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I've looked through many of the threads on the common EVAP problems but couldn't find anything that matched what I was trying to find. I was filling my 98 Blazer up with fuel and when I took the gas cap off I noticed there was a lot of pressure. I didn't think that much of it and drove probably 50 miles. I heard a humming noise coming from the rear end and pulled over and could hear it was coming from the gas cap. I unscrewed the cap and air came rushing out. A lot of air for probably 15 seconds then the engine died.
It started back up fine, I unscrewed the gas cap again and it had the same amount of pressure and shut off again. I drove another few miles and parked it and it sat all night.The next morning it started fine and I drove home and stopped a few times and checked the gas cap and no pressure. Through this whole time no codes. I've been driving a lot of rough forest service roads lately and its pretty dusty out. I'm not sure if that contributed.
This is a generic view of what your evap system looks like:
Under normal operation the purge solenoid near the engine is closed which isolates the evap system from the engine. This solenoid opens occasionally to retrieve the stored gasoline vapors from the evap charcoal cannister and add those vapors to combustion instead of the atmosphere. This is not your problem because the system is sealed enough to increase pressure.
A common fault of the evap system is a leaking or loose gas cap which is supposed to seal the system. No problem here also.
The purpose of the charcoal cannister and evap vent solenoid is to allow air/pressure exchange between the gas tank and the outside air to allow for displacement during tank filling and from temperature expansion (your problem). The charcoal cannister traps fuel vapors during this exchange for later retrieval. This is where your problem is. Either there is a blockage in the hoses or cannister, the evap solenoid is stuck shut or the rollover valve is stuck closed. IOW, there is no clear vent path from tank to the atmosphere. You have to disconnect the system at various points to find what is blocked. Dust and bugs are a common cause. I would start with the vent solenoid at the end.
98 Evap Problems
