Should the evap vent valve make a perfect seal when powered?
I currently have 3 valves (two brand new ones) that when powered they get harder to blow through but do not stop air completely.

(this is for a 2003 E-350 5.4 but it uses the same valve as the 97-99 f150, and 2000-2004 f150 one just has a different elbow on it. Was not getting any responses in the van forum unfortunately)

 

Did you get a code of PO446?

Here is what they do.
The Evaporator System (EVAP) canister vent control valve is located on the EVAP canister and is used to seal the canister vent. This solenoid valve responds to signals from the Engine Control Module (ECM). When the ECM sends an ON signal, the coil in the solenoid valve is energized. A plunger will then move to seal the canister vent. The ability to seal the vent is necessary for the on board diagnosis of other evaporator emission control system components. This solenoid valve is used only for diagnosis, and usually remains opened.
Other words it is closed to check system when the system is first engaged to see if the system is working correctly and then is opened to vent by as much as the system needs. If they are fully closed all the time pressure would build up and something is going to give.
(One of the checks) at the one over the wheel well is to blow air in with about 5psi (just blow through the canister) and see if all of the air line hookups will let air flow through the canister just setting. If it does not then you have problems and need to get a new canister or see if it is plugged and if you can clean it. (Air has to come out each hose connection with lines disconnected and no power to system or you have a problem).

 

Are you talking about the vent or the purge valve? Don't confuse them...

The vent valve should seal completely when energized while the purge valves usually have a bit of bleed-through.

It might be a good idea to have assistance with actual diagnostics as you may have veered down a rabbit trail.

 

The purge valve (the one that attaches to the engine) seems to work perfectly... it seals completely when 'off' and allows flow when 'on'. I am asking about the vent valve (the one that attaches directly to the charcoal canister) and have included a picture below just to avoid confusion.

I have a p0455 code Gross leak/no flow. The vent valve I have off the vehicle currently just applying 12v to it on the test bench. When i do apply voltage to it you can hear/feel the plunger move and it gets harder to blow through but still allows some leakage. The one off the car does this as well as the 2 I have purchased from two different local auto parts stores.

 

Did this test and was able to get air all the way to the inlet of the purge valve. Also got air coming out from the fuel tank when the cap was removed. So i don't believe anything is clogged, but there may still be a leak somewhere in the lines in between.