MPG's need help


Aug 10, 2003
I started driving my T-Type a little more and noticed that my mpg's arfe nowhere close to where I think they should be or maybe I'm just dreaming. I'm getting an average mpg of around 12-14. On a full tank I get around 200-220 miles. I have a TT street chip for 60lbs injectors, new stock maf, new ngk-IX4's. Any ideas on what to check first? I have a wideband guage and at idle it reads 13.5-13.8 which I think is way rich for idle (should be 14.7) Cruising it reads about 14.5-14.7 which looks good.

Any help is appreciated.
I'd pull three easy to get to sparkplugs and have a looksee.

Check the fp.

And watch the blms at idle and cruise.

My TT chip car with bosch 42's gets the normal 17mpg combined mileage.

Make sure you check mileage on these cars the proper way by NOT using the gas gauge just fill it to the neck visually and then drive it and fill it back to the neck again and use the pump reading for gallons divided into the miles you went.

Just in case that's a factor.
Your chip is idling in open loop. Not a concern and doesnt cost much mpg unless you idle all day long. If your at 14.5-14.7:1 cruising your in the correct range for the best mpg unless you get the lean burn option which runs in open loop over 15.0:1. You need to look at driver technique the most if you want mpg. No AC on, and front end alignment good you should average around 18 mpg. If you have a real pu$$y foot on the highway you could get 25 mpg or more. If you want more mpg next time you build an engine go up in compression ratio, use gapless rings with a plate hone, and have the pistons thermally coated. Ive hit 30 mpg with a 9.0:1 engine with a lockup converter. Ive hit 21 mpg with the car in my sig and thats with a non locking converter. If you step on it a few times it will bring the potential mpg down a lot.
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) 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.