STFT pegs on coast down cannot figure out why

[Roller75]

Tuning a 14 Caprice, LS2, E38 ECM.

Everything is done and looks good except for some reason I cannot figure out why when driving highway speeds if I let off the throttle the STFT peg to 29.8% and the O2 sensors go pig rich. Closed Loop Active PID switches to no but Fuel System Status 1 and 2 both say CL - Normal. When I force closed loop in the scanner it does the same thing. DFCO is disabled. If I keep my foot on the throttle at around 5% it'll coast down and stay in closed loop. As soon as I completely let off the throttle pedal it switches to open loop and maxes out the STFT and goes rich again.

It's causing the idle to surge on quick decel down to a stop from highway speeds because it's so rich it just blubbers trying to catch the idle.

Attached log files and the tune file. First instance is 2:27 into the log.

Dakota Mixer 2014 Caprice Police 6.0 Drive 13.hpl

Dakota Mixer 2014 Caprice Police 6.0 Tune 23.hpt

Any help is much much appreciated!!!

[PGA2B]

Try these DFCO settings. I know you have it set to 400*, but maybe this might help.

Make it:
Enable ECT 493
Disable ECT 492
Enable RPM 8192
Disable RPM 8191
Disable RPM Rate 1800

[GHuggins]

You can change the speed settings too.

[Roller75]

Do you think the DFCO is the road to go down with it? Only reason I ask is it's odd that not only does it go into open loop but it doesn't zero the injector PW. It just throws STFT at it and goes rich. I normally keep DFCO on when done but I just turned it off to see if that was causing the odd issues. I will definitely try those settings and see if it helps. I'm honestly willing to try anything.....

Thanks guys!

[Roller75]

Also I noticed when it goes into that mode if I pick up the throttle to around 5% it'll stay in the OL rich mode. This can be seen at the 10 minute mark

[rabbs88]

Just asked the same question about a week ago and figured it out on my own. Go to Engine -> Fuel -> Oxygen Sensors -> and disable "LTFT Idle Cells Thresholds"

https://forum.hptuners.com/showthrea...l-in-open-loop

[Roller75]

Awesome, thank you I will give that a shot!

[rabbs88]

Let us know if that works out for you. I'm surprised this issue isn't more commonly known

[Roller75]

Well I disabled that and it seemed better but was still doing some weird stuff specifically when I fully let off of the throttle pedal. It would work correctly at first then slowly get worse. So I started looking at the mechanical side closer and found that the guy that did the work on the motor must have lost the original oil fill cap and took one off of a different LS valve cover and put it in but this car has 2 tabs inside the filler neck and the new cap only has a spot for one so he couldn't get it on all the way and just left it sitting half sideways on the valve cover causing a nice vacuum leak thanks to the PCV system..... I believe that was 90% of my problem. Unfortunately the customer had to have the car back today so I sent it as is and he will be bringing it back in a few weeks for an injector upgrade so I will mess with it more then and report back.

Thanks to everyone that chimed in!!