1998 Ranger 4.0 v6 4wd. I’ve had an issue with vacuum leaks for a while now. But this one has me stumped. When I’m idling my max ac blows through the vents just fine but when I get going and get over 2,000rpm it goes straight to defrost. When I turn the the switch over to HOT the heater control valve doesn’t move to let the coolant go through. I’ve checked all the vacuum line even the ones behind the glove compartment. I’m stumped. I’m wondering if it’s the vacuum canister inside the fender? But idk how to check it? Any advice would be great

 

Welcome to the forum

Yes, the vent/air direction in the cab is control by vacuum "motors", no vacuum at these motors = Defrost vent, so its the default vent/air direction

The vacuum for the cab vents comes from the Vacuum reservoir(canister) in the front fender area, you will see a Black hard plastic vacuum line from firewall to the Reservoir, it often cracks so leak, check it carefully.
The heater valve has a grey hard plastic vacuum line that also runs to firewall, it is ONLY activated when control in cab is set at MAX AC, it is not used for HOT/COLD adjustment, thats done be the electric Blend Door inside the cab

Both the vents and heater valve use the vacuum reservoir
Vacuum reservoir is also connected to the intake manifold, the vacuum source

The vacuum reservoir has a Check valve, either built in, or on the vacuum line that runs to the intake manifold

At idle vacuum is high so vents work, under acceleration vacuum in engine drops, the vacuum reservoir should hold high vacuum, but isn't, so its leaking out somewhere, you need to find out where