AdsoYo

I’ve been having this problem since doing 24x on my LT1 where the car will intermittently sound and feel like it’s missing on one or more cylinders. Like it will be missing bad then suddenly correct itself for a few seconds, then start missing again. It does it without fail when I give more than ~50% throttle, breaks up bad and doesn’t correct itself in that situation. I can notice it during other driving conditions too such as cruising, acceleration, and idle. A member of this forum emailed me a tune with some aggressive timing and fueling and it covered it up well. Made it more difficult to notice.

While troubleshooting I found a file in HPtuners repository for an Impala SS with a 396 who did 24x. Timing was a few degrees less across the board and fueling was way different due to smaller injectors than mine. I copied the timing and MAF tables but left my current fueling and changed a few minor things myself like DFCO, some idle stuff, and fan temps. After uploading the file, the miss was more prominent. Even during normal driving where before I could barely notice anything, now it felt like it was missing most of the time instead of part of the time.

I tested continuity of the harness for the injectors and they’re all wired correctly to the PCM. The injectors also worked fine before 24x. I pulled the spark plugs and noticed 1 and 7 were black. 8 was almost there but not as bad. The plugs are only a couple hundred miles old. Am I looking at bad coils? There was a seller on Amazon who had 4 new LS2 coils in stock for $19 each so I bought 3 of them. What else could contribute to plugs like this?

Injuneer

Almost sounds like you need someone with more experience with the 24X conversion. I'm not aware of too many people here, maybe one or two. Have you tried LTxTech.com? A few people there have it working, as I recall, although they aren't the friendliest bunch of people for "outsiders".

AdsoYo

I guess I'll ask those guys. I figured since I'm getting the same symptoms at varying intensities with 3 different tunes (pcmforless, hptuners guy, Impala 396) it's not in the tune. And the plugs didn't foul before 24x so I'm confident about the injectors. The only other variable is the coil packs.

ACE1252

You could interchange the coils on 5 and 1. If the problem does not move then the coils are fine.

A trip to the dyno(with constant speed/load test options) with an ignition scope and oscilloscope equipment could help rule out ignition issues. At least you'd be able to load it up and try to replicate the problem with test equipment on the car.

AdsoYo

Well I tried joining LTXtech and they never sent an activation email. And they never responded to my message asking for help. So whatevs.

I swapped coils and plugs on 1 and 3 to rule out the harness and injectors. Plugs look unchanged.

I swapped plugs only on 5 and 7. The blackened plug from 7 started to clean up after being in 5 for a bit. The practically white plug from 5 is much darker after being in 7. New coils should be arriving soon. Not sure what else it could be.

Also whenever I reset fuel trims with hpt or unplug the battery for a while, the car won't idle unless I hold down some throttle. It will idle around 500 and die pretty quickly until it relearns.

I would love to put this thing on a dyno but there just isn't anything close enough. I want to get all the 24x bugs figured out at which point I'm gonna do AI heads and cam. THEN we'll do the dyno

AdsoYo

I replaced coils on 1-7-8 since those were the darkest spark plugs. Car runs PERFECTLY now. I almost can't believe it. No knock retard anywhere, fuel trims within 5% of each other at all times, no jerkiness or bucking like I had with the opti and old PCM.

I feel like the odds of receiving 3 bad coils out of 8 should be pretty bad. Hopefully that isn't a testament to their reliability. Either way, right now I feel like 24x is totally worth it.