Hi all. I have a 2000 Blazer 4wd 4.3l Vortec. Lately, when the fuel tank gets below 1/4 full, the service engine soon light comes on and doesn't go off until I refill the tank. It also stumbles a bit on acceleration and pings and knocks going uphills. I went to Autozone and had them check what code was coming up. It was P0300, multiple misfires?? Anybody have any idea what could be going on? I have read the fuel pumps are notorious for going bad...could that be it?
Any help is greatly appreciated.
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There might be a couple of possibilities. As you have stated could be fuel pump but you should check with pressure guage both static and running pressures...should be 60psi static and 55psi running +/-...

Then there is the fuel tank pressure sensor and/or the Evaporative Emissions System.

Since the SES comes on at <1/4 tank then it gets really wierd to diagnose...this is an unusual set of circumstances...