I have a 1994 GMC Sierra 5.0 305, 2wd auto trans. It's givin me fits and some outside help would be greatly appreciated.

The truck was idling rough and would stall under load (in gear). It showed only codes 33 and 34(MAP) so I replace MAP sensor, reset computer and tried again.

It Still ran terrible and now showed only code 35 (idle Air Control), so I replaced IAC valve and for kicks I replaced distributor cap and rotor because they didn't look in the best shape and also the fuel filter. I also have checked all vacuum lines and connectors.

Still wouldn't run under any load or much throttle and NOW showed code 43 only. I tested knock sensor. It tested ok but the connector and connector pigtail at the sensor seemed ratty and raw in places so I replaced connector and pigtail.

At this point I decide to check the timing. I did it the right way: unplugged the timing bypass wire (tan with black stripe wire). The timing showed WAY advanced. I set timing, unplugged negative side of battery, reconnected timing bypass wire and then reconnected neg. battery cable.

What I have now is this: With timing bypass wiring disconnected the truck will start right up and idle like a dream but stumbles and gags with throttle or load. With timing wire connected, it will start after coupla tries but surges up and down to almost dying and up tio idle, then dies after a few times of that.
Any help would be appreciated. Thank You.

 

What is your fuel pressure?