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Will not start in rain

2000 GMC Savanna Van 5.7 gas. Van sits overnight in rain, it will not start until rain ends. Changed vapor canister, fixed for about 3 months, problem has returned, What next?

I would suspect that the insulation on your spark plug wires is cracked and is admitting moisture.

Not starting in rain is usually a problem with the ignition system.

First place I’d look is the distributor cap…then wires and then the coil.

Simple test…when it’s nice and dry outside and the van starts fine…get a spray bottle of water…and start spraying ignition components one at a time…Try to start after you spray each component. The last component you sprayed is the most likely culprit.

On a dry, dark night, you might be able to see some sparks if you lift the hood.

The warm distributor cap is probably absorbing moisture after the engine is shut off and I think the vapor canister is a coincidence.
When it does this again remove the dist. cap and see if it’s wet inside.

The inside of the cap can attract moisture just like the inside of a warm window on a cold, damp day.
You can try spraying the cap down with WD-40 and see what happens. That won’t remove the moisture that’s in there but can prevent it from happening again; at least for a while.

It is not the ignition. It rained last night, the van would not start this morning unti I took off the gas cap. It started 5 minutes later

Okay, I’ll bite.

How do you know it’s not the ignition?

Along with that good question, why did you decide to take off the gas cap?