I had a Vanagon sit for a year,once. When I started it up, the rear bumper
got so hot it could not be touched.The engine sounded and felt like it was
choked up.Maybe a plugged up catalytic converter. The Vanagon had run good
before sitting,though.

I shut it down and was going to drop the muffler next morning.

Next morning the Vanagon ran fine again. No evidence of a  plugged up
whatever. I think what happened was that a mouse built a nest inside the
muffler and the twiggs,grass and such needed to literally burn before the
plug went away.