Vacuum check valve?

I have a '98 Ford F-150 XLT, 2WD, Short bed, Ext. cab, 4.6L, automatic trans. with 65,000 orig. miles. I bought this truck new.
When I have the A/C or heater or just outside air coming in through the dash or floor vents, the air will switch to coming out of the defrosters when I accelerate and then the air will return to it’s normal position when I let up on the gas. Is this caused by a bad vacuum check valve and if so where is it located and what does it look like??,Jamesk

The check valve is most likely near the heater and AC lines. It often has three ports, one to the intake manifold, one to a holding tank and one through the firewall to the dash control head. If they fail a generic check valve can be installed. All the McParts stores have the generic check valves in the HELP brand… Look here at vacuum components… for the after market part which is somewhat similar to the OE part.

Thanks,I’ll give it a try

Check your vacuum hoses before replacing the check valve. For the price of vacuum hose maybe just replace them first.

There are also vacuum pods used to control various doors in the ductwork. A leak in a pod diaphragm can also cause a problem like this. Some of those pods are easy to access, others are not.
I’m not real familiar with the layout on this particular model but with many Fords the pod that controls this function is often behind the glove box. This makes it one of the easier ones to access; if needed.

This is where the diagnostic footwork starts but I agree with checking the one-way valve first. A vacuum gauge/vacuum hand pump can also help to some extent in tracking these things down.

This sounds like the classic symptom of a vacuum leak. Keep checking all hoses and connections until you find the leak.

Usually, by removing the glove box, you can gain access to the heater controls

ok,I’ll look behind the glove box for something,I can’t seem to find what I’m looking for under the hood or I just don’t know what the thing looks like or where it is on the engine,

Ford used the “coffee can” for many years and may still. It can rust out just like its can cousins and have the same net effect.

Does anyone know where the “can” is located on my '98 ford F-150? I have looked all over and I can’t see a vacum can any where under the hood. Can’t see a vacum check valve either. I haven’t taken the clove box out yet, but I’m going to soon, also the cruise isn’t working anymore. Could also be a vacum leak problem ?

The vacuum hoses in that truck are likely hard plastic tubes the diameter and stiffness of the cartridge in a ball point pen. The tube likely passes through the fire wall on the passenger side near the AC evaporator housing. Locate that and follow it back toward the the intake.

My Jeep uses the same type of system, but of course all the parts are in different places. I had the exact same set of symptoms, and in my case there was a cracked rubber fitting between the vacuum line and the reservoir (the “coffee can” mentioned above) that allowed the line to pull 7-8 in of vacuum but not the full amount; this was enough to hold the HVAC in the right spot for normal cruising but it would change to defrost on acceleration. FWIW my Jeep’s ‘coffee can,’ which is a black plastic tank about the size of a pint or quart, is behind the passenger side fender. Doing a google image search for “ford vacuum reservoir” might help you visually ID the part.

So, in my case it was an extremely cheap fix; the challenge for me was not only tracing all the vacuum lines, but finding the leak which was literally a hairline crack in a rubber fitting.