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Old 01-26-2014, 01:21   #1 (permalink)
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Why No HEAT??

I have a 99 F250SD with a 5.4L, base package w/o AC, standard cab. I cant figure out why my heater doesnt work full time. I purchased this truck with the heater not working at all really, but I was thinking no big deal...easy fix. I guess I was wrong. So far I have flushed coolant through all the lines about 3 times since I switched out the heater core and replaced the intake manifold, so I know the heater core is working and good. I checked the blend door and that is working properly. If I start the truck up and let it idle it will heat up to about 65 degrees when it is around 35 degrees. If its colder out then it will heat up slightly less at idle and parked. The problem is once I start driving down the road, cool air starts coming out of the vents, enough that the truck slowly starts to get cold. I have tried cardboard in front of the radiator and it doesnt matter, which it shouldnt because my temp gauge is reading where it should be at all times. The heater core will burn the shit out of your hand when its parked and at idle but then cools off quickly driving down the road. Just dont understand how the heater core would cool off driving down the road? Its not like air cold air is blowing on it, its inside a box. Anybody have any ideas what would be wrong? Thanks
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Old 01-26-2014, 07:21   #2 (permalink)
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Re: Why No HEAT??

Is the radiator cap the right pressure and functioning properly? Hears a stupid and I don't know if it applies, but I had an old truck many years ago that, if I had the core hoses backwards, I didn't get as good flow though it as I did when I'd flip them. Just a thought. Try getting a to and from temp at the firewall and compare that to the radiator.
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Old 01-26-2014, 12:07   #3 (permalink)
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Re: Why No HEAT??

Thanks for the response. Like I said, the heater core will burn u to the touch when parked and at idle so shouldnt be a reason to check input and output as long as the hot water is getting back there. The hoses are hooked up right, Ive checked the manual like 5 times on that. What u mentioned about the pressure at the radiator kind of makes since, but shouldnt there be enough pressure going the the core if the thermostat is opening up and pushing coolant through the block/radiator and my truck doesnt over heat, or doesnt that have anything to do with hot coolant getting pushed back to the core?
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Old 01-26-2014, 13:52   #4 (permalink)
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Re: Why No HEAT??

Well, I'm more concerned about flow. If the impellar is worn, at higher speeds it will cavitate and you won't move any fluid.
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Old 01-28-2014, 11:41   #5 (permalink)
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Re: Why No HEAT??

What is the impellar? R u talking on the water pump? I forgot to say the truck will heat up when at idle as long as I dont turn the fan speed above the second notch. If I turn it up to the 3rd or highest fan speed, the air coming out will be cold. Basically not enough hot coolant is making its way back to the core or its not circulating fast enough through the core.
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Old 01-28-2014, 13:31   #6 (permalink)
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Re: Why No HEAT??

Tried bleeding cooling system ?
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Old 01-28-2014, 14:08   #7 (permalink)
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Re: Why No HEAT??

I had a similar problem with a GM/Vauxhall , all right till driven for a while.Finally found the cause was a very slight blow in head gasket that caused an airlock in cooling system , hence heater blew cold .
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Old 01-28-2014, 14:47   #8 (permalink)
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Re: Why No HEAT??

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Originally Posted by DevonD View Post
What is the impellar? R u talking on the water pump? I forgot to say the truck will heat up when at idle as long as I dont turn the fan speed above the second notch. If I turn it up to the 3rd or highest fan speed, the air coming out will be cold. Basically not enough hot coolant is making its way back to the core or its not circulating fast enough through the core.
Okay, try this. Let it heat up to normal with heat on blower on low, then turn the blower on high, let it start blowing good and cold for a few minutes and get out and feel both heater hoses near the core. Are both hot, one hot one cold or both cold? That will tell the story.
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Old 01-30-2014, 00:00   #9 (permalink)
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Re: Why No HEAT??

Ill check the hoses today and get back to you on that.

I have bled the lines…so pretty sure it’s not air locked, especially after a year of being like this, and it would have had to of been air locked before I purchased the truck and done anything to it, beings it was like that before I put the new intake manifold and heater core on it. Bugs the crap out of me!! I’m a pretty meticulous person that’s why I fix everything myself so I know it’s done right. I have the problem of fixing things before they go wrong, so nothing does go wrong. The truck has run great since I have owned it and replaced the things I have. Doesn’t use any oil or coolant and has 90k miles on it. Ive stuck 20k on it in the last year for work mainly in the summer and half of that is pulling a skid loader around. I really hope its not a head gasket...I prob would just leave the way it is if that was the case. That was a 12 hr job out in the open doors of winter last year replacing the heater core and intake manifold. Dont feel like digging in that far again.
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Old 01-30-2014, 07:21   #10 (permalink)
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Re: Why No HEAT??

Okay, thanks. To me it sounds like the classic symptom of a bad water pump. Over time the impellar in the pump can deteriorate and lose the ability to pump efficiently. Even though it may not leak, and the engine maintains normal operating temperature, there still may not be enough flow for the heater core to maintain a constant temperature, especially when the engine is at a higher RPM or the fan blower is at a higher speed.

I forgot to mention to also check the hoses when the heat is blowing hot inside the cab and the again a few minutes after its starts blowing cold and note any difference. If both hoses are quite a bit cooler a few minutes after it blow cold, you more than likely have a obstruction or a flow problem.
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