Got a frustrating one here! We could really use some expert help figuring this out... BACKGROUND: 1992 K2500 Sierra SLE 5.7l TBI w/147K miles Bought in 2017 for son and I to have as a work and "project" truck We have been repairing, upgrading, and restoring the usual - A/C compressor, alternator, pulleys, belts, bearings, shocks, brakes, bushings, catalytic converter, and complete tune up. ALWAYS started and ran strong given age, mileage, and obvious abuse by former owners We are barn/driveway guys with basic knowledge/skills and decent home shop setup/tools No, we do not own ALDL cable or use winALDL software, yet. FORMER ISSUE: Began running rough/low power/poor fuel economy with intermittent check engine light at speed. The loss of power and backfiring at speed finally got so bad we began tearing into it. We updated/replaced/performed the following with NO CHANGE to the original symptoms before finally diagnosing leaking intake manifold gaskets: Vacuum was steady and at the low end of "normal" on our cheapo gauge - 17-18lbs? Replaced O2 sensor, CTS, MAP sensor, IAC, distributor, knock sensor, PCV valve, both valve cover bushings, all vacuum hoses and the EGR valve and EGR solenoid. Rebuilt TBI - She was really dirty/carboned up good. Good spray pattern before and after rebuild. Changed fuel filter as precaution. (Had swapped it when replacing fuel pump last summer) Replaced distributor after noticing timing not holding steady - it was very worn and had "slop" in it - basically fell apart when we started poking around Cleaned and/or replaced/reconnected all ground points and cables on entire truck - they were a mess - amazing it ran at all Sent Jet Stage II chip to be tested- all good Replaced ECM as a precautionary measure - the old one was very "crusty" but seemed functional Set base timing to 0 TDC with ECM bypassed - found it advanced 3-4 degrees BTDC While attempting to calibrate IAC and base idle, we discovered the engine would race when IAC disconnected - we could not set base idle Engine would start right up cold or hot, run rough, and get worse as it got hot - very little change from any of the upgrades/repairs listed here. This lead us to realize we had intake air issues, hence intake manifold gasket job Tore it down and performed the intake manifold job... RTV had failed terribly at rear of manifold Engine looked surprisingly clean for age and being worked hard by previous owners We replaced the oil pressure sensor while we were in there, just because it was ancient and easier to get at while apart Set timing by positioning the distributor where it came out - we had made marks and taken pictures to guide us Went back together great - without a hitch... UNTIL NOW! CURRENT ISSUE/STATUS: As our post indicates, motor will crank for days and almost start - sounds "flooded" - like a stuck choke Oil pressure gauge reads 25-30 while cranking - goes up a little when engine almost starts Good spray and pattern coming from both injectors - exhaust smells like gas after cranking a while Checked timing with ECM bypassed while cranking - adjusted slightly to get to 0 - was only half a degree or so out Produced a single loud backfire from exhaust while cranking when I tried WOT to clear fuel out Firing on all 8 plugs: #6 + #7 are notably wet/darker - #3 is clean and dry as a bone - remainder are wet/tan, not fouled, though Starting fluid does not help ATTEMPTED SOLUTIONS: Triple-checked firing order with plug wires - it's correct with spark on all 8 Pulled #1 spark plug and confirmed TDC compression stroke is correctly in position with distributor and #1 pin on cap - twice - it's perfect Unplugged MAP and cranked Unscrewed IAC a few turns to let air in during cranking Unplugged IAC and cranked Checked voltage to every sensor power wire- all getting required 4.8-5V Checked TPS voltage throughout range - 0.48V at closed to 4.8V at WOT - no fluctuation No codes except those produced when disconnecting sensors and cranking Reset ECM / cleared codes Let sit overnight and cranked it again Gave up and decided to ask for help here! We really appreciate any guidance or advice!