You could have a catalytic converter that is almost stopped up, even though this is on the exhaust side of the system, could be causing excess pressure on the intake side.
Could someone tell him how to check to see if it's his catalytic converter, I know one way is if it's really bad you will see it cherry red when you look at it. But I think they have some kind of tester for them that tests the temperature of them not sure exactly what it's supposed to be for sure, there maybe some other way.