SAFC to the Rescue!

[FORSFED]

Well, here's something for all of you to ponder over.

I've had the high rpm misfire problem for some time and some of you know what I'm talking about and have helped me along the way. With a virtually stock (only a 2.5" exhaust) 91 Talon I was getting a consistant misfire from 4500+ rpm's. A week ago I reconnected two vacuum lines which helped at first, but the problem gradually came back.

Tonight I installed a Walbro 255 pump thinking that I wasn't getting the volume of fuel necessary at high rpms. So after installing it and driving, the problem was actually amplified from before. I did some checking on the logger again and trims were at 119, 119, and 104, no knock, 18 deg timing, and 89% duty cylce. Now I don't know exactly, but from my knowledge this sounded awfully high for being stock. So I installed a friends SAFC and tried leaning the car out a lot on the high rpms. From 3k to 7k I went from -5%, -14%, -20%, -22%, and -18%. After one run, the car pulled to redline like the day it drove off of the lot. The DC is now down to 75% and Base FP used to be at 36psi and is now at 41psi. It seems as though it was getting way too much fuel (even before the larger pump) causing the misfires, but I don't know how this could be considering I replaced everything and nothing fixed the problem. Any ideas? I can run the car like it is since everything seems to be in spec, but for a stock car I can't see what would cause it needing to be leaned out so much in order to run properly...

[MATCHBX]

This may sound stupid and you probably have done this already, but it bears asking. Did you do a tune up (ie, plugs, wires, FUEL FILTER, AIR FILTER and PCV valve)? The reason why I stress the 2 filters is that, while the air filter is usually changed to a K&N, the fuel filter is one of the most overlooked pieces during a DIY tune-up or DIY upgrade. In a 22 year old car there's bound to be a lot of shit at the bottom of the gas tank just waiting to be stirred up. I've seen alot of cars that still have the original fuel filter on it from the factory.

I'm not saying that this is your problem but it's also not something to be overlooked. Check you plugs for wear, discoloration and proper gap. Check (or have someone check) the resistance of your plug wires.

[john]

Maybe the previous owned didn't tell you about injector upgrades?

[FORSFED]

AWD1dr - Yes all of those items in the car you listed are new. The guy I bought the car from said it had a new fuel filter in it, but as a safe measure I put one in afterwards. Also looking into the tank with the pump out, there wasn't even a spec of dust in it.

92tsiawd84 - There isn't an injector upgrade on the car, injectors are stock, and I even tried replacing them thinking they were causing the problem.

[FORSFED]

Well after doing some logging on 1st, 2nd, and 3rd gear pulls I'm starting to see that leaning out the safc isn't the answer but only hiding the problem... At the top of each gear here is what I saw::

1st 2 knock counts 0.88V O2
2nd 18 knock counts 0.78V O2
3rd 38 knock counts 0.74V O2

I had no misfires like before, however on the top end towards redline I'm getting really lean by having the afc leaned out so far. My conclusion is that there's too much fuel in the mid to high range and leaning the afc fixes this until 6000+ rpms when the o2 voltage drops and knock rises. When I tried to richen this area back up on the afc, then the misfire came back.

[A//// Guy]

That is way to lean, I hope to god you are using some sort of race gas....

I dont think I would ever go below .87 for O2s on pump gas... You can see yourself your getting loads of knock... I dont think leaning will help as if you lean anymore you will blow it up...

[FORSFED]

I only did one run to take the log I have. I have since zeroed the afc out and am dealing with the misfire again. The only reason I tried the afc was to try and come up with some answers. I know better than to run it that lean.