The problems I'm having with my truck on 1995 Isuzu Pickup

The truck backfires when let up off the gas?
Motor run up and down when idle.
Foul smell coming from tail pipe?

Asked by for the 1995 Isuzu Pickup
Back firing in the exhaust on over run sounds like excess fuel being present in the exhaust. If there is unburned fuel in the exhaust then the catalytic converter works exceptionally hard causing it to smell badly and will eventually damage the catalytic converter. Have the Pickup checked out as soon as possible.