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I have a 94 S10 ,4.3 V-6 5 spd as a regular driver.I have owned this truck since 1996.Has 100k miles .When driven on the freeway at reasonable speeds, the truck gets 27 mpg, day in day out. Lately the gas mileage has dropped to 25 mpg.I checked the throttle body injection fuel pressure,no codes stored in the computer and a vacuum guage says everything is fine with the engine and no clogged converter.The engine runs just the same as always, same power etc.The mileage drop has been sudden, not a gradual thing.I Buy fuel at the same Sunoco station down the road. Maybe their fuel formula has changed?

Winterized fuel just show up? If you keep records - check last year for a similar drop. Did the regional Air Nazis dictate a change in formulation recently? Effective 1-1-02?

Round here they use methonol mixed with the gas in winter.It's supposed to keep emissions down or something, all I know is it takes 1.5 times as much gas to go same distance as before. Price doesn't change though! As for your problem, if it was sudden like you say and everything else is fine, I'd say look for a leak.

Actually I do check regular.The normal winter drop in mileage is about 2 mpg.This has happened in the warm weather we have had up till yesterday.No alcohol in the fuel that I know of.Maybe they switched to winter blend early??

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?

I know the carbon canister isn't monitered by the computer so a problem there won't set a fault code.I'll give it some thought and check the mileage again

Tony,

The 4.3 intake plenum and combustion chambers are notorious for filling up with carbon if you don't drive them hard and run some fuel system cleaner through them every once in awhile. The Chevy dealer wants $800 for an EGR valve change and intake manifold boil out. There is also a "new" EPROM that the dealer will put in some rigs with that motor that is supposed to help out with the carbon build up. This has been a problem with our 95 Astro AWD . . . Lisa just putts around in it too much. Usually you will get a sudden drop in milage along with a code that will tell you the egr is stuck open. Heck, just go buy a new one . . . the economy could use a few folks splurging on a few new cars.

are your front brake calipers dragging at all? Just a thought

The Egr seems fine, no code displayed. If you apply vacuum to the Egr while the engine is at idle, it stumbled,so it works.No dragging brakes either.Buy a new truck? I'm too cheap.Besides money for a new truck would cut into funds for more old ones.I read that you should replace the O2 sensor occasionally ,even if the computer says all is ok.Looking back a few months, I replaced the plugs.This time I used AC plugs.Before that I had used NGK since I got the truck.I don't believe the same heat range in a different brand would make any difference, but who knows?

Does the engine run really smooth?
I had a truck that ran great except under power, the plug wires were breaking down under a load(?).I replaced the wires and problem solved.
How many miles are on the truck?
I would not think fuel wold cause such a drop in mileage but I guess anything is possible.Maybe swap motors out of your 51 !!!!!

There are a couple of things you can do to "tune-up" an efi, even if there are no codes set. With the throttle closed, measure the voltage on the TPS. To really run it's best, a GM EFI will need that TPS to register .5 volts. If it isn't, adjust it until it is. If your TPS isn't adjustable, modify it until it is. This is really critical - you wouldn't think so, but it is. Also check WOT setting to make sure it is around 4.5-5.0 volts. If it is outside that range when adjusted, replace it.
Next, your manual should have a procedure to reset the IAC valve. If the IAC valve is a little off, it can cause problems with driveability. Check your MAP valve with key on/not running, and with engine idling. Make sure it is within manual specs. A good MAP valve will even compensate for a vacuum leak, so make sure it's good.

These are just the main things to check for. Hopefully that will cure it.

I bought a scan tool last year, and it has helped me to straighten out some cars that wouldn't set any codes, but that didn't run very well. Knowing things that make a EFI car run poorly and being able to check them on the fly made all the difference in the world. If you can get your hands on a scantool (a real one, not those jumper wire code readers) Those Turbo Buick guys have a website that helped me with all the little items that ended up making big differences, you might want to check it out. http://www.gnttype.org/techarea/techpage.html