Distributor

First we will start off by describing the important parts of the distributor.  (That information goes here eventually.  I mean, aren't they all important in their own way)

Look for the following issues:

If you notice white/tan corrosion residue from poor ventilation, go ahead and clean that stuff off, it's conductive.  Could be from engine blowby gasses, could be aluminum oxide,  inspect the distributor for wear as you don't want engine blowby gasses accumulating in the cap.  Also inspect your PCV system as a malfunctioning PCV system can exacerbate the condition.

Moisture Issues:

Common problem for these engines and it could be quite a few things.  This can cause random misfires and even no start conditions esp in high humidity areas(it's amazing how water and water vapor can move)  Signs are rusty screws and other components under the cap, green corrosion on copper components, visible moisture and or white residue or black streaking on the cap when removed, etc.

Normal warming during the day and cooling at night will ALWAYS create some condensation in/around the engine and vehicle, more in humid areas, less in dry areas.  After you get your engine up to operating temperature, 180°f or so things start to dry out, when this is done regularly you have less problems with moisture.

• The already mentioned PCV system needs to be working correctly.

• The AC line that runs directly above the distributor cap can drip moisture directly onto the cap itself.  Ensure there is an insulating sleeve to prevent this from happening.  Depending on the year of your vehicle, you will already have one there as it's a known issue(TSB issued).

• Check for distributor condition, including the bushings and Pancake Distributor Disease.  See above and make sure those vent holes are clear.  Also make sure that the cap is the proper cap for your conditions, the aluminum terminal caps may last longer in these high humidity/corrosive situations.  I know, I know, everyone will say get a brass cap, they are THE BEST.  Just like everything else in life, 'the best' all depends on the situation.

• Make sure the rest of the secondary ignition system is in good shape, to include checking all plugs.  You also want to check all ignition wires for 'firework test(water mist test)' and ohm them out.  When parts of the system are on the very edge of failure, all it takes is a little push and over the edge it goes.

• Now we are getting into the edge cases for when you have already checked and cleared the above conditions: 

•  Pressure check your cooling system for leaks, yes it may seem small, but any added moisture to your crankcase can cause problems, it helps to remember that all of that space is tied together.  You better believe that moisture, like Tammy, has an almost magical way of getting around.  A pressure test will also reveal if you have leaking hoses, which can contribute to added moisture around the engine.

• Have you replaced your lower intake manifold gaskets?  They don't last forever and are generally suspect on ALL of our vans by now if they haven't been replaced.  Usually anything over 100k miles but it can happen with age as well.  It should be pretty obvious from the above pressure test if they are leaking.   Less common would be a problem with the front/rear sealing surfaces that get RTV sealant, but make sure that isn't deteriorated as well.  Even less common, but a possibility is a crack in that area in the lower manifold or block.

• Are you changing your oil at the recommended intervals?  This is not just based on mileage but time as well, and not just for moisture but what does a sitting engine do?  Rust.

• If all of that doesn't work, it's time to build a heated garage for you prized Astro/Safari van or move to the desert ;)

Removing and Installing the Distributor

Seems that there is always some question about the best ways to remove and install the distributor.  It comes up A LOT, especially with replacements due to Pancake Distributor Disease so I will address this here.

1. Mark and remove your spark plug/ignition wires and distributor cap.

2. Ideally you will set your engine at TDC for the compression stroke of the #1 cylinder before you even start.  This is not 100% required, but is the preferred method.  Technically as long as the gears and oil pump drive mesh at the proper location it doesn't matter, however if the engine is at TDC on the compression stroke it makes it a whole lot easier as you can't really mess up the marks.

There are two times that the piston will be at the top of it's stroke, one on the compression stor

3. If the engine is at TDC on compression stroke, the rotor will point to the 6 mark/triangle cast into the distributor housing, and the marks on the timing chain/front engine cover and harmonic balancer will align as well(see pictures below).  Otherwise you will have to mark the position where the rotor is pointing onto the distributor housing.

4. Remove the hold down and remove the distributor, you will notice that the rotor rotates ~42 degrees counterclockwise while you are removing the distributor, make a mark where the rotor points on the distributor housing.  If you were at TDC, you can use the cap mounting tab as a reference, and the rotor should be just clockwise of that mounting tab.  Transfer these marks to the new distributor, if you had it at TDC, you can use the cast in/natural marks instead.  If you are replacing with a new distributor, it's a good idea to check the drive gear dimple, should be relatively close to the rotor, not almost 180° out.

5. Most of the time the oil pump drive will have to be turned just a little bit counterclockwise before installing the distributor to ensure the distributor drive and the oil pump will align as the distributor falls into place(see picture below).  If the distributor isn't easily seated fully, then you remove it again and use your long screwdriver/tool to readjust the oil pump drive and then reinsert the distributor.

6. Once fully seated, double check that your rotor and marks align, if they don't you are probably a tooth or more off. 

7. Reinstall the hold down, cap, ignition/spark plug wires and fire up the engine.

Ignore from here down for now ;)

From:  rhbguy @ https://www.scantool.net/forum/index.php?PHPSESSID=9bi6bpncitmue0uhr1pi07ugi0&topic=10382.0

I may have something for you to try. You need Elm Terminal.

If I recall, CMP Timing is adjusted by the Ignition Distributor and watching the CMP Retard Data on a scantool.  In this case you can use this command with Elm Terminal.  It has been a long time, but I think you may have to disconnect a special connector in order to view the timing or maybe that is on a order system.  Read the CMP Timing procedure on the Service Manual.

ATZ

ATSP2

ATSH6C10F1

Send: 22130101

Receive: 62 13 01 AA BB

Watch AA BB should be near, both should be 0.

Every time you adjust distributor, snap throttle and send command (22130101).  I use my phone ELM Terminal for android for convenience.  

In response to my own post, I have some info that seemed to work for me to read the GM CMPRET angle on late '90's GM V8 engines, as installed in Tahoe's, Yukons, and Suburbans.  Allowing for corrections due to distributor replacement or adjustment to resolve P1345 DTC.     Having OBDWIZ with the ELMSCAN interface, I purchased the Professional Add-On which allows you to configure custom or user defined PID's. In the OBD information column, I used as follows:

OBD Header.....ECM

OBD Mode........22

PID Number......130101

Formula............(B/100)*2.4

Receive offset...1

Priority...............Low.

Clicking the 'Test' button will give a reading of the Camshaft Offset Angle.  Having read the CMP offset with a CRP123 scan tool in a couple different distributor positions and then comparing with the OBDWIZ program, it showed the results to be very close to the actual scanner readings.  I only made a couple comparisons but, it seems, you can read the CMPRET with OBDWIZ and maybe save a trip to the dealer to have it scoped out. I couldn't paste the screenshot for some reason but can send the clip file by email for anyone who wants to see it.