Q: Muffler is loud and the van jumps when in idle. When it jumps the muffler makes a puffing noise and smells.

This may be related to a failing catalytic converter or potentially a dirty or failing idle air control valve. The idle air control valve reads the air intake as it enters the motor and relays this information to the computer which makes adjustments to fuel. When this is not working properly, you may experience an intermittent idle problem as you have experienced. This may also cause a rich fuel condition producing a smell of fuel as well.

To avoid any unnecessary repairs, I would recommend having an expert from YourMechanic come to your location to diagnose the smell and noise firsthand. One the issue has been pinpointed, the mechanic can service the catalytic converter or idle air control valve as necessary. Best of luck.