'97 Astro Random Mis-fire/when humid

I have read post's on this before and have not fixed yet,
110k on van,Mis-fires(0300 code) when humid or drive thru fog, much worse if raining. Problem has existed for some time now and I am hesitant to just replace parts one at time to fix. :banghead:

Van runs flawlessly when dry weather, gas mileage not affected, ever, only get a MIL if raining hard!

On a COLD Rainny day it runs fine?

However,the following has not solved problem: Cap + rotor fairly new and cleaned, New plugs(old ones were normal upon removal), new plug wires installed (they were ready), cleaned EGR valve, cleaned throttle body,cleaned Idle Air Valve, Ignition Coil has no obvious cracks.

Nothing scans(ACTRON CP9145) unusual with MAF,MAP or HO2's

Also if I pick a dry day and all is running well spraying the ignition system down with water will NOT reproduce mis-fire :screwy:

I forgot to add that mis-fire only occurs just off closed throttle,maybe 5% open only,even at highway speed

Have you checked the spark wire from the coil to the distributor, that wire runs close to the tube that allows the dip stick down to the transmission. May have some moisture making things happen there on rainy days. Because it is a random misfire, and not specific code for a cylinder, the spark must be lost before the cap. Just a thought.

Coil wire shorting has beed in TSB's for Astros,
the wires were changed as first step to solve mis-fire. The ignition wire from coil to cap was definitley "done" and may even have shown signs of arcing. However as stated on a dry day when all is running perfect I can wash the coil, plugs, cap ands wires down with water and it doesn't reproduce the mis-fire??

I had exactly the same problem with a different vehcile. It would misfire when it would drive in damp weather and warm. It eventutally go to the point where it would just decided to not even start up or as I would drive at highway speeds it would misfire for no reason, then run just fine and maybe do it again as i continued down the freeway. Finally it misfired one day on the freeway and never started back up. I went through replacing all cap, rotor, coil, Ignition control module, Wires, you name it. I later found out after looking through a Haynes manual that there is a Pick-up coil under the distributor rotor. It is mounted inside the cap on the top of the shaft. I had to pull the shaft, but the rest of it came out nice and easy following the Haynes manual for distributor removal. The new pickup coil costs about 12 dollars and I replaced it and never had a problem again, then engine seems to run smoother as a result. The way I under stand it is the Damp, warm air causes condensation to form in the cap and on this pickup coil linkage. The damp provides the moisture and the warmth provides the lift it needs to get into the housing. Dont know if this will help but I thought it will at least give you another option troubleshoot.

I think the "pickup" they call a "hall effect" switch in my service manual, sound reasonable, and OK for a $12 repair :grinyes: but why can't I duplicate problem with a garden hose on all ignition parts?? :screwy:

Perhaps maybe since the switch is so well enclosed but not airtight, the water may not be able to get directly to the switch. I observed in the switch I pulled out, the corroision that had formed on the switch appeared to short out the switch once it had asorbed enough moisture from the atmosphere around it. Eventually the corrosion ate through the lead wires to the switch and bang the truck stopped running and would not start until replaced.

Well now I have the same problem with my 97 Astro been running good all summer and last week we had a rain storm. boom back to the miss fire I proble drove 10000 mles this summer with out a miss hope someone out there can help I am going to look at the crank triger next becouse it is down there where it can get wet easy I had a Villager I was working on a short time ago and it had a miss that got so bad it would quit running it was the crank triger maybe I will get lucky on the parts replacing soon Thank Randy