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Old 10-28-2010, 09:14 AM   #1
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Yj weird thermostat reading

So I just bought my first YJ and the thermostat is reading weird, is will stay cool when I start and stop drive but on the interstate it will rise all the way over to hot, but when I pull over and wait a few minutes it will fall back to half. So my question is, is it the radiator that needs flushed or is the thermostat sticking? I'm planning on doing both, flushing and replacing but what do you think is the real problem? Thanks guys

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Old 10-28-2010, 10:26 AM   #2
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my 88 was doing something along those same lines. i flushed the radiator a few times and it solved my problem. i'd recommend doing that first seeing that it doesnt cost anything. if the problem keeps up then you've lost me, but maybe you do need a new thermostat

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Old 10-28-2010, 10:29 AM   #3
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You might not even have a thermostat in it. Check and or replace, cheap enough.
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Old 10-28-2010, 10:38 AM   #4
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I just took the thermostat cover of the top of the engine, and viola, no thermostat, I bought a new one for five bucks but which way does it go in? Does the spring end go into the engine or vise versa, thanks for the help so far
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Old 10-28-2010, 11:53 AM   #5
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Check your bottom radiator hose and make sure it has a spring in it or it will collapse at high rpm and restrict flow.
If it has a spring flush your cooling system with Preston super flush. Then fill with 50/50 anti freeeze.
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Old 10-28-2010, 03:21 PM   #6
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Alright so here is what going on. I put a thermostat in the jeep and replaced the housing gasket. I filled the radiatior back up after flushing it a few times. But now it overheating even quicker than before with out the thermostat installed. I did add a bottle of super flush and topped of the radiator and am trying to work the coolant through the system but the temp gauge keeps rising all the way to hot before I have to shut it down. Now one other thing is that the radiator, cap, and hose's all feel cool to the touch when the engine is running, but when I shut it off, the system all the sudden becomes hot. And the heater system is working really good. So if anyone has any suggestion, please feel free to share cause im all out of ideas other than removing the thermostat all together. Thanks to all that have posted suggestions and pics.
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Old 10-28-2010, 03:36 PM   #7
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Alright so here is what going on. I put a thermostat in the jeep and replaced the housing gasket. I filled the radiatior back up after flushing it a few times. But now it overheating even quicker than before with out the thermostat installed. I did add a bottle of super flush and topped of the radiator and am trying to work the coolant through the system but the temp gauge keeps rising all the way to hot before I have to shut it down. Now one other thing is that the radiator, cap, and hose's all feel cool to the touch when the engine is running, but when I shut it off, the system all the sudden becomes hot. And the heater system is working really good. So if anyone has any suggestion, please feel free to share cause im all out of ideas other than removing the thermostat all together. Thanks to all that have posted suggestions and pics.
make sure you burp her real good after that flush and fill. buy a couple of extra gallons of coolant.

keep the rad cap open until operating temperature and then at 210║, raise her up to 2000-2500 RPM and slowly add more coolant until it comes to a full level. keep the throttle open until you fully screw the cap back on the rad. that will make sure she's burped right. air pockets can develop in the system especially after having the thermostat housing off, and that will cause exactly what is going on.

after overheating on the freeway like you said, it might be possible that your sending unit for the guage might be fried. you can pick those up pretty cheap (10$) at idiotzone.

good luck!
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Old 10-28-2010, 04:44 PM   #8
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great advice and ill try that when I get home, does anyone think it could be the water pump? and if so where is it cause I think its on the front right of the engine but im not sure
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Old 10-28-2010, 05:05 PM   #9
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great advice and ill try that when I get home, does anyone think it could be the water pump? and if so where is it cause I think its on the front right of the engine but im not sure
water pump is right underneath your thermostat housing. it has a pulley with a fan and clutch attached. usually when your water pump goes out, it leaks from the weep hole. you can take it out and examine it to make sure its not boogered up inside. i wouldn't rule it out as the culprit. not too expensive to change out either.
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Old 10-29-2010, 01:08 AM   #10
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Like carlyj mentioned it sounds like you have air in the system now. You need to fix that before you can diagnose your original problem of overheating on the highway. Its possible you could have a bad water pump. The large hose at the bottom of the radiator leads to your water pump. Don't forget to check the bottom hose for a spring.
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Old 10-29-2010, 05:12 AM   #11
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Well I came home and burped the system, and it didn't overheat as quickly as before but it did get all the way over to the hot side. But like before, if I stop and wait for a few seconds it will cool to about 3/4 on the gauge. But then when I got home and was sitting there letting it cool down with engine idling, the temp gauge falls all the way flat out to the cold side and now won't rise. So I think the sending unit just took a crap so now I'm gonna replace that and see if that helps.
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Old 10-29-2010, 08:07 AM   #12
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Change the sending unit and if it still acts the same make sure you don't still have air in the system. Spring?
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Old 10-29-2010, 10:10 AM   #13
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Change the sending unit and if it still acts the same make sure you don't still have air in the system. Spring?
I guess you referring to the spring in the bottom radiator hose. To my knowledge there is not one in or on it. Should I replace it?
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Old 10-29-2010, 10:28 AM   #14
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dumb statement, but you added the fluid to the radiator right, and not to the reservoir? some idiot i know (ahem) did that, but quickly learned to add it to the actual radiator, as it doesnt take from the reservoir unless it is cooling down/off (thanks dad, i mean that idiot thanked his dad...)
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Old 10-29-2010, 10:29 AM   #15
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I guess you referring to the spring in the bottom radiator hose. To my knowledge there is not one in or on it. Should I replace it?
Just like Fweaky said. Some mistakingly take the spring out. Replace the bottom radiator hose. The new one will have a spring in it to keep the hose from collapsing. While your at it, replace the top hose (no spring) they're not expensive.
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Old 10-29-2010, 01:25 PM   #16
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Ok, I've now replaced the coolant temp sensor, and no change, still over heating. I still don't see any water leaks in the system, and the bottom radiator house does have a spring in it. Now tell me if this sounds weird to anyone, when I turn it off, it sounds like there is water boiling in the radiator and the top radiator house is very hot, but the rad cap is cool, so I'm thinking that the radiator is toast. Tell me what you guys think, thanks to everyone for the help so far.
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Old 10-29-2010, 02:18 PM   #17
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Ok I take that back it not boiling in the rad it's boiling at the thermostat housing. The top rad hose is very hot as well as the top of the rad. Don't just read and run leave a comment please, any help is good help. Oh one other thing and a total newb question, is there a wrong way to install a thermostat? What I mean is I have it in correctly but does it have to be turned a certain way to work correctly? Thanks again
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Old 10-29-2010, 02:56 PM   #18
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Mine makes that same boiling noise lol it shouldn't matter if the thermostat is sideways or what ever as long as it's facing the right way then it can open up when it hits a certain temp, I would take the water pump off and see what it looks like becaue it may not be pumping water through your system, it's like 27 bucks and if you bring in your old pump they will give you some money back, I just recently had to change mine out, it's really simple. Also if you can take it down to the auto parts store and look for someone with grease under his finger nails and ask him to come take a look, I do that all the time.
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Old 10-29-2010, 06:44 PM   #19
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Ok well the water pump is off right now and everything looks good. Everything feels solid and looks good so I'm not gonna replace it right now. What I am gonna do is replace the radiator, and if this don't work, its going in the shop. I am running out of options, and a new radiator is prob the last one. I have a feeling that when the previous owner toasted the last motor that he probably damaged the radiator. The motor was replaced but I'm betting the radiator wasn't. So I'm gonna try a new one. Anybody else got anymore suggestions? Thanks guys
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Old 10-30-2010, 01:22 AM   #20
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Oh one other thing and a total newb question, is there a wrong way to install a thermostat? What I mean is I have it in correctly but does it have to be turned a certain way to work correctly? Thanks again
Place the new thermostat in the housing with the thermostat spring facing into the engine block. If the thermostat is placed the wrong way your engine will overheat.
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Old 10-31-2010, 07:27 AM   #21
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Is there any moisture in a coolant or in the oil?

After all these symptoms I would suspect Head Gasket.

To check it. Run the engine, that the termostat opens (radiator is going warmer and warmer) and then put the engine to 3000rpm. And when the hoses are going tight then there's a pressure in the cooling system and you need to change the Head Gasket.

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