1G Rich misfire

[Anthony Boni]

Hello all.

Need to pick your brains real quick. At idle I can hear a hollow, misfire sound. Doesn’t affect my AFR, idle is steady.

While cruising, I hear the same hollow misfire sound, my AFR goes rich (12.7:1-12.2:1), and the car stutters for a split second. It’s random while cruising at partial throttle. During heavy acceleration, WOT, or any boost whatsoever, car pulls like a champ. No hesitation, no stuttering whatsoever.

I know most misfires cause a LEAN A/F ratio, but this one goes RICH. Which is why I’m confused.

I don’t want to make assumptions before I hear your guys’ answers, but I have an idea as to what the problem may be. I don’t want to make a recommendation and everyone focus on that one thing, so I’d like to hear some answers before I comment.

Mods:

I’m using FIC 1250cc low-z injectors, BPR7ES plugs gapped to .018”, datalogging with DSMLink V3. I’m also logging fuel pressure through a fuel pressure sensor, and using a fuel lab AFPR. Fuel pressure is rock solid, no spikes/drops throughout the misfire 450HP E85 fuel pump, currently on 93 octane pump.

NGK blue spark plug wires, stock transistor and coil pack. FP76HTA turbo, FP2 cams.

Base timing is set at 5°BTDC, base fuel pressure is set to 43PSI.

I’m currently at work so I can’t post a log from my laptop (posting this thread off my phone).

 

[Anthony Boni]

So,

I think found the fix.

I upgraded from low impedance FIC injectors, to high impedance FIC injectors. Plugged in the resistor box, dialed in global and my deadtimes. Solid idle, amazing partial throttle input. A tiny bit of misfire here and there, but I think I am just being very sensitive to any sort of hickup at all. I'll replace the plugs, grab some idle time on them, pull them and check them, cruise, pull and check, then change the plugs again if that all checks out. Do a WOT pull, read the plugs, and be done with this anomoly.

ALSO, I added an ISC motor and got it properly working. After TWO YEARS of no ISC motor, this made driving a DSM much more enjoyable. I bought this remote mount ISC adapter from a buddy of mine for dirt cheap and decided to give it a go. Drilled and tapped 2 1//4" NPT holes, one-pre throttle body and one post, plugged in the ISC and messed with my BISS/throttle stop screw on my S90 throttle body, got it MINT. I love this car now.