it started in 1996 and in 1999 they went to a differnt design that doesnt use a distributor. But yes the 4.3L V-6 5.0L V-8 and the 5.7L V-8 have to be timed this way.

 

It started with computers! The OBD I system that came in the 86 S-10, and others, still allowed you to use a timing light BUT you had to disconnect the computer first. You couldn't see the timing mark if you failed to pull the computer's plug...

Then the Feds decided that OBD I was too easy for hackers to over ride and the pushed the auto makers to make things more difficult and OBD II was born.

 

It's just ridiculous! I realize things are supposed to improve through the years, but lemme tell ya, I had a 1990 1500 series Chevy and I could EASILY get 20mpg on the highway, about 15 in the city - give or take. The truck had around 250,000 when I sold it and bout my 97. What a mistake. I should have stuck it out with the little bit of rust the 90 had and one or 2 wiring gremlins vs what I am going through with my 97. We waited about 2 days to get the timing done after the intake and heads were done, etc, and that was the BEST mileage I got, there was a little bit of a spark knock but she started, she ran, just lacked her giddy up. Once it was timed, a tank of gas later, I got 10 MPG!!!!! So, it is going back in to be timed again but that is not right - I was getting at least 14 on the highway. But really, shouldn't it be doing better than what my 90 was? Just silly. I read somewhere that on the 97 you could time it without a scanner by disconnecting the ECM wire, but sounded too risky for me just to wing it!

 

Nope that is why i said the 1996 up had to be timed with a scanner period.. But it sounds like the shop didnt actually get it to 0 degrees TDC. before they used the scanner.
as to the milage, it all depends on the differences between the model ie: gear ratio etc etc.. but 14mpg is to low to begin with.