[RESOLVED] Could fuel pump wiring be the cause of my misfiring?

[soldave]

I know some people might have read about this problem I've had before but for most of you let me summarize. Let's say I'm drag racing and I launch off the line. I gun it in 1st gear and before I manage to shift I just touch the rev limiter at 7750rpm. When I shift into 2nd gear the car just doesn't pick up at all and sounds like it's misfiring. This goes on for about 2-3 seconds before the car starts to pick up again and goes on as normal. This can occur in other gears too. My AFRs don't go majorly lean but go to what they would be when I'm out of boost (around 15). If I shift at around 6,000rpm-7,000rpm I have no problems at all.

I know the problem is partially my ineptitude at being able to shift, but there is something wrong with the car too I think. Someone just brought up the question of fuelling and it got me thinking. I have a Walbro 255 in my car, along with a Sard FPR and fuel pressure gauge in the engine bay (useful for adjusting, not for monitoring). But the fuel pump is not rewired. Could the fact that I've not done the rewiring modification be causing the problem I'm experiencing? I don't go really lean at any point with my AFRs but I just thought it might be worth a shot.

If you reply and say "Yes, that could definitely be it", I'd really appreciate it if you could say why. Fixing my car is nice, but knowing the technical bits behind it is even better. Thanks for any help you can offer about this

 

[juanpac0]

That could very well be the case as (especially at your power level and with a 255lph pump) there would be significant voltage drop through the tiny stock wire as the fuel pump is trying to supply enough fuel for you. The thing that befuddles me is why it only happens when you touch the rev limiter. The recovery from a voltage drop should be near instantaneous whereas yours takes 2-3 seconds.

 

[soldave]

How about this thought then. It is possible that my car is overfuelling on shifts, meaning that when I get into the next gear the cylinders are flooded and I misfire. If it is flooding my engine on shifts then my ignition system has a problem somewhere along the line. If I could turn the rev limiter ignition cut into a fuel cut, would that help things or would it make me dangerously lean if I hit the limiter?

I know the best answer might be a better ignition system but I want to try the cheaper alternatives first.

 

[soldave]

Well, after a year of having this damn misfiring problem, over the weekend I finally solved it. And I am a little embarrassed to say it was something simple. A guy here in Okinawa mentioned that it sounded like when the rev limiter was touched the ignition was being cut but not the fuelling, meaning that when I changed into the next gear I would encounter a cylinder full of fuel that I was having trouble getting rid of. Hence the misfiring.

I had thought about this before but had always looked at it from the tuning side, as I'd played with spark plugs and wires and it hadn't really helped at all. But this guy mentioned something like a new ignition system or COP setup to cure my problems, which got me thinking about the ignition side of things. I thought "May as well borrow a friend's coil pack while his car is down and make sure it's not that". Lo and behold, I was pinging off the limiter and smoking tyres in the next gear with no hint of a misfire at all.

Can't believe after all that it was just a coil pack problem. Hopefully my experience and solution can help someone else in the future with the same problem. Just don't spend a whole year like me in trying to diagnose it and being frustrated that every time it matters, your car ends up misfiring.