Intake backfire/pop CRUISING. vortec/tbi $0d 5.7

[helo]

About every drive lately I get occasional intake pops under steady cruising. Only happens when fully warmed up (CTS 200-205*F). Somewhat sporatic behavior, not really predictable. EGR is disabled.

Think it might be my fuel getting too hot prior to TB due to lack of return line? My regulator is back by the tank... I doubt this, but I can't deny it's possible.

Timing? I isolated 1 pop in a datalog, 17* advance, 2100rpm 70map (~65mph), ~2.3msec bpw, 32%tps (everything steady before & after). o2 voltage took a huge nose dive for 14 data samples.

Is 17* too far retarded? I'm not sure how this would cause an intake backfire, but lately I have been backing off the SA more than I probably needed to. Vortec heads (stock '96 motor, TBI).

[1.721afr]

I would check for lean conditions, does your $0d have hwy mode enabled?

[helo]

No, Open Loop AFR Enable is not set; I assume that's what you mean by hwy mode (since "hwy mode" is 7747/$42). Link: Lean Cruise Mode (HaulinA$$ @TGO) (http://www.thirdgen.org/techboard/posts/diy-prom/270266-tbi-guys-its-time/3132488-post258.html)

I'm sure it's not commanding lean because the bpw stays nearly constant above 2msec, which is a lot of fuel (@30psi, stock 5.7 injectors). My only indication of improper combustion is the stumble/pop and the nb o2 sensor swings lean for almost a full second.

On the other hand, I thought the PCM is supposed to spike the bpw when the o2 goes lean?? I'm nowhere near PE. Perhaps there are settings for this I haven't learned about yet. I'm aware of the o2 windows, but I haven't modified any of that stuff yet.

[95nProgress]

17º seems a little low to me. From my experiences intake backfires can be from too little timing. Are you sure it is 17º or is it 17º plus the spark bias (~9.8º for a total of 26.8º)? May want to get the timing light out to verify. If it is 17º and at cruise you could probably safely add 4º at that area of the spark map and see if it goes away. Additionally, since it is at cruise you might as well take the time to find out what it really wants at this area of the spark map. Find a consistent patch of road that you can go at a constant speed resulting in this area of the map, play with different amounts of timing ~3º or 4º at at time and see what allows the least amount of TPS for the same speed. The amount of timing that allows the least amount of TPS for the same speed is your sweet spot. Be sure to ramp up on it and not go past.

HTH

Brian
BHall71 @ Thirdgen

[helo]

Alright, thanks. I have SA dialed back at the moment from 65kpa up. I didn't realize being only a few degrees less advanced would be so adverse. I never knew what I was missing running the stock tbi tables on my vortec heads...

It's been tough trying to hammer down my sweet spot for SA. 5* more felt really good, but had some knock around 90kpa that was causing 7* retard. Even more SA triggered audible pinging above 85kpa going up hills.


Find a consistent patch of road that you can go at a constant speed resulting in this area of the map:lol:
I don't have any flat roads within an hour of where I live to get consistent data =). I'm pretty sure I can get it dialed in a lot better by feel, though.

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Update: Ultimately I believe this issue was a result of PE not coming on soon enough coupled with lame timing. Adjusting SA to be a bit more aggressive gave me phenomenal part-throttle torque improvement, and now I'm lowering my PE-TPS table to bring on more fuel when I need it based on logs. I figure if I cruise up slight hills alright at 25-30%, then 30%-35% is reasonable for PE entry.