racecougar

First time posting here after a LOT of searching. I swapped a 5.3L L59 into a '65 C10 back in 2017. The engine has been great, but it has an intermittent no-start that occurs about once a year, always when I'm far from home without any diagnostic tools of course. Here's what happens when this occurs:

1. The fuel pump primes for ~ 2 seconds at key on like normal.
2. The starter spins the rotating assembly as usual, but it's clear that the injectors are not pulsing. It's as if it's in clear flood mode. If you've ever cranked an engine with the throttle wide open you know that the cranking is audibly different/quicker.
3. Cycling the key, trying different throttle positions, etc., makes no difference.
4. After ~30 attempts, the engine will fire right up like nothing ever happened. It'll then go another year before doing this dance again.

So this happened when trying to leave a car show last night. I tried unplugging the DBW pedal, TPS, and throttle actuator one at a time while "trying everything", but to no avail. As usual, after a bunch of attempts, the thing fired up like normal, and I drove home. Diagnosing it this morning, I had a P0452 code for Fuel Tank Pressure Sensor Low Voltage. I have this DTC turned off in the tune file, so I'm not sure how that code was generated. Either way, I cleared it and it's remained clear today. I also pulled the elbow off the throttle body and had a helper operate the throttle pedal with the key on. At initial key on, the throttle blade opens and shuts once quickly. Pressing the pedal results in a pretty smooth actuation of the throttle blade, though I'd say that the actuator is louder than I expected. The strange thing is that pedal actuation with the key on, engine off, causes the tachometer needle to fluctuate. Video link below.


Any ideas? I don't expect this could be the crank sensor, as it would likely flake out while driving. From what I've read, if the TAC module determines an inconsistency between the pedal position and the throttle blade position, it won't fire the injectors, and made not generate a code. Most TAC module failures I've read about show up with the engine running; this truck has never had an issue when the engine was running. My only guess right now is that the actuator on the throttle body gets hung up with the blade closed once in a blue moon, and isn't able to move freely to verify operation at key on. That doesn't explain the tach behavior though. Any help/experience is appreciated greatly!

Here's a pic of the engine in question.

rpturbo

iTrader: (47)

So think about this, does it seem to happen when it gets colder out? Normally, if the cam sensor is having a problem, on the second key cycle, it flips firing order, but perhaps it's a combination of a cam sensor issue, and perhaps how the tune is setup.
I would definitely check, clean the ground on the back of the heads, even though it's probably unlikely a issue, I mean you gotta start somewhere.
I'm no help on the DBW tac module, I don't play with those.

racecougar

Nope, it's done it in hot weather and cool weather. I'm familiar with the cam sensor failure mode, and this sure isn't acting like that. At least with a failing cam sensor, the engine will try to fire, as the injectors will pulse. I really believe it's a DBW-related issue, but I'm looking for anyone that has experienced something similar.