stmpulr

As you can see I am new here so I say hello to all.
I am sure someone has encountered this problem. My air conditioning air or panel vent air (what ever setting it is on) switches to the defrost mode when I accelerate slightly (vacuum drop) I think it must be a loss of vacuum causing the system to default to defrost mode (still very cold air when on A/C).
Does anyone know where to look for the problem. I can't here any leaks and it seems to run OK. It never really had much power IMO.
Next problem is I am hearing a thumping noise that can be heard in the truck as engine speed changes (moving or not) and it can be heard coming from what sounds like center lower dash or floor and from the rt front near the A/C pump area. It does not do it with any type of consistent pattern. It comes and goes randomly.
It is a 2001 F150 Super Crew 4x4 with a 5.4 automatic trans with 3.55 axle with 65K on it.
I had several people who seem to be familiar with Fords and they can't pin point either problem but the have made many suggestions.

Any further help would be appreciated. Thank you.

rtg143

It may all be related to the vacuum problem. The leak is not easy to hear because they're small. Check under the hood for all the vacuum lines and also the check valve and reservoir. Then inside too. After that works, then see if the thumping is gone.

westtnfx4

Yeah it's a vaccume leak and the thumping your hearing is the blend door changing positions on you beacuse of the leak.Check all hoses behind the panel control and under the hood around the firewall/batery etc.

stmpulr

The thumping noise is quite fast and gets faster or slower with engine speed. It does not happen all the time, but the air coming out of the vents changes with every acceleration to defrost mode. The thumping noise can also be heard from the right front wheel well area near the lower right side of the radiator. Again, not all the time. I am not sure if there are two different problems here.
Could the vent problem be a vacuum actuator leaking that opens and closes the door under the dash?

I looked for a vacuum reservoir but could not find one. And where does the vacuum hose go through the firewall to the inside of the truck?
There are two solenoids on the right side of the firewall near the battery, what are they for?

Sorry I sound like such an idiot.

westtnfx4

The system defaults to defroster. Normal occurrence when there is a vacuum leak in the system. Not an intake leak of the engine but the vacuum system operating the servos. Look for a leak at the tiny line going through the firewall, its on the passenger side of the truck near where the heater hoses connect to the heater core. A tiny black plastic tube not much bigger than a small 20 ga. wire.

As far as the thumping goes it could be anything from a air pocket in the cooling system to a thermostat to a bad power steering pump....water pump....who knows.