Set timing on a S5 na
 

The '91 fsm says to ground the diagnostic port before checking / setting the timing. Haynes manual has no mention of grounding the port. On my '91 it makes several degrees difference. This is at idle of 750 - 800 rpm.
No ground on the diag port, my timing light shows the pulley mark clearly and it stays stationary.
With port grounded, my timing light shows the mark on the pulley several degrees over but it starts skipping back and forth about half the time in a 5 degree range.
Any input from those familiar with S5 timing?
Thanks.

the FSM method is the correct one. its been my experience though that if the engine is under 1100rpm, and the TPS isn't way off, the timing is the same with or without the thing grounded.

are you sure you're grounding the right plug? single pin green one by the battery.

Yes, same plug you ground to pull engine codes.