David976

I have a 97 f150 with the 4.2 and i recently underwent the daunting task of changing the heater core. All was successful. But after about a week of being driven, the panel stopped putting out air and it only comes out of the defrost vents and i cannot figure out why. Any suggestions are appreciated.

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Bigronnie

I bought a used 2002 F150, and the air blows from the vents when on vent, but cycles back and forth through defroster, then vent. Also, when you switch to heater, it will only blow through defroster, and not through the heater section. Can anyone pass along a suggestion??

backlash1

Hi
Both of you guys, your problem is more than likely a vacuum leak. The default position of the air flow system when there's no vacuum is 'windscreen' (or defrost).
So, David, since you pull the dash and the heater box out before, it's possible that one of the connectors has come off or (hope it's not this) you kinked or punctured one of the vacuum lines.
Bigronnie, you must have a slow or partial leak, since you're able to use the 'face' position at least sometimes. What happens in your case is that when the engine is at idle (highest vacuum) it will probably work in most positions but when you accelerate (less engine vacuum) it will go to defrost and everytime you stop it will revert back to whatever position you selected previously.
My suggestion for both cases is try to listen for hissing noises at idle or even immediately after switching the engine off, or visually inspect the vacuum hoses both in the engine bay and inside the cabin, including the connector on the firewall. I've also seen the selector switches fail but these are one in a million, so don't go changing it unless you're sure your vacuum lines are all ok. hope this helps. cheers

marc

Bigronnie

Thanks. I have been searching for a vacum leak, but can't find. Yes, at idle, it will blow through vents, but at highway speed, it cycles between vents/defroster, but never heater. Two of the vacum lines disapear into the backside of the mixing box. Will dig deeper.

Bigronnie

Further to this, I have found my problem. A little while back, a white tail deer came crashing through my windshield, sending glass everywhere. When the body shop repaired the damage, and installed the new windshield, they were very negligent in cleaning out the shards of shattered glass out of my defrosters. The glass over time shook down, and filled the rubber diaphram of the heater trap door vaccum motor. Over a short period of time, the shards of glass splinters punctured at least 6 holes in the diaphram, thus making it unoperative, and putting the default up to the defroster. Anyone who has ever had a windshield shattered, beware.......