I was getting on avg about 20-23 mpg for the longest time.Then all of a sudden,I did the TBI mods a few months back and noticed my mph went down a few gallons.Though it was a bit much,I figured it was due to the increase in fuel pressure.But then,I advanced my timing about 4* and now it's hard to get over 10 mpg unless I drive like my great-great grandmother.I put the timing back to 2* and then 0*,but still no increase in mileage.I put in new plugs and wires a couple months ago and inspected the plugs.The plugs looked almost perfect.No loss in power.No fuel leaks. Computer throws code 23(MAT sensor).Would this cause my mpg to decrease that bad? Anyone have ideas? $50-$75 in gas a week is starting to kill me.

 

Did you leave the MAT disconnected? If so reconnect it and your MPG should go back up.

 

Check your MAT sensor, then check the vacuum lines that are connecting the MAT sensor. I had a similar problem, turns out that one of the vacuum lines running to my mat sensor was broken in half. As soon as I fixed it, my MPG went back up and the car ran good.

 

do you know what your fuel pressure is actually set at? raising fuel pressure should be more of an test and tune kind of a thing. on a mostly stock or mild engine raising the pressure will only hurt throttle response and mileage. i tried different pressures on my car and found myself back at the 11psi that our cars are supposed to come with every time until i put on aftermarket heads. even at that, now i run 11psi with 350 injectors. also never put stock tbi injectors above 13psi, the fuel will come out like a garden sprinkler, not in the clean smooth cone that you want.
the moral of the story is that it takes a larger increase in air flow than most would think to really call for more fuel in the engine.

lata
tim

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91 Camaro RS-LO3,Auto.
14.301 @ 94.39mph
The stock cam will go 13's!!!!!!!!!!!
The car is going street strip for next season. 13's here i come!!!!!
Edlebrock TES Headers, Cat Eliminator Pipe, Borla Adjustable Cat Back,Marche Pulleys, HyperTech Chip,Transgo Shift Kit,B&M Convertor(2000stall),Steve Spohn Rear Rig( all rod ends), Poly Front Bushings,Ported TB, 350 Injectors, Edlebrock Performer TBI Intake, Performer Heads, 1.6 Roller Tipped Rockers

 

It's either your mat sensor or a vacuum line I would guess.


NJ SPEEDER sorry but the TBI injectors will work at MUCH higher rates than 13lbs... Stock injectors have tested fine past 40lbs.



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Shane Buss 92 Firebird 355, Holley 750 Carb, Vortec 1.94/1.50 Heads (Ported,Polished,Milled .025), 10:1 Comp Ratio, Cam Specs 224/234 498/520, 1.6 Roller Rockers, Hooker Headers, Flowmaster, !Cat, 3.73 Posi, Msd 6A Ignition Cyb. Speedo box
http://users.ev1.net/~sdbuss/index.html

Note all girls on web page are x's, and will be removed when i get more time

OH i forgot. WEIAND SUPERCHARGER

 

Haven't had a chance to really get in there and look at things up close.I'll have to wait till this weekend.Thanks for the replies.If anyone can think of anything else,please let me know.

 

"NJ SPEEDER sorry but the TBI injectors will work at MUCH higher rates than 13lbs... Stock injectors have tested fine past 40lbs."

i have heard this other places too. have you ever gotten the injectors to a "high" pressure. i have used 3 different sets of injectors at different times in my car and it always seemed like as soon as i got above about 14 psi the spray started getting a bit sloppy. i know it had nothing to do with the injectors since 2 of the sets were cleaned and pressure tested before install. i know when being tested they put about 35psi in them to make sure they will hold.
do you think that i am just in the "sweet spot" for the injectors i happen to have?

thanks
tim


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91 Camaro RS-LO3,Auto.
14.301 @ 94.39mph
The stock cam will go 13's!!!!!!!!!!!
The car is going street strip for next season. 13's here i come!!!!!
Edlebrock TES Headers, Cat Eliminator Pipe, Borla Adjustable Cat Back,Marche Pulleys, HyperTech Chip,Transgo Shift Kit,B&M Convertor(2000stall),Steve Spohn Rear Rig( all rod ends), Poly Front Bushings,Ported TB, 350 Injectors, Edlebrock Performer TBI Intake, Performer Heads, 1.6 Roller Tipped Rockers

 

I have the same problem. I can't get more than 10mpg. I had the fuel filter, and pump replaced. TBI checked and cleaned. I had rough idle too. Now it just "loops" between 600-700rpm. I have no idea what it could be. New rotor, cap, wires. The car only has 50,000 miles on it. I don't drive like my grandma, but I'm no maniac either.

Ethan



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'88 T/A
305 TBI with WC-T5
50,000 miles
Flowmaster 3" exhaust
14" K&N Open Element