I'm completely stumped. My 97 LSS SC, w/80,000 miles has been getting 18 mpg, mostly highway driving. Took it to my mechanic to have the ecm scanned and he found no codes and all parameters in tolerance. I changed the tranny Fluid, plugs, PCV, fuel filter and put in a K&N but there was no improvement. The plugs that I took out btw were fine, and the engine starts, idles and accelerates fine. So I knuckled under and took it to the dealer to see if they could fined anything and the tech says everything scanned within tolerance except #4 was missing and the plug wires needed to be changed. He also said that the fuel injectors were aftermarket because the tops are white and GM injectors are black; probably higher flow injectors that the first owner put in for power gains. So I replaced the wires with Delcos and still 18 mpg. My wife was driving this car until three months ago so I don't know when the mpg deteriorated, but I recall a few trips when we first got it where I was getting 30 mpg. Any suggestions?

O2 checked out OK

Here'* an update. I changed the plug wires with Delcos and drove the car for a week. MPG did not improve so I replaced the o2 sensor with a Delco. Prior to replacing the o2, I disconnected the neg battery terminal to reset the computer and reconnected it about 30 minutes later. I don't know if it was disconnected long enough because the radio presets were not erased, however the trip odometer was. The o2 that I took out was chalky white. MPG still the same. Does it take a while for the pcm to relearn before I see an improvement? Did I have it disconnected long enough? Is there a procedured as far as how I'm suppose to drive after resetting the PCM? Also, the engine revs at 2000 @ 65 mph, but I recall seeing posts where members said they rev 1800 @ 70
Please help.

Thanks. Once the AFR re-learns will the MPG improve? What color should the o2 have been and what does the white indicate?