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Help..Heater blows cold then warm then cold?

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Old 10-29-2011, 09:56 PM
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Help..Heater blows cold then warm then cold?

my dad has a 98 f150 and the heater will blow real cold for awhile then switch to warm for awhile then back to cold? we replaced the thermostat thinking that could be it.....but that did not fix the problem

could it be the heater core? any help would be appreciated
thanks!
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Old 10-29-2011, 10:29 PM
7700Rob 7700Rob is offline
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Just thinking outloud here... Does the truck have automatic climate control? Since you have no different results from before changing the thermostat, I'm guessing the coolant system is full and purged of air. It doesn't seem like the heater core would cause this. If it does not have a.c.c., try moving the temperature control around just a little bit. I once had a Taurus that when I turned the heat all the way up, the temp. mix valve would fluctuate, but if I turned it down just a bit, it would be fine. Try doing a forum search as well, if you haven't already. Rob
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Old 10-29-2011, 11:50 PM
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thanks rob i will def check that out
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Old 10-30-2011, 07:59 AM
LLADNAR LLADNAR is offline
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You could have a problem with your blend door inside the dash. To rule out the heater core, get the truck warm and touch the heater core lines running into the truck, if they are both hot generally you have good flow.
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Old 10-30-2011, 09:33 AM
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LLADNAR is probably right. There is a motor operated plastic door inside the duct that mixes hot and cold air to get the temp you want. Its called the "blend door" Gear box on the motor is known to strip teel and cam on outisde is known to break. I think if you lay ont he drives floor and llook up toward the radio you can see the motor. You can take the plastic dash off under the radio and look see what is going on. Some have gone thru the glove box for access.
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Old 10-30-2011, 09:35 AM
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http://www.ford-trucks.com/forums/10...lend-door.html
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Old 10-30-2011, 10:40 PM
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thanks guys that sounds like that is probably the problem
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Old 10-30-2011, 11:03 PM
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im not sure if this is related or not but dad was driving his truck tonight and i guess the temp kept shooting up sparaticly and the engine would act like its running on 4 cyls with i think is a safety feature? would a junk thermosat cause that? thanks again
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Old 10-31-2011, 03:32 PM
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that is very possible,, i have seen them in two pieces inside the housing,, i guess if it was completely broken it would run cold and if the pieces got lodged and cut off flow then if would make it run hot.
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Old 10-31-2011, 06:55 PM
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well i think we kept the old one... guess we ll out that back in and see if that fixes the prob
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Old 10-31-2011, 09:57 PM
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make sure the radiator overflow tank is up to proper level. Low waster liwll make the temp spike, and heater core run cold.
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Old 10-31-2011, 11:37 PM
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thanks steve ill check that out too
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