Vacuum pressure when removing fuel cap - solenoid?

[iamdub]

That makes total sense. I'm gonna go ahead and order a replacement plug for the next oil change. Excellent advice. Can we be neighbors? You might get tired of me hollering across the fence for advice.

Sure! There's an empty lot next to me and I've been needing someone to split the cost of a fence with me. lol

Another likely cause of vacuum in the fuel tank is a lack of venting when the vehicle is driven. As fuel is used, volume of fuel is reduced, volume of air is increased. If the vent system is not functional, vacuum can result, especially over longer trips, between tank fill-ups.

Right. Well, almost. As the volume of fuel is reduced, the volume of SPACE is increased, not air. The lack of air results in vacuum. The system is supposed to remain sealed for emissions except for when the PCM commands a purge.

His system is clearly sealed as he's getting vacuum. The problem is too much vacuum along with a DTC showing a failed purge solenoid. If the valve is stuck open, the engine is constantly sucking the air from the sealed tank the entire time the engine is running, causing the excessive vacuum. The problem seems to be intermittent now.

 

[02Lightning]

iamdub,
So, I get what the valve at the motor does. It just opens to send fumes from the tank to the manifold to burn them off. But I’m not sure what the valve at the tank does? I just assumed it opens to pull air from the charcoal canister, so the tank wouldn’t be under a large vacuum?

 

[iamdub]

In short, the tank is under positive pressure (ever saw what a gas can does after riding in the back of a truck in the sun)? The vent solenoid lets the built-up fumes escape and collect into the charcoal canister. It doesn't need to be open all the time as this wouldn't allow the system to build any vacuum or pressure and the PCM wouldn't be able to tell if the tank had a leak because the MAP sensor on the tank would always report 0 pressure (1 atmosphere/14.7 psi). It's not just a matter of letting the fumes go somewhere, but ensuring that they are going into the canister. Otherwise, a leaking gas cap would let the fumes escape just the same, which is the exact opposite of the EVAP system's purpose. It's how the system keeps itself in check while keeping the fumes in check.