp sensor
X2 Hopefully someone by you has a Infra Red Temp gun you can borrow. Nothing worse than wasting money on a guage or sending unit when you could spend that on Good Stuff like a new Tire Fund, Locker Fund GAS FUND
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Old 06-08-2012, 01:12 AM   #16
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Quote:
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X2 Hopefully someone by you has a Infra Red Temp gun you can borrow. Nothing worse than wasting money on a guage or sending unit when you could spend that on Good Stuff like a new Tire Fund, Locker Fund GAS FUND
Already took care of the Locker and tire fund. Goes straight into Gas!
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Old 06-08-2012, 08:35 PM   #17
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Replace the thermostat with a new 195 degree thermostat like from Stant, problem fixed. Avoid using a fail-safe thermostat which causes more problems than they are worth.

If you can find a thermostat with an air bleed hole in it like the below on the left, that will help keep air from becoming trapped behind the thermostat which can cause crazy temperature swings. If you can't find one, you can drill your own like in the pic on the right. Be sure to position the hole at the very top so any trapped air can escape into the radiator.
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Old 06-09-2012, 12:07 AM   #18
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Quote:
Originally Posted by Jerry Bransford
Replace the thermostat with a new 195 degree thermostat like from Stant, problem fixed. Avoid using a fail-safe thermostat which causes more problems than they are worth.

If you can find a thermostat with an air bleed hole in it like the below on the left, that will help keep air from becoming trapped behind the thermostat which can cause crazy temperature swings. If you can't find one, you can drill your own like in the pic on the right. Be sure to position the hole at the very top so any trapped air can escape into the radiator.
Thanks Jerry. Where is the thermostat located? I have heard that it is pretty easy to replace.
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Old 06-09-2012, 12:28 AM   #19
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There are two ways to check your gauge for accuracy. One is to remove the sensor from the engine, hook it up with a couple jumper wires to the engine ground and the hot lead and dip the business end of the sensor into a pan or can of boiling water. You can use a torch or other heat source to keep the water boiling. If the gauge is accurate it should show just a couple needle widths above 210F.
The other way isn't quite as accurate, as already mentioned you can use an IR thermometer on the thermostat housing or the top of the radiator. You can buy an inexpensive IR thermometer at Harbor Freight. I've had one from them that I've used for several years for trouble shooting cooling and heathing systems.
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Old 06-09-2012, 12:38 AM   #20
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Thanks Jerry. Where is the thermostat located? I have heard that it is pretty easy to replace.
Follow the top radiator hose down towards the engine and what it is connected to is the thermostat housing. Held to the engine head by two bolts, it's easily replaced. Buy a new gasket with the thermostat, and don't use any sealant or RTV (silicone) on the gasket. Just be very careful to get all of the old gasket off. The two surfaces must be surgically clean to prevent leaking.
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Old 06-09-2012, 12:44 AM   #21
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I put in a cooler thermostat to stop my radiator from busting every two years. My last (2nd) rebuild was in 2005. I think I just went 10 degrees cooler though. My Jeep is getting around 20 mpg.
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Old 06-09-2012, 03:20 AM   #22
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be very careful to get all of the old gasket off. The two surfaces must be surgically clean to prevent leaking.
Or it will leak like a Russian built sub. Ask me how I know
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Old 06-09-2012, 02:31 PM   #23
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I put in a cooler thermostat...
Not recommended. If your radiator isn't holding up, it's not from running the proper 195 degree thermostat. Not to mention your engine computer is programmed around the thermostat opening at 195 degrees.
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Old 06-09-2012, 05:51 PM   #24
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Side question... to clear air bubbles in the system do you just pop the radiator cap (when it's cool of course), start the jeep, and waiting til the system overflows a smidge replacing the cap when it does? Or is there a different way?
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Old 06-09-2012, 06:32 PM   #25
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try installing a aftermarket temp gauge i had the same issues and finally tracked it down to stock gauge issues good luck

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