Thread: afr questions?

Yes the exhaust leak will throw the wideband off. After you fix it check your cruise A/F.

I'm not talking about WOT. If your vacuum is 15 @ idle it must be a very rough idle. Do you have cams? Normally vacuum is 18-20.

yeah bc stage 2 cams

was just about to reply with unless he has cams. Damn beat me to the punch.

I don't think a car will idle properly with an AFR of 21. It's just too lean to support combustion. So, if that's what you are reading, you have an AFR measurement problem. Either poor calibration or an exhaust leak could cause this.

Haha, well I think there are issues to be corrected/verified before continuing.

1) IF you have any exhaust leaks, fix immediately and don't bother trying to diagnose until then.

2) Re calibrate your sensor just to be absolutely sure it's not just giving you bogus values for no reason.

Is that how it works? I thought they just injected very little fuel because of the closed TPS reading. I've always sort of wondered because it still sounds like it's firing when you do that, and I even get occasional back fires engine braking

What wildmane said about engine braking is true for the most part. When engine braking, the fuel injectors are off completely in almost all modern cars, and most standalone ECUs offer this functionality. However, before going completely lean factory ECUs (and most good standalones) dump fuel for a split second (which can cause the backfires). This is called deceleration enrichment and it prevents the car from jerking you forward in your seat when you let off the throttle. It provides a smooth transition from engine operation to engine shutdown.

If a car is backfiring excessively during engine braking it is typically a sign of a poorly tuned car. For one reason or another the ECU isn't cutting fuel completely during breaking. It can be a TPS problem, or someone who forgot to turn the "overrun fuel cut" on in their standalone settings, etc.