Rich decel

[SSpdDmon]

I got to thinking about why the car goes rich during decel. This one does it, my last one did it, and if you're reading this, yours probably does it too. But why? I've tried tackling this issue thinking the bigger injectors I have just couldn't perform well at such low pulse widths. I tried adjusting the voltage correction table, the short pulse adder table, etc. Nothing seemed to really fix the issue. Besides, the injectors will perform a certain way given voltage, pressure, and vacuum. And that's when it hit me....vacuum!!! Look at the injector tables. They stop at 80kPa. That means, whatever you have in that 80kPa cell is what the PCM assumes is true from that point on - which is not true on unreferenced fuel systems that are designed to maintain a steady AFR. In other words, if you hit 90kPa of manifold vacuum during decel, the PCM commands a pulse width based on 80kPa of manifold vacuum and you go rich.

How do we fix this? Well, I'm working on that. My initial/current thought is, how do we prevent the car from going into scenarios where the manifold vacuum exceeds 80kPa???? Since the car is rolling and you're off the throttle during decel....that leaves one obvious place to start. Throttle cracker. I set up a map to log mph and rpm like the throttle cracker table with manvac as the data point. Then, I started looking for where manvac exceeded 80kPa. Bump up the throttle cracker in those spots aiming for 80kPa as a target and guess what.... ...yup, much less pig-rich decel situations.

Granted, I'm still figuring this out and you can only go so far with the throttle cracker before the idle RPMs want to run away from you. But, it's a start in the right direction that I thought I'd share. I figure I play with the decay rates a little more and make a few tweaks here & there....I may just crack this puppy.

[joecar]

Jeff, good find...

[405HP_Z06]

Where does deep decel mode fit into this? Are there some other tables we don't have access to that control this?

[white01ss]

I know this is an old post, but I've been trying to lean up my cars decel as well. It would usually run about 9-11 AFR on decel and changing the VE never seem to make any difference.

The entire tire I've been on the custom CD OS and have been on closed loop in the vacuum cell. For grins I set all the columns 25 Kpa and lower to 15.0 to take it out of closed loop. Whal-la! It worked! When I let off the gas it would take a few seconds then I would hear a pop sound and my AFR gauge would spike lean. Now it's just a matter of tweaking the high vacuum VE cells so it doesn't lean out so much.

Dan