Slamrod

iTrader: (11)

Can anyone suggest some of the more unusual suspects for a random misfire? The last couple of weeks Ive been having an issue with misfires coming and going with no rhyme or reason. Im seeing somewhat erratic AFR’s as well, it seems like the ECU is struggling to maintain stoich while cruising, im seeing swinging numbers from my wideband - normally cruising is at 14.7 or sometimes a tad higher but always stable, but im seeing it constantly swing back and forth, sometimes even dipping to the high 13’s. At WOT, my AFRs are no longer pegging at 12.6ish like my tune is set to do, but instead its going to around 13.5 - WAY more lean than its supposed to be.

While driving longer distances, its likely to suddenly start misfiring, almost like a cylinder isnt firing with how rough it is - this is accompanied by the flashing CEL. This doesnt throw a code though, at least not unless I keep driving it. This misfire can sometimes be remedied by giving the throttle a blip, and sometimes by downshifting quickly, and other times Ive even seen going over a bump magically cure it. And then other times, nothing will make it go away, not even pulling over, or restarting the car - itll go away when it feels like it suddenly and without explanation.

-Ive replaced plugs (NGK Iridiums) which had no effect, the originals looked perfectly healthy as well.

-I tried shutting down each cylinder individually with NDS2 but I couldnt find one that made a different effect than the others. O2 sensors are relatively young (bosch brand as well) and im not getting a code for them - the fact that my AFR is lean during WOT where the O2 sensors dont control fuel mixture suggests to me that they are not the culprit.

-Ive considered coils, but they/the motor has under 25k miles on it, so considering how young they are I dont know that they are the issue either, Ive checked all connections and tried tapping on them and jiggling wires while idling as well as physically inspecting them and couldnt find any issues there.

-MAF is new (04+ slot style, OEM Hitachi) and my MAF voltages seem quite stable so I dont suspect that is the issue.

-Battery voltage is good.

-Have not done a compression test yet, but considering the issue is intermittent, I dont see that being the culprit, especially with how young the motor is.

-Ive removed and inspected my cat (headers so no precats) and it looks healthy, so I ruled that out.

-I cannot locate any vacuum leaks - however, the symptoms im seeing, in my opinion, are practically screaming that this would be the culprit. Ive inspected all my hoses and everything is snug and where it needs to be. But swinging AFRs, lean WOT, and misfires would all be perfectly explained by this - I just cant find any leaks. I tried the spray method looking for leaks while idling but had no success there.

So, thats where Im at with this, pulling my hair out. Whats left? Injectors? They are relatively young, bought them from Admin Tuning, and I dont think they would cause a random misfire, idk though. Maybe something EVAP related? I know literally nothing about EVAP so idk if that could affect anything, but is it possible for a vacuum leak to develop within that system?

Any other ideas? Thanks guys - hoping someone can rescue me on this because this dumb thing is kicking my ***.

CichlidKid

iTrader: (1)

This literally just started happening to me. It's missing quite heavily but no code is flashing. I put on a Sri recently and I think the maf maybe going out. Gonna grab a couple of junkyard ones and give those a try and see if they fix it.

The Wizard

iTrader: (43)

Just because parts are relatively new doesn't mean they don't go bad. I would still focus on the usual suspects and rule them out first before looking for the unusual suspects. Flashing CEL is a dead giveaway that it's likely an injector issue. FSM even states so. Can try a smoke test for vacuum leaks, get two new coils (front + back) and start swapping them one at a time to see if you can find the culprit.

For an unusual suspect, would a failing IACV cause this? Not sure.

User1

Plug in an unmolested ECU and rule out data corruption.

Slamrod

iTrader: (11)

I dont think its the injectors since its so intermittent and (seemingly) not isolated to one cylinder, as per my cylinder power testing. However Wiz, you did just give me an idea - fuel pressure regulator? Would explain unstable AFR despite MAF voltage being stable, and why I cant find vacuum leaks yet im seeing lean conditions. I think thats my next area of investigation.

As far as unmolested ECUs, my car cannot run without me molesting it lol. It likes being touched but in all seriousness, I havent retuned anything recently so I dont see what could cause data corruption spontaneously.

Amave

It sounds like one or more coil packs are causing your hesitation, probably not a hard misfire but intermittent enough to where you can feel it but the ECU doesn't pick it up. I have noticed that even when the misfire is hard like a completely dead coil it takes awhile for the ECU to pick it up .

Try swapping a known good coil pack and drive it under the conditions when you feel the misfire , one at a time

CichlidKid

iTrader: (1)

Ended up being a coil pack for me.