Poor gas mileage
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?