I repaired this problem on my 1998 F150 4.6L. It had a bad vaccum line between the vaccum check valve and the vaccum storage tank. The vaccum check valve is located between the battery and the firewall. The vaccum storage tank is attached to the rear of the battery case.
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Hi, Most Ford products have vacuum actuated control valves in the heating and a/c system. Ford designed a fail safe system into their cars and trucks that if all vacuum is lost that the system would default to defrost so at least your windsheild would stay clear. Typically there is a canister under the hood mounted to the fender well that looks like a metal tomato juice container from the 60's. That cannister is the vacuum reservoir that is supposed to hold the vacuum under high load (heavy acceleration) conditions. If that "tomato juice can" is either disconnected or if the check valve in the vacuum line is bad, then whether you are heating or running the a/c, under hard acceleration air will come out of the defroster vent. Check all your vacuum lines running from the intake manifold to that canister and back to your heater and a/c system for cracks or leaks, Also check that the check valves in the vacuum lines are in place the right way (they should only flow one way- toward the engine.) Listen under the hood when the engine is running for the hiss of a vacuum leak. Fix any leaks that you find. I hope that starts you off in the right direction.
you may have an a/c compressor clutch and or clutch bearing going bad. may be time for a new a/c compressor or compressor clutch assembly. the a/c comes on when you have the defrost or recirculation on in heater mode.
This sounds like a vacuum control problem. Check on the top side of your engine (Manifold or throttle body area) for the vacuum feed for the heater controls. You will find a plastic vacuum line that goes thriugh the dash. Follow this line from the engineto the firewall. Somewhere along this line you will find a check valve. Replacing this check valve should fix your problem.
For verification that this COULD be the problem do this: drive the vehicle and make a point to notice whether this switching is taking place while the vehicle is under moderate to heavy acceleration. If this is the case, the check valve is definately the problem.
It sounds like a vacumn problem. When the heater controls loose vacumn it defaults to the defrost position. When your car decels your engine vacumn is at its highest. Check your vacumn lines for damage. Check the control panel for a vacumn leak. If you take out the controls and hear a suction noise than the control switch is leaking.
this is very common problem, theres a vacuum leak.check the vacumm lines under the hood also check the one that goes inside to heater controls, whats happening is that it looses vacuum when you accel because the motor takes it,theres a one way valve that holds the vaccum when this happens but with a leak it cant hold vacuum,heres a vacuum diagram to help you check the vacuum lines,i dont know which engine you have in that 150 but this will be close. take good look at the circled areas in the diagrams,good luck, hope this helps you.
thats normal, you are taking vacume from the heater system when you are under a load,but it returns when you let back up on pedal,if you wanted to alter this affect you might try installing a vacume ball to the heater vacume hose,