Fuel Back Pressure

[thsmd4u]

Thanks for help on replacing fuel pump/fpcu. Not sure if this is a related problem or not. Occasionally I notice the smell of gas and when I investigated, found that there was gas puddled around the fuel filler cap area. Apparently the back pressure had forced it back through the vent in the gas cap. I know there is a vent in the gas cap but I don't think it is supposed to work this way. Would appreciate your thoughts.

[DAEDALUS]

Doesn't sound possible. You're sure it was gas?

[maxnix]

Check your carbon cannister and I think there is a check valve associated with it.
Modified by maxnix at 9:03 PM 5/8/2005

[thsmd4u]

Where do I find this canister and check valve?

[DAEDALUS]

There is a vacuum solenoid on the driver's side head (just a guess, since I don't know what kind of car you have), which controls purge vacuum through the cannister, which is right in front of it. Not likely the problem.

The gas cap is a check valve. It allows air to come in while preventing vapors from escaping. I suppose it could pass a liquid if it malfunctioned, but the fill tube is not submerged in gasoline. Not sure how gas would get up that high.

[thsmd4u]

I think you are on to something. 94 Q45-90k miles. Problem could have started before replaced fuel pump b/c of dreaded hummmmmm.

Can you give me a hint where to look for this carbon canister and check valve. A local mechanic mentioned this also. Thanks for help.

[DAEDALUS]

Look for a large black plastic cylinder just behind and lower down of the airbox. Follow the vacuum hoses--one will lead you to the solenoid on the engine head. Any cracked hoses would allow gasoline vapors to escape.

[thsmd4u]

Can you tell me a little about what this canister and check valve does? Does this valve regulate pressure in the fuel tank?

[DAEDALUS]

The EVAP system traps hydrocarbon vapors in the cannister, which are then sucked out into the plenum to be burned while the car's cruising. In effect this is a small vacuum leak. The solenoid ensures it only happens at times you're least likely to notice. The purge system should keep the fuel tank internal pressure fairly close to atmospheric.

[squeefoo]

At WOT tip in there is a lag while the processor computes what the FI sys. needs to do vs. throttle position/gear selection/RPM/ ETC. The evap canister purges at this point to supply fumes via the plenum to help with the potential lag. (like an accelerator pump) -THX QTech