A customer with a '97 Astro (4.3, AWD) is trying to rule out a stretched timing chain as the cause of some problems: mainly, when placed under load it intermittently stumbles and then will run awful (missing and carrying on) until it's shut off and restarted. Codes: P0337 "CKP sensor A signal low frequency" P0339 "CKP sensor A signal intermittent" P0341 "CMP sensor circuit out of range" Watching the live data as it idled in my shop, ignition timing was running right around 20 while "cam retard" was showing -6 or -7. How exactly do these two numbers correlate? Is the cam retard number comparing crank sensor to cam sensor, saying that it sees the cam (and therefore, distributor shaft) running 6 degrees behind the crank? I had no time to do any physical checks, and the weather was going downhill fast so I didn't take it for a drive at the time - we agreed I'd do some homework on the codes and follow up later. He has changed the cam and crank sensors but says he's not moved or removed the distributor. Are the 4.3 dizzy's prone to excess wear on the driven gear like the Vortec V8's are?