Hey Tony, I too have a S-series (the GMC one) with 5spd and 4.3, and have tried to do what I could to up the fuel economy. They have batted around this topic a good bit on the site, s10-4x4.com, and their suggestions included, vacuum leak, carbon-caked egr valve, o2 sensor, pcv valve, emissions canister (!). Having said that, I don't know that any of these would account for a sudden drop in mileage ... you know tons more about this stuff that I do, but I'm just pointing to that site. Don't suppose you've noticed that its trying to rev higher, as in throttle position sensor?
