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Old 04-28-2013, 08:10 PM
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significant overheat 4.0. diag?

Hello all,
Punched the gas pedal very swiftly and the throttle 'grabbed' where the pedal did not let off. This became immediately dangerous, I applied brakes hard and shifted into N. The rpms were super high and redlined before I killed the ignition. I pop hood and release stuck throttle lever. Throttle Spring seemed to have stuck..start up drive into nearby gas station.

Things seem ok, but its gets hot. Then very hot, I turn on heat full blast, no heat. Wtf. Coolant everywhere. Overflow reserve was completely backed up and bursting out the overflow tube. Lower Crowl covered in coolant, and rad cap seems to have sprayed a bit...
Got jeep home shortly avoiding overheat.

Next day top off now empty reservoir and start jeep. Runs nice, good idle no leaks, warms to op temp normal fan on @ 210 then temp continues to climb and climb...crank heat and only cool air blows out. I then park jeep in garage awaiting ideas.

Water pump was replaced by po. My thoughts were heater core and radiator. This is a 4.0 stock Laredo. I have researched pretty in depth the repairs, if you have any ideas of diagnosis or any info regarding this cooling failure please help out. Thanks for any thoughts.
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Old 04-28-2013, 08:54 PM
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Re: significant overheat 4.0. diag?

Did you change the thermostat and radiator cap? you have the symptoms of stuck thermostat.

I would also consider a Coolant flush and heat core flush.

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Old 04-28-2013, 09:00 PM
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Re: significant overheat 4.0. diag?

That the heating doesn't work, is or there is no(t) (enough) coolant in the cooling system, an air pocket preventing circulation of the coolant or a waterpump failure. Did you also put coolant in the radiator and not only in the overflow reservoir? If both radiator and overflow reservoir are full, do you see any air bubbles in the overflow reservoir when the engine is warm and runs?
But the fact that the coolant came out of the overflow reservoir, makes me think about a blown head gasket.
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Old 04-29-2013, 05:29 AM
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Re: significant overheat 4.0. diag?

Thanks for the ideas. No I did not check to see air bubbles while jeep was running. I can check in the near future. I only topped off the reservoir, the radiator was full. The overflow tank backed up once the jeep was getting hotter than 210 and continued to rise to overflow as the heat increased. Otherwise it didn't leak unless under the high pressure of the overheat. I think water pump is good still as all other meters read good ie : voltage oil pressure and it was surely replaced by past owner. I'm hoping head gasket is good still for obvious reasons. The oil looks perfect was just changed last week. Jeep ran and performed tip top shape before this. Thanks to all the feedback.
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Old 04-29-2013, 05:46 AM
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Originally Posted by kgingerich View Post
Thanks for the ideas. No I did not check to see air bubbles while jeep was running. I can check in the near future. I only topped off the reservoir, the radiator was full. The overflow tank backed up once the jeep was getting hotter than 210 and continued to rise to overflow as the heat increased. Otherwise it didn't leak unless under the high pressure of the overheat. I think water pump is good still as all other meters read good ie : voltage oil pressure and it was surely replaced by past owner. I'm hoping head gasket is good still for obvious reasons. The oil looks perfect was just changed last week. Jeep ran and performed tip top shape before this. Thanks to all the feedback.
Change thermostat on the 4.0 is 20min job.
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Old 04-29-2013, 06:20 AM
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Re: significant overheat 4.0. diag?

would a malfunctioning thermostat make my heat only blow cool air? Thanks again!
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Old 04-29-2013, 06:23 AM
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Quote:
Originally Posted by kgingerich View Post
would a malfunctioning thermostat make my heat only blow cool air? Thanks again!
Yes coolant isn't reaching the heater core because thermostat is closed shut.

Battery side of the motor near firewall are the heater core hoses these should be to hot to hold. They are below the ac lines.

Also the top heater hose should be super hot also once thermostat opens. Start the jeep let the temp get up see if that hose gets super hot or stays cold get a mopar or stant super stat 15 to20 bucks $3 dollar gasket

You also haven't mentioned if you have the electric and Mechanical CLutch fan or one or the other.

If you have factory tow package you should have both.

Check make sure the E fan comes on Easy trick for this is Press the A/C button this instantly kicks the e-fan on.
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Old 04-29-2013, 07:07 AM
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Re: significant overheat 4.0. diag?

thanks for the quick replies! i will continue to look into the issue this week. Please keep ideas, tips and advice coming as they arise. I will keep all posted with the status. Those videos made a lot of sense, and the thermostat being stuck would explain why coolant didn't circulate into the heater core and thus was pushed back up and through the reservoir over flow tank, making a mess and overheating. The fan kicking on around 200-210 seems like a good sign though. Thank you very much! more to follow. No tow package, but electronic fan does kick on around 200-210 degrees automatically.
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Old 04-29-2013, 10:06 AM
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Re: significant overheat 4.0. diag?

On the 4.7 i´m sure and for the 4.0 it shouldn´t be that different, that the flow to the heater core is not regulated by the thermostat, or in fact not regulated at all. With thermostat closed the coolant circulates witin the engine block and the heater core at all times (when engine runs at least). Imagine a very cold day, where the thermostat will open only a bit, then you would not have any or at least very poor heating, when the flow to the heater core was regulated by the thermostat.
With the thermostat closed, there will be no circulation to the radiator, which will cause overheating and boiling of the coolant. The boiling will cause the volume to increase and the coolant flows to the overflow reservoir. Maybe that an air pocket caused by the boiling is preventing the circulation to the heater core now.
So changing the thermostat is a good first step. I only wonder why the thermostat, which was working ok until the engine redlined, suddenly would fail closed? I don´t say it is not, only trying to find the reason behind it.
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Old 04-29-2013, 10:29 AM
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Re: significant overheat 4.0. diag?

thanks for the insight Frango. Your theories make sense. So we are basically in agreement the thermostat seems closed/failed causing my boiling and overflow of the coolant itself, and engine overheat. But we are not sure about the heater core functioning. The Passenger carpet is not wet, and the heater core and all options worked perfectly before the redlining of the engine. I have called my local family operated shop. I will pick up the 195 stat, stat gasket, and rad cap. They are coming to tow away the WJ to the shop to install the stat, and thoroughly flush the entire cooling system including the heater core. They would need to tow the jeep to the shop to flush it all anyway so i will have them install the stat and troubleshoot from there. They will be checking the Water pump and Head Gasket as well. I am hoping this will solve my problems. Jeep idles nice, sounds normal, and oil is clean. so at worst im thinking heater core/head gasket. Damn i wish i didnt punch the gas pedal like that. That is what got me in this mess. I mean I really laid on it and the throttle stuck down. scary. Thanks all, and keep the ideas coming please.
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Old 04-29-2013, 07:58 PM
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Re: significant overheat 4.0. diag?

Quote:
Originally Posted by Frango100 View Post
On the 4.7 i´m sure and for the 4.0 it shouldn´t be that different, that the flow to the heater core is not regulated by the thermostat, or in fact not regulated at all. With thermostat closed the coolant circulates witin the engine block and the heater core at all times (when engine runs at least). Imagine a very cold day, where the thermostat will open only a bit, then you would not have any or at least very poor heating, when the flow to the heater core was regulated by the thermostat.
I don't follow because I always thought it was regulated by the thermostat. To my knowledge the thermostat regulates the coolant to/from the water jacket around the cylinders. So the thermostat is closed when the coolant is cool. Once it heats up enough, the thermostat opens to let the hot coolant out and get replaced by cool coolant. This hot coolant then flows to your heater core where heat can be extracted into cab. So what am I missing? How is the hot coolant getting from the water jacket to the heater core if it's not controlled by the thermostat?

One thing I do know 100% is that if your thermostat is stuck open, it will take your heat FOREVER to warm up (had this happen on a couple vehicles). With the thermostat stuck open, it doesn't allow the coolant to sit in the water jacket and warm up (and then go to the heater core) before passing it out of the engine. This results in cold coolant to the heater core until all of the coolant gets some heat.

So you can see where I'm confused when you say it's not controlled by the thermostat.

*EDIT* I pulled out my Haynes manual and looked at the coolant system. The thermostat housing has 2 "exit" hose on it. One of them goes to the radiator and the other goes to the heater core. If the thermostat is stuck closed, it would prevent coolant (or at least hot coolant) from flowing down this hose and to the heater core. So a thermostat that's stuck closed would cause the engine to overheat and the heater to stop blowing heat.
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Old 04-29-2013, 09:33 PM
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Re: significant overheat 4.0. diag?

A thermostat is there to regulate the temperature of the engine. But with a cold engine, the coolant circulates within the engine AND the heater core. When the temperature of the coolant is hot enough, the thermostat opens and the coolant flows to the radiator, where it gets cooled to prevent overheating.
My jeep service manual also states as follows:

Engine coolant is circulated through heater hoses
to the heater core at all times.

the small tube on the thermostat housing is the supply to the heater core, but its not regulated by the thermostat..

There is no reason to control the coolant temperature to the heater core, since you already have the blend door(s) taking care of the required hot air temperature regulation inside the cabin.
Also on cold days the thermostat will open less, because the radiator will function better with cold air passing through it, so cools better. Then just on the days that you need the heating the most, you would get less heat from the heater core, because the thermostat opens less .

The problem with a stuck open thermostat is that all coolant will flow through the radiator all the time. The radiator will cool down all coolant and for that reason the temperature will not rise enough. You will see that on the engine coolant temperature indicator, which will be much lower then normal and the cabin heater will not have hot air. (warm only, depending on outside temperature)
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