spontaneously. I can't find any Vac leaks as my research said they are vac operated. Are there any other solutions? It seems to be doing the same thing with the heater on too.

 

Sound like your having a problem with the "blend door" inside the heater box.

There are several write ups on this problem, a search should help you.

 

What exactly is it doing or not doing? Temps changing on their own or air flow directed without change of the function control?

 

Airflow changes all by itself. from dash to floor to defroster

 

Okay then this narrows it down considerably. FWIW the heater and A/C system share the air flow ducts which is why this occurs in both functions.

Typically this is a loss of vacuum which in turn is almost always attributed to a leak somewhere in the air flow control system. The system is built to default to the defrost position when vacuum is lost

Blend door issues tend to be centered around an air temperature that cannot be adjusted via the temp control ****.

Vacuum loss such as you describe will be found along any one of the various rigid plastic tubes that are part of the air flow control system. Many times the original vacuum reservoir and/or its check valve have failed. The fact it will change says to me vacuum loss is sporadic, most possibly in sync with the engine RPM's.

This is a factory schematic for your heater air flow system:

 

Thanks. I'll look into it deeper.

 

That's only prudent!

 

I have a similar situation with an E450. The dealer explained that there is a vacuum chamber designed to keep vacuum for this system constant at different throttle openings. If this chamber leaks it can cause this sort of thing.
It's also difficult and expensive to have replaced.

 

There are plenty of work arounds for this situation---its fairly common on this type air flow control system.

The biggest reason its so costly to replace the reservoir is its labor intensity. Having owned three '97 and newer E-Series I've yet to find or locate the actual reservoir itself.

Said work arounds are so simple and cheap it would be crazy to spend so much money to replace a $30 part.

 

Reading along here and this is also my problem. just wondering what you might do to work around a leaking air tank or what ever the problem might be. I have tried looking over the hoses. Mine only changes from vent to floor or defrost, when you accelerate. I guess that is due to low vacuum at that point. I notice you pointed out defrost is default position.
I hope this isn't considered a thread hijack. Just trying to stay on subject.

 

One can wear the proverbial anti-flaming protecting by first engaging a "search engine", works very well here on FTE in fact!

Here's a thread or two that cover this in good detail:

https://www.ford-trucks.com/forums/1...efrosters.html

(In that thread look at Post #35 as it has a VERY simple and elegant solution to this problem.)

https://www.ford-trucks.com/forums/1...-problems.html

Read through both threads even though it seems like a lot to digest---it will make solving this problem much easier and labor-saving too.

HTH

 

thanks for the great lead on this subject.
I didn't do a search on this because I was at this post reading. I guess a search is in order.
thanks again...................