97 Astro misfires but no OBD codes

I own a 97 Astro with 105,000 miles on the clock. It's been maintained with AC Delco parts only. Recently it started idling very rough and seems to run rough in cruise. Cold starting, acceleration and fuel consumption OK. It only happens when driving on very wet roads, no engine light comes on and no fault codes are stored in memory. Problem goes away as soon as road dries! It could be a number of things, where should I start?
Note: Except for plugs, wires, rotor, distributor cap and fuel filter everything else is original. Vacuum hoses & PCV checked OK. EGR valve will sometimes trigger a P1406 code.

My neighbours' astro had the same problem, turned out to be the coil assembly.

I had the same trouble, I cleaned and oiled the stem of the EGR, Cleaned or replaced distributor cap and rotor and electronic module (modules's two connectors to sensor pick up coil were very corroded, since then I clean them every 6000 miles) replaced spark plugs and Delco wires two of them were defective; open resistance infinity, and I had to replace them this time in pep boys Delco is a good brand made in china but there are better long lasting parts in Pep boys", also I found in the Vaccum system a Tee conector cracked and britle, I replaced as many vaccum hoses as I could as some of them are impossible to reach.
also with an allen key Y accelerated to 750 RPM in iddle. I have been driving for 24,000 miles with no probl;ems.

only in the wet leads to wires or cap suspect

Thanks guys for all those pointers. Next week I'm planning to remove the cover and I'll check all.

Well, I'm running out of options. I've sprayed the entire ignition system with water and the engine ran just fine. I've also sprayed the crank sensor and nothing.

I'm starting to believe it was a coincidence that the two instances happen on wet roads. Took a look at the EGR to verify that pintle operation is free - nothing obvious there.

Anymore pointers?

Make sure your spark plug wires aren't arcing against the aluminium heat sheild on the inside of the dog house, wires should be tied back. Wire from your distributor to your coil should run down and around rather then up and over. There is an actual service bulletin for the running of the coil wire.

Read this:

http://www.obdii.com/ubb/Forum1/HTML/001357.html

Thanks CD for the link. I can't find the previous post referenced in that post to find the GM service Bulletin. Does anyone have it?

I'm removing the dog house again today to inspect coil & plug wires again for evidence of arcing as suggested.

Checked coil & plug wiring routing and also for evidence of arcing with dog house but nothing!

Later yesterday afternoon on a short trip on dry road, the engine light came on for about 4 or 5 minutes. During that period idling was very rough. Hook-up scanner but no codes stored in memory! Engine running fine for now.

A couple of months ago I got a P1406 code which translate to the "EGR pintle run out" Would you invest in a new EGR or could this code be triggered by another component?

EGR could be the cuplrit. I would go buy a new gasket with a screen for it first. Take the current one off and clean the passage as best you can. I used a screw driver and a shop vac to work the big pieces of carbon loose and suck them out.

Oh, also got to the local GM dealer and buy a can of GM Top Engine Cleaner. Get the kind in the aerosol can. It's fairly new so they might not have it. Follow the instructions on the can. It will help work some of the carbon loose.

If you can't find that. Go to the local Dodge dealer and get a can of Mopar Combustion Chamber Conditioner. Same basic stuff. It works good too.