1991 Z28 5.7 TPI. So I hate to be that guy since there are a hundred threads floating around with the exact issue, but my fuel is boiling, twice now just after a fillup, and my tank is pressurizing so badly that it is overpowering my fuel tank vent valve (white valve mounted near rearend) and dumping half of my fuel into the street. I've got a basic working knowledge of how the pressure from the fuel tank vents, basically air comes in that white valve, moves through the tank, collects vapors, and runs down a line to the evap canister.
Seems to me that if the valve in front of the evap canister is plugged or has failed, that pressure will be unable to escape the tank.
My idea is, why can't I just bypass the vapor canister purge valve? I believe that its only function is to open once the car starts. Can anybody see why this might be a poor idea?

 

Hi Rockin RS ,

I can .

It would be a terrible idea since without that solenoid your manifold vacuum is gonna end up pulling raw fuel from the purge line into the engine .

Here's how ;

When you drive , and the fuel sloshes around in the tank (especially if you have a mostly full tank) the fuel that gets sloshed around will find it's way into the vent line , tank baffles be damned , and end up with the untimate result of burning more fuel than required (poorer MPG)

I have seen the above happen to a friend's camaro that the purge solenoid simply stuck open . It was pig rich , dirtying plugs and black sooty exhaust pipes , and had had several other "attempts" at a fix (new carb , dist , and a WHOLE host of other expen$ive parts before he brought it to me) ...

I pulled the purge line from the charcoal can to the carb and found it full of fuel . Replaced both the can and the stuck valve and the MPG went up by 5 , the pipes turned a nice light grey , and all was well in camaroland ...

The moral of this story is to REPLACE failed parts , rather than just eliminating them . Your car's emission control system functioned perfectly well when that car left the General's factory , and it's now up to you to restore the proper functioning of that system .

 

That line gave me quite the chuckle, but then again I suppose that's a point for another day. I can see how bypassing a purge solenoid would pull raw full as the manifold will be basically directing applying vacuum to the vent line at all times, however I was talking about bypassing the VALVE that's only function is to let fumes out of the tank and into the carbon canister.

I do believe I found the problem though, I tested the valve and it functions as it should. Closed under STP and open under vacuum. The vacuum line however, had been routed to the fuel vent line instead of a vacuum source. Basically, the valve could never open. Upon further inspection, the "AIR" line on the canister which is supposed to dump off somewhere, was plugged off. I think this is just a problem which was waiting for a hot enough day to happen.

So lerng stery shert, I routed the vacuum port on the valve to a vacuum line, unblocked the AIR port on the canister, drained the fuel from the fuel tank vent valve and connecting line, and inspecting the rest of the evap system for blockages. Just filled up the tank again, and I suppose time will tell.