I'm trying to troubleshoot an HC emissions failure in a 94 Chevy Suburban 5 .7 liter with TBI and one heated O2 sensor. Over several minutes idling inpark the long term fuel trim will creep down to around 100 (with 128 being stoichiometric). So this works out to like -22% or so if I'm reading it r ight, cutting fuel to correct a rich condition. But idling in drive it's a lways around 128. Scope traces look almost identical between the 2 cases; 5 seconds or even onger between switching at idle, but switches much faster at higherrpm and while driving. Why would the car run rich in park and OK in drive, both at idle? It's not that big an idle speed difference. O2 s ensor is new; acts same as the 1 week old new one I just replaced.