Guys,

After a lenghty cruise a couple weeks ago with temps in the mid 90's I unscrewed the gas cap to fill up and the pressure was overwhelming. It releaved pressure for atleast 3-4 seconds and was pretty forceful flow of fumes coming out of the filler neck. I've checked the lines to and from the purge solenoid to the char canister and everything is clear and flows freely. The lines are good and there is no vacuum leaks, but can I test the purge solenoid somehow to make sure it's functioning properly? Also, I'm getting no codes.

Lastly, is there anywhere else I could check as the tank should not hold that much pressure? Thanks and any advise is appreciated.

 

There are no codes related to fuel tank pressure, or EVAP system performance in OBD-I, other than DTC 26 for the electrical circuit to the EVAP purge solenoid. To test the purge solenoid, apply a ground to the dark green/white wire. With the key on, that should open the solenoid and you should be able to blow through it.

There is a pressure/vacuum relief valve that should relieve any pressure level that might damage the tank. In Shoebox's photo it's called the "vent valve":

http://shbox.com/1/fuel_pump_wiring.jpg

There is also a pressure control valve between the tank and the canister:

http://shbox.com/1/evap_system.jpg

The question might be "why is the system generating so much vapor?". Is the fuel pump possibly on it's way out, and heating up the fuel?

 

Thanks Fred, very helpful as always. The pump is new (Racetronix setup) and probably has about 300 miles on it (installed it last year) and works flawlessly. My pressure is spot on, so I'm sure my problem is not there. I'll check the solenoid to make sure it's operating properly this weekend. Could the vent valve between the tank and charcoal canister be the culprit? If so, how can I check this out?

 

Anyone else?

 

Okay, got an update. Pulled the Charcoal canister and all looked good, no charcoal and lines are clear all the way from the tank to the purge solenoid in the engine bay. The solenoid works just fine as I put 12v to it and it opens/closes just fine. Also, the evap pressure control valve connected to the charcoal canister works just fine and flows. I replaced all the hoses as they were pretty dry rotted, but I'm still unsure why the tank is pressurizing so much? Could the low vacuum from my cam cause the EVAP system to not work very well? As that's the only thing I can think of?

 

Mine has pretty much always done this and even more so when the tank is closer to empty. I consider it normal. One of my other GM cars does the same thing.

 

Guess, I'll live with it, I'm just worried that with the high pressure and heat of the summer that I could be boiling the fuel? Not sure, just want to make sure it's functioning properly.