Join Date: Mar 2002
Location: Austin, TX, USA
Re: Erratic RPM readings on 4l80e
It's possible the ECM gives your diag tool priority and the instruments have to sleep between data packets. Modern day instruments are often driven by the ECM. If the ECM's busy, it may presume you can live without knowing RPMs accuratly.
Did it not do it with the old coil?
68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI