[Superman]

Okay, I'll elaborate. When those trusted mechanics get told "no, it doesn't have an exhaust leak, but it does pop on deceleration," the mechanics scratch their heads and ask "well then how is air getting in there."

You see, when combustion is finished, there is not enough oxygen in those exhaust gasses to support further combustion, even if there is fuel (not unusual for there to be enough fuel left). That is why an exhaust leak always results in popping, and also why a secondary explanation is hard to find.

It also happens to be the reason flames can be seen coming from the pipes of race cars. The fire is not burning in the exhaust pipe, but the mixture leaving the cylinder is pretty rich and hot. It ignites at the end of the tailpipe when it hits the atmosphere.