When I first bought my GN 2.5 years ago, I started a thread about my poor mileage. It has just about every possible source of bad mileage in that thread. Do a search with my user name and the search word (my mileage sucks) and you'll be able to figure out your issue.
I spent a LONG time trying to figure it out, and many of the "obvious" sources of bad mileage actually didnt do anything to improve it. All these things are a little out of order. I had a 3800 stall non lockup converter in it. I figured that HAS to help if I throw a stock TC back in it. It didnt help AT ALL. The guy who owned the car before me had stuck a non GN, 7.5" rear with 4.56's. Another source of high rpms (this guy seemed to think the GN motor has a redline of 9,000rpm with the drivetrain mods he was throwing at it. Spent a pretty penny getting a worn out used 8.5" GN rear with stock gears in it (the rear went peg leg after a month of just cruising)....my mileage didnt improve even 1mpg. I had a pretty bad part throttle misfire issue. I replaced the intake gaskets (got rid of the crap sheetmetal intake gasket and went fel-pro), all the hoses and gaskets, dialed in my fuel pressure and all my sensors. I replaced check valves with good ones and a new PCV valve. Also replaced the vacuum block gasket (had a significant leak there) and installed a hose clamp everywhere I could. I also replaced my injectors because 2 of them had cracked o-rings and were leaking mad air. After that, my 8-10mpg issue went up to about 15mpg. (this is 90% higheway driving). Misfires/vacuum leaks/clogged exhaust are major sources of bad mpg. I tried a million things but only a few actually made a difference. Royal purple 10w-40 gave me another 1-2mpg. Bosch platinum plus 2's? gave me another 1mpg..this plug has a positive electrode smaller than the iridium..its like a tiny speck, but my idle quality improved alot and gave a definite power increase under boost. Ive tried every plug there is and these were the only ones that i will get again. Inflating my tires to 40psi instead of 32 gave me another 1-2mpg. I never did replace my cat converter, but you can rev the motor up in park and watch the vacuum reading. If it drops at all bringing it up to 4000 or so, the exhaust is clogged. Mine seemed to be ok. Its a stock restrictive setup, but still ok. Driving like a grandpa, my best ever has been 19mpg, with my average 16-17 with the way I drive it. And my TC lockup doesnt work. Ive been through every troubleshooting exercise there is and narrowed it down to either a bad TC or a bad lockup solenoid. All symptoms point to a bad TC...I bought it used and it was pretty beat up. But I was getting 15mpg with the 3800 stall non lock art carr TC and 4.56 gears, and then 15mpg with a stock TC and stock gears. I didnt have lockup, but I did drop my cruising (highway) rpm down 1000rpm and surprisingly it didnt help at all.
So ohm check your coil towers and your plug wires.
Check all sensors and set them. Replace the o2 if it doesnt seem active enough.
Replace the intake gasket if you have a sheetmetal one or suspect anything at all is wrong. Get a hose and listen all around for any sucking sounds (vacuum leaks)...propane doesnt work and neither does anything else. The hose trick works better than anything. Listen at the base of every fuel injector for vacuum leaks.
Replace all suspect hoses and clamp all hoses you can.
Replace plugs.
Get a new gas cap, vented. (the o-ring can cause evaporation).
Get your BLM's happy however you please.
Adjust your fuel pressure in the process. Mine likes 32psi line off, believe it or not, with 50's.
Check the EGR for a torn diaphragm. Check the EGR solenoid to see if its stuck. Replace the EGR filter.
If you replace all this stuff, your mpg will improve and itll run alot better as well.
