I am posting this here hoping for some input. I have a 91 D21 4cyl, 5spd, 2wd, short bed. I had the fuel pump go out and replaced it from the selection of 10 or so different ones there are. Ever since i have no pressure at the gas cap. Then it started this intermittent miss, like while sitting idling at a stoplight you can feel it miss, then clear, then miss , back and forth. When I accelerate from a stop , sometimes it misses then clears up. So i did all the normal stuff, plugs platinum NGK, wires, cap, rotor, cleaned the MAFF, checked sensors, replaced the O2 sensor, checked for vacuum leaks. Took it to a shop, the guy replaced the intake thermo sensor, it was dead, replaced the air filter housing seal as it was trashed, rechecked the sensors, and it still is doing the on and off crap, just enough to drive you nuts especially if you have been driving this truck for almost 6 years with no problems. I mean I know how this feels to drive and it's not right. I also hooked up a fuel pressure gauge inline with the filter so I know it has 35lbs pressure and pressure drops after you turn it off, but cannot find any leaks. Could it be the check valve in the tank? Hate to pull the tank again, but oh well. Any help is appreciated as I am running out of options.

Might try checking the fuel pressure regulator. Test injectors. Could have a bad or dirty one? Check for vacuum leaks. Not sure about the 1992 but The later models have a vacuum line located on the bottom of the air cleaner. The pressure regulator also has a vacuum hose check for leaks? Might also try an injector cleaner before taking the fuel rail apart.

Ok well I forgot to include all the stuff I have checked , like new injectors installed. New plugs Platinum NGK, new wires, rotor and cap, replaced a few vacuum lines I found were leaking and have a fuel pressure gauge hooked up inline with the new fuel filter so I can make sure it has 35lbs pressure, 40 when the pressure regulator is unhooked and plugged off. It does bleed off overnight. All this started when the fuel pump gave out and i replaced it. Ever since there is no whoosh you get when you loosen the gas cap, that's why I got a pressure gauge to test it with. Also have checked almost all the sensors and replaced the O2 sensor yesterday and it still is acting weird. Even checked the spark with a inline spark checker, those things are great for small motors too like weedeaters and mowers. I changed the injectors after I tested them and found one with a low ohm reading. I'm just about out of options here and ready to pull the bed and get another pump, because that is where all this started and i don't know what else to check. I am reading that the check valve in the tank could be bad letting the fuel pressure bleed off. And this looks like a double post, damn. Sorry about that.