Re: Thoughts on backfire?
 

The modified PROM is back in the car. Backfire problem is solved. Which leaves me with an entirely new issue...

The fuel that is now being added on deceleration is manifesting itself as a huge puff of gray smoke being blown out of the tailpipes. It is most evident a moment or two after an acceleration run, when I have mostly or completely lifted off the throttle. Can't decide yet which is more embarrassing: the backfiring or the smoke. The challenges of modifying this car are really starting to wear on me.

Re: Thoughts on backfire?
 

You run it to Closed Loop?

Got a data log program?

Re: Thoughts on backfire?
 

DFCO could clear this up, if you get it enabled quickly enough.

Glad you got the backfire fixed, reducing backpressure/scavenging characteristics can affect a lot. Glad Mark got your fueling correct to eliminate the issue.

Re: Thoughts on backfire?
 

Yep, I went out and ran it hard for 15 minutes or so. It was definitely warm. No data logging program, unfortunately. I just emailed Marc to ask whether he just disabled the DFCO, or if he may have also added more fuel on top of that.

Re: Thoughts on backfire?
 

Marc just disabled the DFCO, which is what stopped the backfire and started the gray haze.

Re: Thoughts on backfire?
 

I though DFCO was already disabled on your last backfiring calibration?

Re: Thoughts on backfire?
 

We thought it was. Marc couldn't figure it out. Then I got an email from him on Monday AM telling me that he had been going over my file trying to find an answer, and found that he hadn't disabled it. So I sent it to him and got it back today. Which puts me where I am right now.

Re: Thoughts on backfire?
 

Forgive me, but challenges like this are minor. And they've been solved by any number of others before. I wouldn't get discouraged. You'll be glad you didn't.

Re: Thoughts on backfire?
 

Sorry, I thought that his Anti-backfire changes were seperate from from DFCO.

On my experiences with DFCO. On stock calibrations that a work with, there is a delay timer that starts counting the moment the PCM/ECM detects a closed throttle. On my trucks its set by GM to 4 seconds, so 4 seconds after I let off the throttle, DFCO kicks in cutting the injector pulsewidth essentially cutting fuel to the engine. When DFCO does engage, there is a marked increase the rate of deceleration, compared to when the engine is idling with the throttle closed. I was running LT headers and 4" sidepipes at the time, and after the computer entered DFCO, I could hit throttle and a loud "CRACK" would be heard as the fueling recommenced as DFCO was dropped.

Here are a few DFCO parameters from an LT5 ECM

DFCO RPM Enable Threshold

DFCO RPM Disable Threshold

DFCO MAP Enable Threshold

DFCO MAP Disable Threshold

DFCO RPM Decrease Disable Threshold

DFCO Enable Delay

DFCO Enable TPS Threshold Hysteresis

DFCO Enable Coolant Temp Threshold

DFCO Enable Vehicle Speed Threshold

My question is, can anyone tell us what the DFCO Enable Delay is in a stock calibration?

What I am wondering about is that you were getting a backfire as soon as you depressed the clutch. Unless the DFCO Enable Delay is set to "Zero" seconds, DFCO engagement "shouldnt" be the source of your push clutch then immediate backfire issue. I could see DFCO being an issue during coast down from high rpm, but my suspicion is that the part of the "anti-back fire" calibration mods that you received today were mostly fueling, possibly spark, but not DFCO. I can 100% understand a DFCO engagement causing a crackle, but not as soon as the clutch pedal is pushed.(again unless the DFCO enable is set to zero seconds)

Knowing the stock enable times would help. If mu suspicions are correct and DFCO is not the primary issue here, you should be able to have a non-backfiring ZR-1 that also doesnt spew gray smoke. This would be accomplished by using whatever cal. changes were used that stopped the backfire, in addition to a more thorough calibration of the DFCO settings. This however, may be beyond the capabilities of mail order tuning. DFCO recalibration is very trying at best, as aposed to Enabling or Disabling its function entirely.

On GM trucks with the tow/haul switches, I can take have the truck shift normally when the tow/haul button is off. But when you press the T/H button, I change a whole slew of ECM/TCM tables and make the same truck accelerate harder, shift harder(bark tires if that is desired) all with the flick of the T/H button. Many tuners, call this collective group of numerous transmission parameter changes their "Tow/Haul *** Mod", rather than describe each and every change that was made to the entire calibration. My point here is that it is much easier to decsribe a group of calibration changes as "Tow/Haul ***" or "Anti-BAckfire" modes to a customer, than it is to list the 10's or even hundreds of changes you made, esp. when it really wont make a difference.
This may, or may not be the case with Mr Haibeck's marketing of his excellent ECM calibrations and the hard work and dedication he puts into all the work he does for his customers.. But if I were in his shoes, it's what I would do. It simply keeps phonetime, and internet time to a minimum. It would be different if these professionals got paid for their question/answer sessions, but usually they dont. (eg. It almost took me an hour to formulate and type this post, but theres a difference between business and pleasure)

Sorry for the book, but its a complicated issue IMO.

Re: Thoughts on backfire?
 

Stock is 250ms for Delay, 3 sec for a repeat. But Hog, DFCO has been disabled. So now there's fuel continuing to flow even while throttle is closed. Don't forget that when you hit the throttle, you are invoking Acceleration Enrichment. Too big a pump shot and...