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Old 05-14-2016, 09:30 AM
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Runaway radiator fan at idle

My 2004 4.7 L JGC Laredo radiator fan runs faster and faster at idle after a couple of minutes in park when warm, while the rpms stay put at about 650. Fan gets running so fast that I shut jeep off in concern. When restart it takes another couple of minutes of idling for the fan to start racing again. Any ideas on what is causing this?

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  #2  
Old 05-15-2016, 01:07 PM
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Re: Runaway radiator fan at idle

what's the temp gauge saying?
the valve the runs the Cooling fan is stuck open, It is ran from your power steering pump.

the PCM will command the hydraulic fan solenoid valve more open, to ramp up the fan speed.
Check your temp sensor.
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Old 05-15-2016, 01:31 PM
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Re: Runaway radiator fan at idle

Eventhough I have never seen/heard my fan speed going up on hot days, it is in fact the way the system is designed. The fan runs at around 500 rpm on a cool running engine, but will speed up as soon as the engine temperature becomes too high. It can be that the coolant level is a bit low, causing it to run hotter then normal and causing the fan to speed up. A faulty coolant temp sensor could indeed also be the problem.
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Old 05-15-2016, 08:37 PM
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Re: Runaway radiator fan at idle

The temperature gauge shows normal and the coolant level appears full. I'll test the coolant temp sensor if I can figure out where it's at. Appears that it's probably under the first electrical connector (from front to back) between the alternator and the AC? Thanks for your help!
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Old 05-15-2016, 10:44 PM
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Re: Runaway radiator fan at idle

Yes, the temperature sensor sits in between the alternator and airco compressor. When the fan goes to high speed, what is then the indicated temperature?
Regarding the coolant level, you will have to check on the radiator itself on a cold engine. Donīt remove the radiator cap on a hot engine, because hot coolant will be blown out and burn your skin. It can happen that the overflow reservoir stays full, while the radiator level is low, when there is a leak in the system. In case for example a leaking head gasket.
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Old 05-16-2016, 07:29 AM
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Re: Runaway radiator fan at idle

I'll check the temp gauge when I run it today, but my recollection is that it stays steady. How can I tell if the radiator itself is full, as I don't see any markings and I can only see into the throat leading to the radiator itself.
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Old 05-16-2016, 09:01 AM
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Re: Runaway radiator fan at idle

The coolant should be up to the cap. As soon as the engine heats up, the coolant will expand and an internal valve in the cap (18 PSI) will open and let air and coolant escape to the overflow reservoir. If you donīt see coolant, its low.
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Old 05-16-2016, 09:41 AM
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Re: Runaway radiator fan at idle

Ran the jeep, going down the road the temp gauge was about 210, when the fan sped up the gauge increased toward about 230. Once it cools down, if I have to add coolant directly to the radiator do I need to open the bleeder valve to get rid of the air after that? I don't see any fluid leaking anywhere, so I'll have to have a mechanic find the problem.
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Old 05-16-2016, 10:18 AM
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Re: Runaway radiator fan at idle

Just discovered that the interior heat did not come on until was a mile or so down the road even though the engine was already warm. Might this all be a heater core problem?
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Old 05-16-2016, 06:22 PM
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Re: Runaway radiator fan at idle

If the heater was working before this all started, I presume that there is air trapped in it. Best is to remove the bleeder screw and fill it until it starts flowing out of the opening. Put the screw back in and fill the radiator until it almost overflows. After closing the radiator cap you can squeeze the upper radiator hose to try to dislodge trapped air. There might be a head gasket leak, causing the coolant to be burned of. Do you see any white smoke from the exhaust or does it have a sweet smell when warm?
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Old 05-18-2016, 07:30 AM
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Re: Runaway radiator fan at idle

I followed your steps and everything is now working correctly with no issues. There were a lot of air bubbles that came out. I did discover blockage in the overflow line too. No white smoke or sweet smell in the exhaust and no leaking apparent, so I'll just have to keep a close eye on the coolant level and try to determine where it's going. Thank you so much for your help!
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Old 05-18-2016, 02:06 PM
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Re: Runaway radiator fan at idle

Glad to hear you got it sorted. I wish Frango100 worked at my local dealership. Two thumbs up!
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