"From what I can find you can't adjust the timing. Is there some type of vacuum advance? Or is the ignition timing determined by the ECM?"
The ignition timing is normally controlled by the ECM. The "vacuum" signal comes to the ECM via the MAP sensor.
I said "normally", because when you crank the engine, but it hasn't started to run yet, the timing is controlled by the ignition module. The switchover point is about 400 RPM. If the ECM detects the engine speed is more than that, it asks the ignition module to hand over the control to the ECM. The ignition module responds by changing the voltage on one of those control signals, so the ECM knows its request was heard, and allows the ECM to control the advance. If the ECM does not detect this voltage change when it expects one, it sets code 42.
On the DIS system, you cannot set the base timing by rotating the distributor because there is no distributor. You cannot set it, period. My scan tool (Diacom) displays wild advances like 30 degrees or 50 degrees. I have been told that these numbers have some magical offset included in them, and this offset is supposed to be printed on the VECI label. No matter how much I study my VECI label, I cannot find it there, it only says no adjustments can be made.
From: Jukka Alve