Took the car out for a spin yesterday. Car ran perfect for 45 miles, then the fuel pump started screaming, no loss in fuel pressure, but loss in power - car leaned out. Got home and noticed a loud hissing from underneath, but couldn't pinpoint it. Removed gas cap and it was like opening a shaken can of coke. Does anyone know how this system works? I know the gas cap is a ventless type and the tank is vented through a rollover valve in the tank (I really don't want to drop tank if I don't have to). What is a charcoal canister and how does it work?? Can I push air through vent hose without messing anything up?
Thanks for any help!!

 

The charcoal canister stores fuel vapors that evaporate while the car is not running. Then, when you're driving the car, the ECM uses a solenoid valve to purge the fumes into the intake. If your canister is clogged, or if one of the lines is clogged or the solenoid is broken, it can cause the fuel tank to build up pressure.

 

and what effect would that have on the car?

 

Shouldn't the tank vent via the gas cap also? Anyways, take the hose going from the tank to the canister and drive around and see if that problem goes away.

 

According to the chevy shop manual "The fuel tank filler neck is equipped with a non-vented threaded type cap" Anyway, I just verified the line from the tank (from where the sender assembly meets the lines going to the front) to the canister with my air hose. The vent seems ok - made the rollover valve make noise in the tank. Couldn't get anything from the line under the tank into the tank (canister line). Anyway I'll disconnect the line to canister and go for a ride and keep you posted. Supposed to rain so may take a couple of days - Thanks

 

ok, just got back from 55 mile ride with line off canister. When I first left I was getting max hp (FUN!), then the power fell off slightly again, but the fuel pump stayed quiet. When I got back I checked the fuel tank by taking cap off. There was only a slight psst, but there is a smell of fumes lingering about. BTW fuel pressure never drops off either. Any ideas?? Does disconnecting the canister affect performance?? Thanks

 

I think I recall somthing about a replaceable filter in the canister? Anyone?

 

Of course, there will be fumes lingering about. BTW, when you did this run, did you plug off the air hose going to the TB? That way, the engine doesn't suck in more air than it should.

 

Guess I wasn't thinking clearly about the fumes - DUH!
I did not plug the line into into the intake. I'll have to try it again, but I thought I tried that and the car wounldn't idle properly - would explain the lean condition though. Thanks

 

I would plug it up just to make sure the car isn't sucking any air into the intake that it shouldn't. Just remove the hose from the TB going to the charcoal canister and plug up that port with a vacuum cap.

 

i have the sameeeee problem with my iroc. whenever i drive around, my idle will start to get retarded until i take off the gas cap. it'll hiss like a mother***er for a few seconds, and stop. then i put the cap back on, get in the car, and notice the idle is now perfect.

i was thinking my charcoal canister wasn't working right... aklim, should i remove the charcoal canister, or is there a better solution to all this?

 

Look up in the in the rear fender well area on the drivers side near the tank, you will see a round thing, that is the vent canister thingy, can't remember what it is called, but that may need to be replaced if you have pressure in your tank.

Here is a picture of it on my car, it is sitting right next to my fuel pump,

 

I would check the item Zepher pointed out as a probable cause but I would also remove the charcoal canister and let that portion vent out. Actually, I already did.

 

what do i need to do to remove the charcoal canister? ill replace the "vent canister thingy" when she comes back this week

 

Just remove the lines and plug them and then put a small filter on the hard line.