89 TPI L98 , was running rough, stalling, took it to mechanic he said there was wierd humming from gas tank, and a lot of pressure in the tank. He pushed open the flap and said warm, moist air came out for 30 seconds, and pressure kept building. I did fuel pump about 6 months ago, and cat back exhaust about then. Heat shields are all in place. Another unlrelated problem is the key is stuck on and won't turn back. I need ideas, I can't afford the mechanic to keep guessing! Thanks.

 

Sounds like an issue with the charcoal can, the pressure control valve, a blockage in the lines, or some other problem with the evaporative emmissions system. Sounds like the system isnt capturing the gas that evaporates off. The moan you hear is the pressure releif valve letting the gas tank vent before its damaged.

 

And perhaps you could find a less confused, more versed automotive technician. GM has been using essentially the same tank pressure and evaporative emissions control systems for 20+ years. About ten years ago, there was a slight change that added pressure/vacuum monitoring by the PCM, but even at that, GM has more vehicles on the road than anyone, worldwide. One might presume that a technician would have trained on or seen at least one of them.

 

On the EEC i believe it is the center hose, it is labeled gas tank or vent or something of the sort. Pull it off and go for a few rides.

If it still does it then i would say blockage in that line but if it works fine the EEC is hooped.