Some cars run rich and idle funny when off, some do not. I tried it on both my cars and they ran fine, mpg did not drop, but the GP would hickup when idling sometimes. The Monte acted the same. I decided to believe in GM engineers and put them both back in.
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03 GT, STB, GM HD sway bars, 35% tint, GE893(37watt)fogs, Flowtech Terminators, 180 T-stat, ZZP DP & PCM, Ceramic PLOG & Crossover, 18" glasspack, 2007 17" Mustang Bullits 235/55/17 Pirelli PZero Nero, 12" R1 D\S rotors, gutted air box, GTP radiator