Excessive fuel tank pressure

[afrostydog]

hey guys,

I have a 03 Tahoe 5.3 flex and lately I have been having a lot of pressure in the gas tank when I remove the cap to fill her up. It has been doing this intermittently for about 8-9 months, but in the last week or so it has gotten really bad and my MPGs have dropped significantly. I put $20 in it last night and have drove 25 miles since and I'm already on low fuel. i believe it may be the Evaporate vent solenoid staying closed as sometimes I would have trouble filling it up and other times I wouldn't and I think it may have finally locked shut. Anyone else have any input or have had this problem? Thanks

 

[iamdub]

I'm sure it's actually vacuum (exact opposite of pressure), right? Pressure would be blowing the cap off as you loosen it.

Anyway, a number of things can cause this, but I'd suspect the EVAP purge solenoid or one of the other EVAP system components is faulty. If the purge solenoid was stuck open, then the engine would be sucking on the tank the entire time it's running instead of periodically as commanded by the PCM.

After running the engine for a little bit, pop off the hose going to the purge solenoid and see if it has wet fuel in it. This purge solenoid is located on the top front of the intake manifold. You'll have to remove the cover (8mm bolt, I think). It will have a black plastic tube that runs along the driver's side of the IM going to it. Squeeze the grey plastic clip and wiggle/pull the connector straight up off the valve.

 

[fig865]

Mine is doing the same, and I can see how it can be confused with "pressure". When I pulled the gas cap off of mine this morning, it made an almost honking sound for 5-10 seconds. There was so much air moving I couldn't really tell if it was going in or out. So I'll check my purge solenoid. Should the hose not be wet up by the intake?

 

[iamdub]

Mine is doing the same, and I can see how it can be confused with "pressure". When I pulled the gas cap off of mine this morning, it made an almost honking sound for 5-10 seconds. There was so much air moving I couldn't really tell if it was going in or out. So I'll check my purge solenoid. Should the hose not be wet up by the intake?

There's really no way for the tank to get pressurized unless the vents to it get clogged and the gasoline sloshing around and heating up releases vapors that can't escape.

The pump is constantly removing gasoline from the tank, so it has to have a fresh air supply somewhere because air is what replaces the gas that is being removed. If no air can get in, then it would be just like you sucking the juice from a Capri Sun pack- the pack collapses in as the juice is removed. The fuel tank doesn't collapse, but a vacuum can still build up.

The hose to the purge valve should be dry. The OP mentioned the pressure sound and poor fuel economy. I'm wondering if the valve is stuck open, allowing the intake to constantly suck the fuel from the tank through the EVAP canister.