The gas mileage for my 1998 Olds 88 has decreased from about 30 hwy to 23 over the past year or so . The vehicle has 71K miles ( even now gets light use). The car got a new fuel rail and FPR under recall a couple of years ago , has new plugs, air filters and oil is changed 2X year (less than 7k mile per year) . I have been told that it is very common to have a "lazy" oxygen ( O2 ) sensor ($55 ) and /or "lazy" mass air flow (MAF) sensor ( $150 rebuilt ) that will cause mileage drop but not trigger codes . Is this correct ? Any suggestions on what to do next ? - Thanks

Do you have the ability to use a scanner? You could watch the O2 toggle and determine if it is getting lazy.

On the MAF, I would suggest some maf cleaner and not replacing it.

Thanks - I hope to get with a friend who has a scan tool or get an ELM327 scan tool on fleabay (only about
$ 35 and I have a laptop) . So ,I guess the scan tool won't tell me anything a bout the MAF . Is it OK to clean the MAF sensor with spray carb cleaner ?

I would use MAF cleaner..don't chance the carb stuff. It'* not intended for maf'*.

On my 96 LSS, the gas mileage slowly decreased and the engine began to studder after shifting into OD. This happened over a span of about 1 year, with the symptoms getting worse the more I drove it. Finally last month my service engine light illuminated, I don't recall the code, but it pointed towards the EGR valve being beyond its limitations. I picked one up from the junkyard for 20$ and that took care of it!
Good luck!
jj

If your millage is going down then I suggest changing your O2 sensor. Most of the time that will get most if not all of your millage back.