Driver, yes and no. All that the IMRC does is open the secondaries that is correct... but at the same time, the CEL and improperly working engine component directly related to airflow totaly changes the way that the computer works. It is no longer in closed loop mode, it runs in open loop (default pre-set fuel tables... limp mode) due to the problem. This directly effects the a/f ratio (which the cats are VERY reliant on being in the proper range to operate normally - hence the egg smell) which directly effects the gas milage.
I pinned my secondaries open due to the SEVERE lack of performance. The car was a total slug when they were stuck shut and I couldn't live driving the car that way, so I pinned them open which made my idle a little iratic at times and gave me big time part throttle issues before the engine reached normal operating temp. Once the car reached normal operating temp, the idle and part throttle were fine (although I was still lacking on torque due to them being open and also the car being in open loop and the a/f being way off and robbing me of power down low), but up top was lean as hell and the car would detonate under heavy accelleration at times and would ALWAYS smell like rotten eggs after getting on it or while getting on the freeway. Me pinning the secondaries totaly threw gas milage out of the window because the car wasn't running right.
A dead sign of "bad cats" is that the car doesn't move down the road hardly at all due to being clogged and snuffing the engine. When your exhaust smells wrong one way or another (like rotten eggs or whatever), it simply means that something is not right with the a/f ratio for one reason or another... common problems are vacuum leak, something like a bad injector, so on. Things like that make your exhaust smell bad.
In my case, it happened to be the fact that I pinned my secondaries open and the computer wasn't compinsating the a/f ratio normally anymore and I had all of those problems. That is why it cured so many things for me.
