Did a Google search for "Negative Timing 1996" and "Negative Timing 1997" and came up with quite a bit. What you are observing is nothing new. In particular, pay attention to post 3 in the first link, and post 4 in the last link (below). If Graveyard has the same negative readings as you, I'm going to say it is normal for (at least some) 1996 and 1997 PCMs, Hopefully we will soon be able to put the timing issue to bed and get on with diagnosis of your P0171 and P0174!

Torque Lite advance timing issue or it could be my truck. | GMT400 - The Ultimate 88-98 GM Truck Forum

ignition timing always reads retarded on scanner? - SaturnFans.com Forums

https://www.gmforum.com/performance-...ce-wtf-244683/

https://ls1tech.com/forums/lt1-lt4-m...what-hell.html

https://www.hptuners.com/forum/showt...ading-negative
post#4

Here's a nice primer I found on Fuel Trim diagnosis. Sounds like you have been through a lot of this stuff before, but I'll post it since it seems very nice and others may benefit as well.

 

I think I got to the bottom of the negative timing issue. From what I can tell, the timing is not actually negative, but it is being read by OBDII scanners as negative by their software. I reach this conclusion using the generic OBDII pid an the GM pid for timing advance. The generic pid in DashCommand calls it "timing advance" and the gm pid calls it "spark advance." And what I think it is is that the GM pid takes the data and makes it into a positive number on its display. I've got gauges for both on the same page in DashCommand and they're always the inverse of the other. The generic is always negative and the GM is always positive. So, I'm thinking that in this first year of OBDII, GM was doing its own thing and that it was so short lived that no generic OBDII scanner inverts the numbers. Crazy, but I think the timing issue was that simple.

I've made no progress on the fuel trim. I watched my IAC valve with the key on engine off and saw it go in and out, but never fully seat. I don't know if it is supposed to fully seat, however. Trims with the junkyard IAC are different than the noisy one that was on the truck -- not better, but different -- so it may have something to do with the problem. I ordered a new IAC just to rule it out.

Edit: Also, on DashCommand, my vehicle needed to be input as a 1996 Chevrolet S10 Pickup 4.3L V6 L35. I had it input as a LF6 and none of the GM PIDS would work. I had contacted DashCommand and they wanted me to buy a new bluetooth adapter, but obviously that wasn't the issue. Just putting this out there for future readers as it was not obvious to me what the problem was with not being able to use the PIDS. the L35 thing has something to do with the head design, but it must carry over into the ECU as well.

 

Great, we are finally on the same page with the ignition timing.

Note that the GM PID and the generic PID in Torque are probably both negative. Probably the GM PID used in Torque is for the later vehicles, while Dash command actually gives you the GM PID for the 1996.

Next time the SES light turns on, read the freeze frame and save the data for what the vehicle was doing when the codes were set, before clearing them again. Looks like you're going to have to try to catch it in the act of misbehaving. That should get you to focus on the STFT's when the vehicle is actually having a problem. I'm kind of wondering if you still don't have a fuel supply problem, even with a new fuel pump - or maybe an EGR problem. As far as the IAC is concerned, once you are off of idle I believe the IAC retracts and just sits there waiting for the RPMs to come back into a range where it will control again. If your problem is happening going down the highway and the counts at idle are normal, I would think that would eliminate the IAC. I think I described the IAC counts on mine at both idle and at elevated engine speed in a previous post in this thread. How does yours compare?

 

Okay, sorry it took so long but I hope this is what you were wanting...
600 rpm
183 water temp...not completely warmed up
cam retard is -.9 degrees
spark advance is 19.56 to 20.94
timing shows -20 degrees to -21 degrees
Hope this helps...
Later

 

Thanks!!

 

I finally got the new spider and time to install it. It looks like the old CPI spider was the problem. Fuel trims with the updated spider are near zero and don't go over 5%. Interestingly, they are on the negative side, whereas the old CPI spider resulted in fuel trims on the positive side. I only have a few drive cycles on the new spider, so I suppose the numbers are likely to change, but they look good at this point.