I've read most of the running rich posts and didn't find this. 2002 Z28, Service Engine Soon light is on. The 02 sensors are fine, had the MAP sensor replaced and now replaced the mass air flow sensor. Same deal. It's running rich about 15 on the scope. It runs fine, doesn't smoke, gas mileage around 20-21mpg combined.....anyone had this problem or have any ideas? Thanks.\BluewaterBill

 

Clogged cats perhaps.

 

What do you mean by "about 15 on the scope"? Do you mean the A/F ratio is about 15:1? If so, that's not rich. Its very close to the stoichiometric A/F ratio of 14.7:1, which is where its supposed to be in closed loop (idle, part throttle/moderate loads). If it's 15:1 at WOT it's running lean.

It is possible for the engine to be running at the correct closed loop A/F ratio, and still set the "rich" codes. The PCM uses the long term fuel corrections to keep the engine operating at 14.7:1. The codes mean the engine would be running rich, IF THE LONG TERM CORRECTIONS WEREN'T BEING USED. The long term fuel correction are limited as to how much fuel they can pull out to correct a rich condition. When the corrections approach their limits, the codes are set.

Have you checked the fuel pressure?