1G Top end missfire need help!!

[Ace_gst]

Hey don't know if this is through right spot to post this, but I feel it's ignition related. I have a 91 gsx with simple mods, ebay 16g, 550 injectors, wally 255, afpr, alky/water injection, was running 18psi with a afc tune with logger and wideband fine for month's. One day the car started breaking up a little up top, feels like a boost leak would. So I BLT and it held 25 for about 10 seconds and drops really slow and holds at 8 ish. There is also a slight miss at idle that will make wideband dip lean with the miss(15.6 with miss). So here is what has been changed and check.

New NGK 7es
New NGK wires
Swapped coil packs
New fuel filter
Compression 165-168-165-167
Swapped injectors
swapped maf
Cleaned injectors

Im lost now and was thinking maybe CAS or Tansistor? Would either of those lead to a miss at 5k up in boost and slightly noticeable miss at idle. I'm sure it's missing while driving and light acceleration but it's not detectable. Logger dosent show anything out of the norm from when it was running right, ie. No knock or weird timing, o2 fluctuates. If anyone has some ideas please help me out, been fighting this one for 2 month's.

 

[Sjd6795]

Make sure ignition timing is set right. You also said you changed and cleaned the injectors, did you make sure the gap is correct? Seems like you have a small boost leak by the way you described it but that isn't causing your problem, al though I would fix it while you are working on the car. Also 15.6 isn't lean at idle, that's damn near perfect. Make sure all plug wires are hooked up correctly as well. It could also be fuel related so check your injectors.

 

[Ace_gst]

I know 15.5 isnt lean but stock map keeps it pretty consistant 14.7 and it dip to 15.5 or lower when it misses at idle. Gap is .028 on the plugs witch is what i think you ment and I have checked the injectors and have tired others. I couldn't find any leaks on the BLT and honestly it's holding better then it ever has. Ignition timing is spot on 5*. Wires are correct and car runs fine other then in boost at wot. Low fuel trims at 95 mid and high 100

Plugs showed miss on all cylinders also. Could a transister cause a miss just at high rpm in boost, or the cas?

 

[bling5tatus]

I think if the cas was inaccurate you'd be seeing misfires all over the rpm/power band. I'd be more likely to think the PTU is the cause. Of course, just changing to another one will be a good test if it's known to be a good one. They can "work" when the car is cold and then intermittently fail when they get hot under the hood. I had that exact problem and it was very annoying to figure out.

 

[Ace_gst]

What I was thinking that about the CAS also. The PTU is the transistor that sits next to the coils plug correct. I only have spare 90 ones so I'm on the hunt for a 91+ to try out. I'm pretty much down to the Transistor or ecu being bad because everything else is new.

Also when driving normally and I shift to my next gear my wideband will dip to 17 for a second as I'm back on the throttle. I seen someone who was having same issues with the missfire and lean after shift also buy he never posted if he resolved the problem.

 

[Ace_gst]

Well little update, I swapped out the PTU and it seemed to have cleared it up some but not totally. It's still missing at idle (it's not bad, just bugs me) and broke up once at 6k during two second gear pulls. Did notice AFR drop to 9.7 before it broke up, I know that's the stock target afr and I haven't had a chance to retune it (swapped injectors trying to find miss) in the high rpms. Could it be missing at that AFR because it's also getting alky/water injection and it just dosent want to ignite? I'm going to pull fuel and let ya guys know anyway haha.

 

[911gdsmboostin]

Hey I'm having this same exact problem... sence i got this car it's always seem to have a slight miss at idle.. also should not this car is pretty bone stock gst fwd auto.. again as long ad I had this car I always seemed to have a miss at idle but it's more annoying then anything it doesn't effect the performance at all now recently I have eliminated the egr except the actual egr valve I get a cel but didn't effect anything also at the same time I also rebuilt the tb minus the biss oring.. but new shaft seals and cleaned well I also installed a mbc.. that being said thing ran well up until tonight I been going up local track for some t&t Last weak ran great now it misses at high rpm and still has annoying mis ar idle ( it seriously happened the last 2 runs of the night just lifted could tell somthing was wrong ) .... I will say I have hit fuel cut a time or 2 but only when I launch and boost spikes over 20.. I will also note I know the exhaust manifold is cracked and possibly other small boost leaks but never has effected it this much. So I'm right here with ya pal keep updated and I'll do the same in trying to figure this out

 

[bling5tatus]

You're breaking up due to the afr being too rich in boost. I've experienced that too. It's not powerful, it's bad for mpg, and it pisses off your gf/wife/bf/husband if they're riding w/ you.