# Would misfiring change the afr?



## Stephen Max (Aug 18, 2003)

Last night I did a second gear run out to about 6850 rpm, hitting a max boost of 11.2 psi. I datalogged the run with an LM-1 wideband O2 controller.

At the end of the run I hear detonation and shut her down.

I had recently filled up and thought I had gotten a tank full of bad gas, but when I looked at the data from the LM-1, it showed my afr beginning to get lean at 6400 rpm (from 11.5:1), and by 6500 rpm it is shooting upward above 20:1.

I am running 370 cc/min injectors at oem fuel pressure and a JWT ecu.

I'm trying to eliminate possibilities for why it went so lean (I've never seen that happen before with my setup).

So one question I had was, if my plugs were misfiring, would that cause a change in afr, and if so, could it cause the afr to go lean because of unreacted oxygen?


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## Stephen Max (Aug 18, 2003)

*here's the data, by the way:*


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## Chuck (Nov 19, 2003)

Hmmm interesting, i doubt a missfire would change the AFR permanantly.. try resetting the ecu, and running the dyno again.. it could have been 1 of many things that caused it to lean out..


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## Stephen Max (Aug 18, 2003)

Chuck said:


> Hmmm interesting, i doubt a missfire would change the AFR permanantly.. try resetting the ecu, and running the dyno again.. it could have been 1 of many things that caused it to lean out..


I just read that engine misfiring will indeed cause a narrowband O2 sensor to register a lean condition due to the presence of unreacted oxygen in the exhaust. I haven't found out yet if the same is true for a wideband O2 sensor, which is what I have. My plugs are gapped maybe a little on the high side (.034-.036") and they're overdue for changing, so I'm thinking (i.e. hoping) it was misfiring and not detonation I heard. Then again, misfiring would trip a CEL, right? Or does the ecu trip the CEL only with repeated misfire events?

If it was detonation, then the afr went lean first, and I have a fuel delivery problem I have to track down.


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## morepower2 (Apr 19, 2002)

Stephen Max said:


> Last night I did a second gear run out to about 6850 rpm, hitting a max boost of 11.2 psi. I datalogged the run with an LM-1 wideband O2 controller.
> 
> At the end of the run I hear detonation and shut her down.
> 
> ...


AFR will usualy go up with missfire. I think you are out of fuel pump!

Mike


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## lshadoff (Nov 26, 2002)

morepower2 said:


> AFR will usualy go up with missfire. I think you are out of fuel pump!
> 
> Mike


Perhaps the 370 injectors were maxed out and couldn't deliver enough fuel at 11psi and 3bar.

Lew


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## Stephen Max (Aug 18, 2003)

morepower2 said:


> AFR will usualy go up with missfire. I think you are out of fuel pump!
> 
> Mike


I talked to the folks at Innovate, they said that misfiring will indeed cause wideband O2 sensors to register lean, so that is what I think is happening, although I think I also had some detonation going on simultaneously, to confuse matters. But I've heard detonation before when logging runs and I've never seen the afr skyrocket like that.

I've got the Walbro 255 lph pump, it's about a year and a half old. I'm running pretty low fuel pressure - mid to high 50's boosting at 10-11 psi, so it should have plenty of capacity at that pressure to supply the 350 bhp I dynoed at. 

Injector duty cycle is getting up to about 90% at that power level, but there should still be some headroom there. Besides, if you're running out of injector capacity, the afr would rise more gradually, I would think. At any rate, I've got a Cartech fmu I'm going to install to raise fuel pressure a bit at higher boost levels.

Z32 fuel filter installed about 25000 miles ago.


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## morepower2 (Apr 19, 2002)

Stephen Max said:


> I talked to the folks at Innovate, they said that misfiring will indeed cause wideband O2 sensors to register lean, so that is what I think is happening, although I think I also had some detonation going on simultaneously, to confuse matters. But I've heard detonation before when logging runs and I've never seen the afr skyrocket like that.
> 
> I've got the Walbro 255 lph pump, it's about a year and a half old. I'm running pretty low fuel pressure - mid to high 50's boosting at 10-11 psi, so it should have plenty of capacity at that pressure to supply the 350 bhp I dynoed at.
> 
> ...


Did you try to close down your plug gap?

Mike


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