Deceleration Popping / Strange WBO2 Reading

[ZBass28]

I noticed that I was getting some bad popping when I would let off the gas
and coast. I took a good amount out of the VE table in the areas where
necessary in the 1000-3500 rpm range down in the low kPa area. This
seemed to help immensely but I still get some various popping and I think
it's due to the next point I will bring up.

If I am cruising and watching my corrections, they are all under 5% either +/-.
What's strange is that as soon as I let off the throttle the WBO2 registers
15.9:1 AFR and the corrections swing to +24.2% and that's when I get the
popping. My question is what causes the AFR to go to the high 15's there
and what causes the big correction. I can't see how the car could possibly
be going lean at this point as the WB is indicating. Is this something that needs to be
addressed in the deceleration enleanment table?

 

[Red Canyonero]

Last I saw, there were no DE tables available to edit.. unless that has changed in the latest firmware revisions ?? Drove me nuts, not having DE tables to tune.

I've had difficulty with this in the past, usually going too rich and having fuel burning/popping out the exhaust. I find it odd you are going lean.

I'd try changing the VE values in that area back and forth and see what helps or hurts, maybe even go to open loop, so fueling corrections aren't confusing the issue, as there can be a lag effect screwing things up here.

I've also seen when I was trying to tune the VE (DE really) at decel, that the motor would tend to die easily when you leaned it out. I'm surprised with yours reading so lean that it isn't dying... does it feel like the motor decels very fast and wants to die? or is it still coming down slow? If you are burning off a lot of fuel, it'd decel slowly I'd think.

Brian Green
89 TTA

 

[ZBass28]

It decels pretty smoothly and slow, no sign of wanting to die out at all.

This is something I noticed last year when logging at the track too. In between
shifts it would go to 15.9:1 and add fuel (24%) and then back to normal. Seems to
do this just on shifts or coasting when the throttle closes.

 

[Craig Smith]

This is normal. When you slam the throttle blades shut, there is virtually no air going through the motor. The lack of airflow across the sensor causes it to read dead lean, and if the closed loop parameters are stil met under these conditions, the ECU will add all the fuel allowed by the correction limits. A lot of that fuel winds up burning in the pipe due to the lack of air in the motor and the result is the popping you hear.

You can lower the numbers in the VE table in the affected areas and this usually resolves the problem. Just remember that you aren't going to get a reading other than 15.94 on decel, so don't worry about trying to get it richer. You aren't going to hurt anything by letting it go lean on decel. If the wideband adding fuel seems to counter your efforts, you can reduce the positive correction limits in those table rows and that should help a great deal as well.

 

[TurboJim]

Originally posted by Craig Smith
the wideband adding fuel seems to counter your efforts, you can reduce the positive correction limits in those table rows and that should help a great deal as well.

You can set correction based on VE cell? I didnt see that in the software. Might make for some interesting emissions settings...especially for those IM240 challenged.

 

[ZBass28]

Thanks Craig, looks like nothing to worry about then. I did drop the VE numbers
a bit and it helped. I also used the DE table and inserted the demo #'s and it
seemed to make a noticeable difference.

Tom

 

[TURBOZ]

Zbass you can also change the correction limits on the bottom row so it does not dump so much fuel back inn decel.

 

[tazgn]

Craig,
Can I recommend that you put a note in the new FAST manual, that I remember you asking for inputs on, about this. I spent a lot of time trying to get the AFR less than 15.9 on deceleration. I didn't realize that it was normal and nothing you could do about it and it's pretty much a false reading. I don't get the popping (probably because I'm in open loop at idle/coast).

Thanks,
Derrick

 

[Red Canyonero]

I wouldn't say it's 'normal' to have popping at decel. No other vehicles do this when properly tuned. It might be normal for a car running FAST, but I still think something should/could be done about it.

On regular GM ECM's DE stuff they pull pw back to zero (or nearly so) on decel, and have an anti-stall system where fuel is added back in at the lower RPM to keep the motor running. No popping, but 02's do read lean.