well this is sort of an odd question but does any one with a 94-98 1500 have an issue with the heat switching from the vents to the defrosters when your approaching any were between 55-70 miles an hour while giving the truck constant gas, but then when you let up on the pedal the heat starts coming back out of the vents? its very strange and any help would be much appreciated
Thank god someone else has this problem. That problem has plagued me for years, even before i got the truck handed down to me. I've had personal and professional mechanics as well as myself hunt for anything suspicious, and nothing explains it. I've gotten one suggestion in the 12 years i've had this truck, and there's a flapper valve in the dash, where it switches from defrost to vent. When the flapper goes bad, it looses control, but I have yet to find a faulty flapper valve.
If you haven't found the problem yet. On the right side of the intake, you will see a round vacuum one way check valve. Replace it. Have had many of these come to me with this problem, that was what it was every time. Because it is plastic, and close to heat. The check valve melts out of it and is unable to hold vacuum, causing the blend door to open and close on it's own.
vacuum leak, maybe check valve but sounds like small leak in vacuum system the the truck goes into a safety default defrost mode when vacuum is not enough
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