SureTrack Real Fix: Wrong Direction

It’s the peak of summer and shops are full of vehicles with air conditioning problems. SureTrack community member SpidellChris had a Ford van in their shop with a problem where the air would come out of the defroster vents when you stepped on the gas. They posed the question to the community, and with the help of fellow community member jhirz and others, they were able to resolve the issue.

The following Real Fix summarizes the interaction between them in the community that led to the solution.

Vehicle
2007 Ford Econoline E350 Super Duty 5.4L

Title
Air comes out of defrost vents under load, replaced vacuum canister.

Complaint
When control is on Vent, A/C or Max and you put the engine under a load, the defrost vacuum actuator starts moving and air flow goes to defrost. Let off on the load and the defrost actuator returns to allow air out the vents.

Diagnostic Steps Performed/Parts Replaced
Visually inspected and smoke tested intake system, brake booster and hvac system. No leaks found. What am I missing? Has anyone ran into this issue before?

Best Answer from Community Member jhirz
When air flow switches to defrost position, it means you are losing vacuum in the system. Check all mode door positions at idle. This makes sure a blend door motor vacuum line isn’t disconnected. There is a vacuum canister under the heater box in engine compartment. It has a check valve in it or on it that goes bad. When supply vacuum goes away (acceleration), the valve closes and vacuum in the canister holds the mode door positions. You can buy the replacement parts or redo the system with aftermarket parts.

Cause
Performed smoke test from control to intake. Found vacuum canister leaking and check valve leaking.

Correction
Replaced vacuum canister and check valve. Vent control works as expected.