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Old 08-24-2010, 09:07 PM
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Instant overheating issues

98 Wrangler with 4.0 engine. After about 20 min. of driving at speeds over 55, the temp gauge will redline and the "Check gauges" light comes on. The "check gauges" light comes on first and then the temp gauge goes from normal to the beginning of the red zone in about one second. If you slow down to 50 or so immediately, within a minute or so the "check gauges" light will go off and the temp gauge will plunge to normal in a couple seconds. If you keep driving at high speeds, the gauge will sudenly jump to the top of the red zone.

At this point you are probably thinking what I was, that it is the temp sensor sending unit or something like that. But the crazy thing is that the engine really is overheating. If you let it continue to run in the red zone, it will blow antifreeze all over the engine compartment.

The radiator and thermostat were replaced less than one year ago to try and correct this problem but to no avail.

Any ideas???

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Old 08-24-2010, 10:29 PM   #2
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Nickm77,

I own a 99 Sahara with a 4.0L that is having a similar issue. I also replaced the water pump and put in a new thermostat thinking the last one that was installed was somehow faulty. No luck! I am stilling experiencing the same problem you are having.

Can anyone help with this issue?

Thanks in advance,
Zippey

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Old 08-24-2010, 10:47 PM   #3
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Sounds like your system may be partially blocked. Try flushing it. Also, did your heater produce good heat last winter? If not then your clog is probably in the heater core which is usually the first part of the system to clog (lots of fun to change). A vacume lock in the system can also cause what you are describing, but can be fixed when you do the flush.
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Old 08-24-2010, 10:51 PM   #4
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Some things to check...
Make sure there is no air in the cooling system. that will make the temp gauge swing quickly.
Make sure the is nothing blocking the grill.
Do you have heat...is your heater core plugged?
Has the block / system been flushed.

...and very important for a vehicle overheating at highway speeds... does the bottom hose have a spring in it to keep it from collapsing.
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Old 08-25-2010, 12:26 AM   #5
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Check the lower radiator hose for softness, better yet replace it with a QUALITY hose. Sounds like it's collapsing.
The lower hose is on the suction side of the pump, the faster you go the harder the pump pulls, When you reach the critical speed, the hose gets sucked closed, no coolant flows - overheats.
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Old 08-25-2010, 09:44 AM
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Thanks for all the great ideas! I burped the system this morning and found that there was a lot of air in it. So much that I think I need to do it again so I'm letting it cool down. I think that might take care of the temp gauge behaving erratically.

And the thought that the overheating at highway speeds is caused by the lower radiator hose sounds plausible. I'm not sure exactly how to tell if it is too soft or pliable, but maybe I should just replace it to be sure. It is very easy to squeeze, just as easy as the upper hose.

Will try to get back with you if I find the problem or a fix. In the meantime, would appreciate any other thoughts you may have that might be helpful.

Thanks again!!!
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Old 08-25-2010, 09:57 AM   #7
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Thanks for all the great ideas! I burped the system this morning and found that there was a lot of air in it. So much that I think I need to do it again so I'm letting it cool down. I think that might take care of the temp gauge behaving erratically.

And the thought that the overheating at highway speeds is caused by the lower radiator hose sounds plausible. I'm not sure exactly how to tell if it is too soft or pliable, but maybe I should just replace it to be sure. It is very easy to squeeze, just as easy as the upper hose.

Will try to get back with you if I find the problem or a fix. In the meantime, would appreciate any other thoughts you may have that might be helpful.

Thanks again!!!
Honestly for the cost of the hose, I would change it out. Make sure you get a good name brand like Gates. Don't let em sell you on all the mumbo jumbo of silicon hoses. Sounds like you are on the right path with getting the air out.
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Old 08-25-2010, 10:03 AM   #8
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if you sqeeze the bottom hose and it collapses the there is no spring in it and you need to replace it.
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Old 08-25-2010, 10:08 AM   #9
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To bleed the air out...With the engine cool, remove the radiator cap and start the engine. Fill the radiator with fluid if needed to bring the level up to the cap and let it run until it reaches normal temperature range and make sure the thermostat is open. You may see bubbles coming from the radiator if you have air pockets and it will probably overflow as the temperature comes up. When you don't see any air coming from the radiator for a couple of minutes carefully replace the cap and check the overflow bottle level and if needed add fluid to bring it up to the proper level.
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Old 08-25-2010, 10:11 AM   #10
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Anything mechanical can fail, reguardless of age.

After the hoses (suggested above) I'd replace the thermostat again.

Have someone rev gradually to 2500rpm and watch the fluid in the radiator to make sure you got flow (water pump). Impeller could be slipping on the shaft at higher RPMs (greater resistance) whch would stop the flow and spike the heat.
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Old 08-25-2010, 10:31 AM
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Here is an update. I burped the system again and got more air. But I also checked the heater...not a bit of even warm air. Just cool air coming through even with the engine at normal temp. So if the heater core is plugged, could that be causing the eingine to overheat? It wouldn't seem like it is the radiator since that is a brand new unit installed in May of this year. What do you think???
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Old 08-25-2010, 10:55 AM   #12
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A clogged heater core can cause your engine to overheat. Try back flushing it to see if it helps. There are 2 tubes with hoses on them protruding from the front of the engine firewall on the passenger side. Pull the hoses off and use a water hose to flush the system. Force the water through each tube until clear water comes out the other. Or take the Jeep to a shop to have it flushed for about $80.
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Old 08-25-2010, 11:35 AM   #13
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"""""A clogged heater core can cause your engine to overheat"""""

Huh?
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Old 08-25-2010, 11:45 AM   #14
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"""""A clogged heater core can cause your engine to overheat"""""

Huh?
Well, if the rest of the system is marginal, then a clogged heater core could be the final straw that broke the camel's back. The heater core (a small second radiator in reality) is most definitely part of the cooling system since coolant flows through it 100% of the time regardless of how any of the heater controls are set.
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Old 08-25-2010, 12:11 PM   #15
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While you have the hoses disconnected from the heater core at the firewall connect the two hoses back together using a piece of pipe and run it see if it runs cooler.
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Old 08-25-2010, 12:32 PM   #16
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Yup - I'm wrong, sorry. I guess TJ's don't have a heater control valve that shuts of the coolant flow to the core. I guess in Chrysler's cost cutting they eliminated it. I just looked in the FSM. (My Rubi isn't here to "not" take a picture of a valve that isn't there.)
Makes sense? Bring in hot then cool it off with AC - must be the German logic.

Most vehicles have a hot water valve that shuts off the coolant flow through the heater core when you turn off the heater? Closing that valve would do the same thing as having a plugged core. On those when you shut off the heater it will overheat? Bypassing the heater core would be a certain overheat.


RE: Air in the system. Since air is thinner than coolant, air can get through a hole that coolant can't. To test for an air leak, fill the coolant overflow bottle full when hot. Let the engine cool off. The bottle's fluid level should be much lower now - as the engine cools it "sucks" coolant from the bottle. If a hose or something is leaking air in - even a pinhole - the bottle will still be full as it pulled in more air through the leak.
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Old 08-25-2010, 02:06 PM
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This gets even more bizarre :-). Just took it for a test drive at high speeds after bleeding the air out and now the temp gauge stays way low, indicating that the engine is not coming up to the proper temp. But the Jeep feels like it has more power than it has in a long time. Still no heat from the heater core. The upper radiator hose seems just as hot as it ever was when at normal temps, but it doesn't seem to be overheating (not blowing antifreeze out the overflow).

Not sure what to make of all this except that I know the heater core is clogged and it seems there has been air in the system. Not sure if my thermostat decided to freeze open or if the temp sensor sending unit just decided to fail...
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Old 08-25-2010, 05:06 PM   #18
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Sounds like you fixed the over heating problem. Congrats. What is "way low" on the temp gauge? Also, what temp thermostat did you put into it last year? As far as the heat goes, ensure that the actuator is actually moving when you slide it into the heat position.
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Old 08-25-2010, 05:09 PM   #19
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Top hose is coolant exiting the engine and will be hotter than bottom return.
What thermostat did you put in when you replaced it. I believe stock calls for 195 degree stat. You can bypass that heater core as long as you connect the inlet and exit hose back together to complete the system.
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Old 08-25-2010, 06:28 PM
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I believe it was the OEM thermostat. By way low I mean about half way between all the way to the left and straight up. I have been told that strange things can happen when it is running too cool, so this concerns me a bit. It usually runs around straight up (200 +).
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Old 08-26-2010, 08:46 AM   #21
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Replaced the bottom radiator hose. The Jeep is blowing very hot air when I turn the heat on and its still overheating. At this point I am gonna have to take it to a specialist.

Anyone further ideas I can forward to the mechanic?
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Old 08-26-2010, 11:17 AM   #22
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Replaced the bottom radiator hose. The Jeep is blowing very hot air when I turn the heat on and its still overheating. At this point I am gonna have to take it to a specialist.

Anyone further ideas I can forward to the mechanic?
Did you check the new lower hose for a spring?
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Old 08-26-2010, 11:31 AM   #23
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You had it to where the temp gauge said it's cold. Replaced the bottom hose and now the heater's working and it shows hot.

The temp gauge won't register if there's air around it. If the heater is airlocked, it won't get hot.

Sounds like you have a big air leak. Check for head gasket troubles - look at the water - any signs of oil in it?

Look at the oil filler cap - water - condensation - in the oil will collect at the highest point - the filler cap. Look for a brown milky scum on it.

Look at the dipstick for water.

That still doesn't mean you don't have a compresson leak into the water.

Have a shop use a Block Check - it's a bulb affair that pulls air out of the radiator through a chemical. If the chemical turns blue, there's combustion gasses (CO) in the cooling system (overheats same as air.) They'll probably charge you $10, well worth it.
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Old 08-26-2010, 11:56 AM   #24
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It looks like zippey interjected on nickm77's thread.
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Old 08-26-2010, 10:19 PM
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Okay, here is the result. I took it in to a radiator shop and they thought it was the water pump. They got into it and sure enough, the portion of the impellers that actually move the antifreeze were completely rusted off...nothing left! So they replaced the water pump and the thermostat (for good measure) and it runs like a champ.
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Old 09-05-2010, 08:13 PM   #26
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Fix

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Originally Posted by nickm77 View Post
98 wrangler with 4.0 engine. After about 20 min. Of driving at speeds over 55, the temp gauge will redline and the "check gauges" light comes on. The "check gauges" light comes on first and then the temp gauge goes from normal to the beginning of the red zone in about one second. If you slow down to 50 or so immediately, within a minute or so the "check gauges" light will go off and the temp gauge will plunge to normal in a couple seconds. If you keep driving at high speeds, the gauge will sudenly jump to the top of the red zone.

At this point you are probably thinking what i was, that it is the temp sensor sending unit or something like that. But the crazy thing is that the engine really is overheating. If you let it continue to run in the red zone, it will blow antifreeze all over the engine compartment.

The radiator and thermostat were replaced less than one year ago to try and correct this problem but to no avail.

Any ideas???
spin your fan blade if it spins fast you have a bad clutch on your fan , also feel the top radiator hose and if its soft then the hose is colapsing
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Old 09-05-2010, 08:27 PM   #27
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Heater core

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Originally Posted by nickm77 View Post
98 wrangler with 4.0 engine. After about 20 min. Of driving at speeds over 55, the temp gauge will redline and the "check gauges" light comes on. The "check gauges" light comes on first and then the temp gauge goes from normal to the beginning of the red zone in about one second. If you slow down to 50 or so immediately, within a minute or so the "check gauges" light will go off and the temp gauge will plunge to normal in a couple seconds. If you keep driving at high speeds, the gauge will sudenly jump to the top of the red zone.

At this point you are probably thinking what i was, that it is the temp sensor sending unit or something like that. But the crazy thing is that the engine really is overheating. If you let it continue to run in the red zone, it will blow antifreeze all over the engine compartment.

The radiator and thermostat were replaced less than one year ago to try and correct this problem but to no avail.

Any ideas???
i see there are a lot of bones giving you advice , take the 2 heater hose's off the engine , put a garden hose on one end and if water comes out the core is fine , if it doesn't then its clogged ! Now, if you don't get heat, then air is trapped in the core and that will make it so water can not pass you must "burp" your heater core and your motor , and by the way when testing your core try to keep the water in it when you put the hoses back on , then you will have no air in the system
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Old 09-05-2010, 08:30 PM   #28
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One other thing

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98 wrangler with 4.0 engine. After about 20 min. Of driving at speeds over 55, the temp gauge will redline and the "check gauges" light comes on. The "check gauges" light comes on first and then the temp gauge goes from normal to the beginning of the red zone in about one second. If you slow down to 50 or so immediately, within a minute or so the "check gauges" light will go off and the temp gauge will plunge to normal in a couple seconds. If you keep driving at high speeds, the gauge will sudenly jump to the top of the red zone.

At this point you are probably thinking what i was, that it is the temp sensor sending unit or something like that. But the crazy thing is that the engine really is overheating. If you let it continue to run in the red zone, it will blow antifreeze all over the engine compartment.

The radiator and thermostat were replaced less than one year ago to try and correct this problem but to no avail.

Any ideas???
if the weep hole on the water pump is leaking thats where the air is coming into your engine bye !!
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Old 09-05-2010, 08:39 PM   #29
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Old 10-26-2010, 03:04 PM   #30
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Great - I have an '02 Rubicon with the same overheating issues as nickm77 that just began a couple of days ago, but very mild. Check Gauge light would come on and within seconds would return to normal. Not today! My question...I am at work, about 35 miles from home and noticed the gauge stayed at the highest, red zone for about 10 minutes on my way in. I was ready to pull over at any second, but made it here. Now can I make it back home? Yes, I totally meant to have it looked at yesterday and it just never happened!

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