No Start/Run Condition Problem......

[NC_John]

Background - LSX swap, 04 blue/green PCM. DBW. Speartech standalone harness. Boyd tank with aeromotive pump. Build has got about 60 miles on it, still working through some new-build bugs but engine has been great so far.

Leaving the house the other day the truck started fine and ran fine but then just instantly died in the driveway like it was keyed off. I figured fuel was too low as it just cranked but would not restart. Pushed it back in the garage. Next day added a few more gallons of fuel to the tank but it would only turn over, no start.

Started digging into it yesterday. Noticed I could not hear the fuel pump cycle on to prime with initial key-on (in-tank pump is controlled by PCM through speartech harness). Got my voltmeter on the power terminals at the tank and had zero volts with key-on. Thought that explained the no-start. Jumpered the fuel pump relay terminals to manually run the fuel pump but still crank only, no start. Ran out of time to play with it for the day.

Today checked that the PCM has voltage, it does. Checked all the fuses (power to pcm, fuel pump, 02 heaters and injectors/ignition). They were good. Manually forced the fuel pump on again and cranked. No start. Checked ignition for spark (old school method- screwdriver in plug wire watching for spark to block). None. Pulled a plug and checked for fuel - none, dry as a bone. So it looked like the pcm was neither calling for the injectors to cycle or the coils to fire.

Went back and checked the signal for fuel pump relay to see if the pcm was even calling for the fuel pump. It was, it read 12 v with key on. Put the relay back in- no fuel pump. Bench tested the relay- was clicking and metered good with my ohmmeter. Jumped the power to the fuel pump again. This time it cranked some and then and fired up. A little more backfiring and popping through the exhaust than I would consider to be normal (its pretty free-flowing exhaust, barely muffled on a built engine). The exhaust smoothed out to a more normal idle within 15-20 seconds. Shut it off. It restarted normally. Pulled the relay jumper wire and put the relay back in. Nothing. Crank but no fire.

Even though it tested OK, I suspect the fuel pump relay is bad (some chinese one that came with the speartech harness) and ordered a replacement (its got a goofy ISO280 base so I don't have one here I can use to test). I'll have a couple relays here early next week.

Here is my confusion. If the fuel pump relay is indeed bad, why was I not seeing spark from the ignition or fuel on the plugs when I was forcing the fuel pump on with the jumper wire? There was no evidence that the engine was even trying to run. I was starting to think VATS had somehow been re-enabled. Do I maybe still have a problem with the PCM (cold solder joint going intermittent or something similar)? I don't think it would take that much cranking to prime the fuel line. I don't have a fuel pressure gauge but did order one from Summit that I'll have this week as well (couldn't do a loaner tool gauge on my fast fuel rails.)

I'm going to be nervous about driving this thing until I know why it tested the way it did. Again, if it is the fuel pump relay, which I will confirm when the new one gets here, why was there no spark or (perceived) fuel injector activity? Is the engine looking for fuel pressure before it tries to start? I don't know anywhere where it would measure fuel pressure from.

anybody know what might have happened/might be happening?