Re: Just wondering
Because of E10/E15 mandated gas which induces engine knock and yanked timing GM biased the tune RICH by about -7% as E10 alone leans up AFR about 7-8%
Add higher elevation AFR would even be richer.
Now use Non Ethanol gas, losing the leanness of E10 and you end up too rich
Add colder weather and more richer on cold starts but as OP stated using a scanner the fuel trims (non WOT) are way too rich and that is a stock Z06 with a 3rd party air-filter adds to super rich on cold startup which is the OP complaint
In the end engine running for some time with dirty air-filter, leaking airbridge, 17-20 % too rich would end up with lots of carbon buildup and also effecting cold starts.
Going to E10 will help reduce that too rich AFR at that elevation and by use of scanner know what AFR is on colder days, cold start and what AFR is on hot summer days using non Ethanol fuel.
Mike M - 2001 / 2008 Z06s