Weird problem with my 5.0 van

Sounds as if you had a lot of condensation in your exhaust. Preceding the rain, the weather was no doubt extra humid. This moisture was pulled into the engine while it ran and exhausted out the back. When you shut the engine down, moisture was trapped in the system. Adding to this, while the exhaust was warm/hot, the gasses in the tube and muffler were expanded and of low density. As the system cooled down, a vacume was created that drew in the moist outside air. This later condensed in the system and when added to the preexisting moisture, created the water you saw blown out when you started the engine. It was rust colored because when water combines with the hydrogen and sulfur in exhaust gasses, it creates an acid that hastens rusting.

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if you need a second opinion, ditto reece... you just never noticed it before.

It is important to note that water vapor is generated from the combustion process when hydrocarbons are burned. You may see lots of water after starting the engine when the exhaust pipes are cold causing water to condense. John

I'll third that opinion. Just a personal observation on my part, I've noticed more of this on vehicles with aftermarket mufflers. The OEM mufflers have drain holes that will allow condensed water to run out as it forms whereas many aftermarket mufflers don't. The water accumulates and blows out of the pipe on startup. Tom

My van has an aftermarket muffler.