Hello, a friend of mine has a '90 Firebird Formula, and wanted me to help him figure out why his car is not getting good gas mileage. So i thought i'd bring it to the experts. He filled the tank up a week ago and has driven approx 85 miles, and now his tank is 1/4 full. He has the stock 305 TPI with roughly 150k miles on it. What do you guys think he needs to check first? BTW, he has the "infl rest" light on. I was thinking the 02 sensors, and he has no cats also. Anyway your help would be great.

 

spark plugs. air filter. start with the basics

 

Spark plugs and wires, fuel filter, O2 sensor, distributor cap and rotor, that sort of thing should be done with regular maintanence.

 

alright will do, thanks guys.

 

Seafoam it and run some techron through the tank... Change PCV and check tire pressure all around.. Make sure car runs at around 210 degrees , if it runs lower fuel economy could suffer... along with all the previous things mentioned up top.

 

V8Slayer----Isnt 210 degrees a little hot? Cruising at 40mph it runs about 170. I never thought a cooler car would hurt the gas mileage. I could use a little education here.

 

Cars run at 210 at the head. 170-190 at the thermastat.

Air filter will not do anything for fuel mileage. the computer adjusts that. Cap, rotor, plugs, wires, fuel filter, things like that. Here is a question though. when it reads 1/4 tank how much fuel does he put into it. Seriously. you never mentioned that. My thirdgen will go from full to 1/4 tank in about 100 miles and sit there for the next 125 or so.. the gas guages in the 3rd gens are absolutely horrid crap. about as inaccurate as you can get sometimes.

 

he said that when he fills it all the way up (holds about $55.00 of gas), the needle will stay up towards the full line for a while but when it gets below the 3/4 mark it just drops very quickly. So youre saying it could be the gas gauge thats inaccurate?

 

absolutley ( and a bad air filter cant affect gas milage my foot) everytime i step on the brakes my gas gauge dips down half a tank. and opposite when i accelarate. there very innacurate. same story on alot of gm cars. my work van sits at half a tank all day long till its empty it shots up to full. i had an alero which ran out with 1/4 tank on the guage.

 

A bad air filter on a fuel injected car will not effect fuel mileage to any noticeable degree. It will effect power. the computer will lean your fuel in order to maintain the 14:1 with the reduced airflow. unless it is extremely dirty (like around 75% clogged). That is different from a carbed engine however where an air filter WILL effect fuel mileage.

And yea that sounds like the fuel sending unit is going but not the problem. 55 $ at about 4$ a pop is only 13 3/4 gallons. we have 15 gallon tanks. I figure most people are probably paying more by now for premium or what not. which would be more like 12 gallons of fuel. Also, about the fuel tanks. they are not square. they are "v" shaped. So really the gauge was manufactured inaccurate. the first 1/2 tank of fuel is a bit more than the last half tank of fuel.

hmm
However you said he drives 85 miles all the time and fills up with 13 gallons of fuel? I would still say do the normal stuff. Make sure all the O2's and do a tune up. Spark, air, fuel ya know lol.

This is all in consideration that it is still fuel injected. And has not had a carb put on it.