Our van has just recently begun to miss under load when it is raining. What's puzzling is that there are no codes. The plugs and Belden wires have about 35,000 miles on them so I guess the wires can't be automatically eliminated but are there any other areas I should be looking at?

 

I'm continually baffled as to why so many out there think you install part X, part X simply can't fail early in it's life. Good to see you're not in that camp. Certainly less likely to fail but far from bullet proof especially at 35,000 miles. That's far from "new".

Wait till it's dark. Grab a spray bottle and fill it with water. Start the engine and spray water on the plug wires/coil (don't drown them). If you see arcing start with new plug wires.

Another thought is the MAP sensor. It's purpose in life is to adjust the fuel air mixture when the engine is under load. Not sure how you could prove it's the problem with your symptoms. For now I would just make sure it's connected securely and the contacts are clean and undamaged. It's mounted on the intake manifold.

No codes? The OBDII fault monitoring system is far from fool proof. Components regularly fail without throwing a code.

 

Deleted duplicate post - again.

 

Interesting comment on the MAP sensor because I just removed the wiper tub when I was working on the struts. When I started the van, it didn't want to idle and in checking around, I found that the MAP sensor connector was off. It immediately threw a code for the sensor but that cleared after a few days.
I'll have to check how tightly the connector is on there.