Timing adjustment for a '92 3.1 Pontiac Trans Sport

[Meridivs]

We have a 92 Pontiac Trans Sport SE with the 3.1 TBI engine in it.

I need to know how to set the timing on this thing since it has the computer controlled advance. Any help is greatly appreciated!

[LMP]

Well in fact, since it uses a rotary distributor, there is SOME means of adjusting timing there. Here is what the manual says:

ENGINE TIMING CHECK -3.1L (LG6) Only (part of scheduled maintenance services)
Adjust timing to underhood label specifications.

ANd in ENgine Electrical, I have:

Ignition Timing
Timing specifications for the 3.1L engine are listed on the Vehicle Emission COntrol Information label in the engine compartment. WHen using a timing light, connect an adapter between the #1 spark plug and the #1 spark plug wire, or use an inductive pickup.

then...
DIstributor Ignition(DI) 3.1L engine
....All spark timing changes in the EST distributor are done electronically by the ECM which monitors information from various engine sensors, computes the desired spark timing and signals the distributor to change the timing accordingly. A back-up spark advance system is incorporated to signal the ignition module in case of ECM failure. No vacuum or mechanical advance is used. .....

.....that's it....period....nothing more....

The "changes" mentioned above are, in my opinion, relative to the initial mechanical position/timing of the distributor....I understand that normal basic timing practice and tools must be used using the label data, ...expecting the label is there. NO example in the book....

Of course, knock sensor will retard timing if ping is heard by the sensor, and a nominal retard status is applied if the knock sensor is disconneceted or found faulty.

FInally, a general tone in various forums sets the base timing at 10 degres BTDC...here is an example :

First off, you need to get a timing light. You will have to hook it up to the number one cylinder plug wire (next to alternator) and the battery. Disconnect the connector for the EST (tan wire with black tracer) that comes out of the main harness by the right strut tower. The timing tag that you point the light at is on the left side of the engine and turn the distributor till it lines up with the 10° mark. Tighten the distributor and recheck, turn off the car and reconnect the EST connector and call it a day.

[Meridivs]

Thank you LMP! I will try to find a qualified mechanic to apply this procedure (I also have the GM Service Manual but it doesn't say much about timing adjustment).

The need for such a job appeared after I let a mechanic "play" with its distributor - after that the car ran fine for a day then it got even worse than before the adjustment: stalling, idle rpm "hunting", even dies at stop lights which never happened before this intervention.

Now I only hope we can restore it to a setup as close to the manufacturer's spec as possible while trying to cure that annoying rough idle once and for all (already changed EGR, ICM, distributor cap&rotor, spark plugs and wires and, recently, the MAP sensor with no effect whatsoever).

[Meridivs]

Thank you for the useful answers!

This weekend we completed the adjustment by the book - now the engine runs absolutely great

There's something new now:
- from time to time the idle rpm drops (usually at a stop light) and the engine struggles to stay running - usually after the engine reached the operating temperature
- most of the time the rpm stays where it should be and the engine pulls great at higher rpm's

Do you know what would cause this issue (note that most of the time it idles good but once in a while it runs really rough/struggles to keep it running and starts to run really rich)?