96 Stalling EVAP system help large in rush of air when opening gas cap

My 96 is stumbling and stalling, it has a new fule pump months ago, now it will start to stumble, hesitate and the fule pressure starts to drop to 45 psi. When I open the gas cap air rushes in. It runs great if I leave the fuel cap loose. I believe the purge valve or something that vents the fule system is not working properly. Any tips on how to check the EVAP system or venting of the fuel tank. Thanks for all replies.

The take should have a vent that is blocked.

Have a mechanic check it out.

I believe in 1996 the system is closed due to OBDII regulations. There is a purge valve on the intake manifold and I think it may vent the tank there at certain times. There is a schrader valve test port for the EVAP system that says 1 PSI max. I am looking for ideas on how to test the system or how does the system vent ? The charcole canister is fairly dry not filled with gas or plugged.

I just did a quick bit of research and guess what, you might need a new gas cap.

When the valve on the gas cap will no longer open, the fuel pump is essentially pulling against the cap trying to get air into the tank.

But first, are you getting any check engine lights?

Get a good quality gas cap with a lock and you will be set to go.

Good luck and let us know.

I really do not get any check engine lights. I hooked up a fuel pressure gauge to the fuel rail, and tucked it under the windshield wiper against the windshield so I could watch it while driving. It stays steady at 60 psi for about 2 miles then slowly drops to about 45 psi. the truck will then be struggling to maintain speed and start bucking. If I keep trying to force it to go it will stall and set a check engine light stating all four o2 sensors lean. before it stalls if I leave it running and open the gas cap there is a rush of air into the tank and fuel pressure goes up to 60 psi it will run perfect with the gas cap open for days. I thought the check engine light would come on if your gas cap is loose, so I shut it when parked. Thanks for the tip I will look at the cap more closely to see if it has a venting mechanism in it.

Don't bother looking, it does.

Get a new cap and install.

BlazerLT thanks I will replace the cap and in a couple of days post the results.

no problem, here to help.

Where Is The Charcoal Canister Located On Your Blazer,thanks For Your Help

My 1996 2 dr 4wd has the Charcoal canister located in the lower part of the rear drivers side fender directly in back of the rear wheel. Imagine if rocks and mud were thrown up by the rear tire they would directly hit the canister, on a 2dr it is directly below the fuel filler neck.

I purchased a new gas cap as BlazerLT suggested and my stalling problem and in rush of air are gone for 2 days now. i bought the cap from a Chevy dealer it was about $16. I looked at some Stant caps at Auto stores that were $7 but looked kind of cheaply made so I decided to go OEM.

So all is well?

I thought so 275 miles without a problem then on my way home from work last night it started to act up on the highway. Step on it to pass and tach will not rev past 3000 rpm and truck starts to stumble. It definitely seems fuel related. Tank and fuel sending unit are brand new due to northeast salt belt, the lines are good. I drive 75 miles a day and get 19 mpg so I believe the CPI unit is good, oil is at proper level with only normal slight gasoline smell to it. The new gas cap has certainly made it better, from a start accellrates strong seems to be more of a problem when at speed. It is very intermitten like I said 275 miles with out any problems sometimes longer, its done it on all different fuel tank levels from full, half, to near empty, fule filter is new when tank was done. Would definitly like to solve it by winter time as do not want to get stuck in the cold.

Just to let you know, you don't have the CPI system. CPI's were only between 1992-1995.

Have you started to monitor the fuel pressure again?

I really cannot see any injectors I guess they may be under the intake manifold. Not sure what a CPI looks like but my motherinlaw has a 92 s10 pickup and I know what that looks like. I was curious about the nut kit and leaking of fuel under the intake manifold I hear poeple talk about. My next step is to hook the fuel pressure gauge back up and drive it while watching the readings. Maybe I have a couple of problems going on.

Again, I said you DON'T HAVE THE CPI AND NUT KIT SYSTEM.

I think you might have a pump on its way out.

It sounds like your vent solenoid is blocked this vents your fuel tank this is usually located near canister vent is a normally open solenoid the pcm only commands it closed for testing email for more info