Rich on decceleration

**MTT700** · 09-01-2010, 12:59 PM

Stock 2002 LS1, drive-by-cable.

O2 is fine in open and closed loop, idle and acceleration, but on decceleration (letting off the gas) the O2 senses super richness and I can't get through emissions .
Any ideas or pointers would be very much appreciated.

**pajeff02** · 09-01-2010, 02:38 PM

What are your mods?

**MTT700** · 09-01-2010, 08:49 PM

No mods although this drivetrain (LS1 + 4L60E) are no longer in the Firebird.
ECU has been updated only for rear tire circumference. All emissions intact per factory.

**0rion** · 09-01-2010, 09:38 PM

strange....the ecu with stock programming should go into DFCO and cut fueling during decel. Is that what they told you when they tested it or are you going by a wideband reading? Sure the stock ecu has the stock tune on it other than the tire?

**MTT700** · 09-02-2010, 05:06 AM

Thanks for this, I wil check. I had forgotten to mention that the exhaust I am using is very free-flowing. I am guessing that this makes a difference.
I just need to know how to tune his problem out if at all possible.

**0rion** · 09-02-2010, 12:19 PM

Originally Posted by MTT700

Thanks for this, I wil check. I had forgotten to mention that the exhaust I am using is very free-flowing. I am guessing that this makes a difference.
I just need to know how to tune his problem out if at all possible.

exhaust won't matter. On decel the pcm goes into Decel Fuel Cut Off or DFCO.......shouldn't be fueling at all on decel. There may be some exceptions. The person you should PM and ask is Frost.....he will know for sure when DFCO is activated.

**mxlplx71** · 09-02-2010, 12:47 PM

would a fouled plug do this?

**Cutlass** · 09-02-2010, 06:34 PM

Originally Posted by 0rion

exhaust won't matter. On decel the pcm goes into Decel Fuel Cut Off or DFCO.......shouldn't be fueling at all on decel. There may be some exceptions. The person you should PM and ask is Frost.....he will know for sure when DFCO is activated.

here's the definition from the service manual

Deceleration Mode
When the driver releases the accelerator pedal, air flow into the engine is reduced. The PCM looks at the corresponding changes in throttle position, manifold air pressure and mass air flow. The PCM shuts OFF fuel completely if the deceleration is very rapid, or for long periods, such as long closed throttle coast-down. The fuel shuts OFF in order to protect the warm-up three-way catalytic converters.