Letting Off Gas - AFR/Wideband/Exhaust Question
 

Someone teach me something here. I always love learning and I have maybe a weird question.

I have an S4 turbo (mild streetport), 3" RB Exhaust, Stock Injectors, Stock Intake, S5 Turbocharger - pretty much stock with an exhaust. Emissions removed though.

I just installed an LC-2 wideband, and the car idles at around 12.7:1 ratio. This gets leaner as I go through the powerband (up to 3000rpm - still breaking it in @ almost 50 miles on the new build) - up to maybe 15.8:1 with sometimes hitting the 16s depending on gear and throttle %.

Now setup for my question - when I let all the way off the gas in gear (foot off pedal for argument's sake), the wideband goes up to 22.4 and stays there until I give it throttle or shift to neutral. My boost gauge, which only reads vacuum when I'm on the throttle but otherwise stays at 0, goes back to 0 when I let off the gas (a totally other issue but may be relevant).

Is this caused by the back pressure from letting off the gas and air moving back up the exhaust to the Bosch sensor even though the engine is still running? Is it anything to worry about or keep an eye on, or is this normal?

I am expecting people to say that it's normal, but since this is not only my first car I have put this type of $ and time into, but my first time working with a wideband, I am not trying to screw it up while taking preventative measures to keep the engine healthy.

Excuse the newbie-ness, and thanks for any insight.

What ECU does the car have? When you let off its likely seeing a deceleration fuel cut where the injectors aren't spraying as the car decelerates, which is why you see that lean reading. Its not technically lean as you aren't spraying fuel, its just passing air through the engine. All perfectly normal and nothing to worry about.

Some people set up the tune to forgo the deceleration fuel cut, which makes for alot of flames and backfiring and hinders gas mileage but helps hold down the temps inside the engine itself. That being said, if you aren't in a full race application theres no reason to go without the cut.

Skeese

Cheers Skeese. That makes sense why it goes rich again in neutral as if it's idling.

The car is still on the stock ECU, so I have no choice on the matter either way.

What do you mean? What exactly is happening with the rich at idle neutral deal?

Well just when I shift into neutral, the engine starts firing again is all. The deceleration fuel cut only happens in gear. It just happens to idle around 12.6 or so which is why I say "rich".