I just finished building a 4.3, lunati 270 cam, 10.5:1 flat tops, marine intake, heads -.020, total rebuild, and getting my tune down pat, and im back to MAF, and doing some logging to track spark. I've noticed that every so often, 15 seconds or so, the TQ MGNT spark advance pulls anywhere from -13 to -18 degrees timing from the engine. Happens with foot feed, and cruise control. Im lost as to why it would do this, the graph on the spark advance looks like a heartbeat monitor on flat highway with cruise set at 65 Mph........ stumped.
2002 sonoma. 4.3 auto.