Chevy 350 (5.7 l)

I am not a mechanic, so I am not familiar with the inner workings of the engine, but I dont mind doing the small exterior stuff. That said, here is my problem. I have a 1995 Chevy 350 fuel injected TBI, which I have had 3 years, and the gas mileage has suddenly dropped. I had been getting 17MPG (highway) and am now getting only 9 MPG (highway). There is no oil consumption, no smoke, no engine light. I do notice small loss of power, but nothing REAL bad. I have run lots of fuel injector cleaner thru it, and have replaced the air filter, spark plugs, plug wires, dist. cap, rotor, and EGR valve. Still only 9MPG. Today I took it to a GM garage, and they found nothing wrong. It was a Pontac garage (only thing open on a saturday), but still a Goodwrench garage. They admitted they dont work on a lot of 350 engines. I NEED HELP!!!

i may be asking a stupid question, but your torque converter is still locking up at highway speeds right? or is it a manual. that sudden drop may be a sign of major problems. do you have 4x4? maybe if you left it on and forgot about it, it could make that bad of mileage like that. im assuming you would have checked that of course tho

Try a new oxygen sensor. It is supposed to be replaced about every 50,000 miles or so.
Usually you get an engine code when it goes, but this does not always happen.

I second the notion of transmission issues. Sounds like a problem in the TCC system. Running in 4th with the TCC inoperable will lead to transmission failure. When you get it to freeway speeds, shift from 4th (OD) into 3rd - do you feel a difference in the power? Does the engine increase RPM? If not, you've lost OD.

You can do a crude check of the operation of the TCC system. When you're at speed, you should notice a 'shift' or a 'bump' when you lightly tap the brake pedal - if you have a tach you'll see a slight increase in engine RPM - if there's no change, the TCC isn't working.

TCC = torque converter clutch

Then again, a '95 will have a 4L60E (electronic shift TH700R4) and if there's a problem, will manifest as a check engine light.

first you need to be specific, we only know the motor not what its in and before you go replacing a transmission change youre fuel filter, has the timing been adjusted this will affect gas mileage and power. dont ever take your car back to that shop if they couldn't even check the timing.
back to the transmission is it standard or auto. does it flare.