I would like to preface by apologizing as this will be incredibly long winded but I believe the full story is relevant, gassers are not my usual MO as I typically own and wrench on diesels both privately and professionally and am fair at diagnosing electrical issues but im no expert.
It starts with a 600$ 1999 GMC Jimmy. Bought the truck after it had been sitting for about a year with a bad fuel pump, previous owner claimed to have changed the lower end bearing due to knocking as well as the lower intake gaskets and the fuel pump. New fuel pump was bad, pulled the tank, changed the pump, emptied the fuel put it back together and it ran, at first a bit rough but cleared up. After a couple weeks it developed a p0300 random misfire although cylinder 1 seemed to be registering the most, eventually became undriveable as it had no power and reeked of raw fuel with raw gas dumping out of the exhaust, replaced the distributor no change, did plugs and wires, no change, clogged cats... nope their hollow and the honeycomb is not stuck in the muffler. Eventually stuck a borescope down the intake and saw the original poppet injectors were just gushing fuel with the key on engine off. Replaced the spider with the updated unit style injectors and it ran much better, still a very slight miss at idle and the p0300 didn't go away.
Regardless it was good enough to drift in the snow. While driving it after replacing the fuel pump with no warning or developing symptoms, it stalled when I let off the gas and would not restart. Was only about a half miles from home so we towed it back and parked it. When I got to messing with it, it would crank and make a sputter or two but would not start, tried some gas down the intake, no difference. Pulled the plugs and had spark. Now as I write, I've been messing with this thing for 3 weeks now and have made no headway.
I'm going to rattle off everything I've checked quickly because its extensive. Starting with the mechanical aspect, cranks about 145 rpm, even and good compression in all cylinders, don't know if it jumped mechanical timing but ignition timing from the rotor is unchanged from since it ran so I'm assuming a chain jump would equal a matching change visible in the distributor rotor alignment, smoke tested vacuum leaks and sure its not totally air tight but its such a small and negligible leak that it should run although not optimally, I don't want to invest the time in fixing a problem that likely is not THE problem. Moving to electrical, checked fuses and relays all good, crank sensor good, ICM and its wiring, 12v, ground, VCM signal, crank signal, all good, coils sparking has power and ICM signal no problems, jumping almost 5/8th of an inch and I have good spark on all 6 plugs, pulled and checked them all, timing light shows its timed correctly at least with the crank. Double checked the new distributor, cap and sensor incase China let me down, all fine. Fuel? Key on engine off 60+ psi, drops to about 50-55 psi and holds, 60+ psi cranking. So it has fuel pressure I believe, how about pulse, back probed every injector connector, 12v on the pink lines for power feed and a signal from the VCM to fire each one. Every injector is pulsing with pressure behind it so it should have fuel, pulling the spark plugs shows fuel on them so its getting in the chamber, but no bang? Its got compression, air, fuel and sparks, not even a fart...strange. Im running short on ideas.
There are plenty of other mysteries that I cannot explain, once while cranking I saw sparks flying from the frame to the transmission lines clear as day, about as bright as a candle, checked voltage at ground with a multimeter and test light and had 0.9 volts, I've experience nearly identical random sparking between grounds on a 6.0 powerstroke that I quickly found had a glow plug module shorted internally with 12.6 volts to ground and I discovered it by unplugging modules until the voltage on the block went to 0. tried the same think on this Jimmy and couldn't find anything, cleaned all the grounds, no connectors have corrosion or moisture in the plugs, did the ground update for the VCM. Checked every visible inch of the harness to try and find a chaffing wiring but I came up empty handed. I no longer have sparks flying between the transmission lines and the frame but I also haven't found a smoking gun explaining the 0.9v on the grounds, I read about a shorted tail light on a Blazer causing havoc but at this point my best course of action to find this short is to strip the truck down to bare harness and inspect every inch? Really don't want to do that.
IT HAS FUEL, SPARK, COMPRESSION, AIR and TIMMING by everything I know about mechanics it should start, no if or buts about it, I actually think technically it is not possible it doesn't start but alas... crank no start. Any sort of security system disables fuel or spark which is not the case, even low compression of massive vacuum leaks tries to run.
Im so frustrated and probably forgot details in my rage filled ramble as at this point, I've been working on this unit for 3 weeks straight, every day rechecking every measure and parameter necessary and relevant with no faults, all the sensor read correct specs, there are no engine codes, this might be my defeat and the death of this Jimmy