This morning, I took my 420 through NJ's emissions test. Judging by the way it was running (slight sputtering, exhaust had an odor to it, but not of rotten eggs), I had a feeling it was going to flunk. As a last ditch effort, I threw in a bottle of 104 Octane boost and a bottle of Heat into the gas tank, opened it up on the highway, and ran it through. These were the results...

IDLE - FAIL

GAS READING STANDARD

HC ppm 1878 220 FAIL
CO% 0.23 1.20 PASS
C02% 7.6
02% 9.5

HIGH IDLE - PASS

HC ppm 217 220 PASS
CO% 0.72 1.20 PASS
C02% 13.1
02% 1.6

I'm almost wondering if the octane boost and dry gas did more harm than good. When I purchased the car, it had a slight hesitation to it when you'd hit the gas, and the idle is ever so slightly on the "sputtery" side. However, it never feels as though it's going to stall out. What should I look for? What should I change? I can't say that I've paid much attention to the gasmileage, as it's not my daily driver. The inspector at the DMV told me to try "the cheapest fix [I] could first", as he's seen people "spend hundreds of dollars, only to still fail". Sage advice.

Any suggestions?