Ever since i bought my formula 5 years back its gas mileage sucks. It a 350 TPI mostly stock except for headers and cat back system. from full to empty it gets 153 miles. and then it near the E. something is wrong here. The car runs well and I don't know what to look for here. what would cause me such bad gas mileage. I gave it a nice tune up last mouth but that did not help.

 

a bad O2 will cause that, its easy to replace and all, use some PB blaster and it will come out easy, a better spark plug will help alot.

do you have a heavy right foot like me? because if so than its your problem, i got a 305 TBI that will pass everything but a gas station if i lead foot it, but if i drive with some sence i can get 18-20 on a city/highway mix, if its all highway and i keep it at 78 mph i get mid 20's easy, im running 93 octane but i got to due to the mods and computer, but even driving with some sence around town will still lean toward low MPGs ( but stll better than you describe my best guess says you are under 10MPG).... another word of advise, dont take off slower than a dump truck to try and save gas, it dont help, i had a caddy for along time and it has the on board computer with instant and average MPGs, if you take off like slower than christmas it reads say 9mpg till you get to say 55, it takes 30 seconds, you take off like an average person it says 9mpg till you get to 55 and it only takes 17 seconds, so you get what im trying to say here

 

I'm having the same problem - no idea what the hell gives me such bad gas mileage.

Guess I can only offer advice that I've been offered.

Check for vacuum leaks, check sensors, check injectors.

Driving style doesn't really seem to affect my mileage that much.

 

well I never changed the o2 sensor. the car get alittle better miles on the highway. I really don't drive the car hard. I do use 10w30 motor oil im sure using 5w30 will help alittle with gas mileage. I will try and changed the o2 sensor and see were I am after that.

 

I average about the same with my car and it's a pain in the *** right now since gas is nearing $3/gallon. When I go out to get some coffee with some friends, it is costing me almost $8 in gas for the 36mile round trip.

 

I was having the same issue right after I purchase my car. I did a complete tune-up and still no help. I had no codes and all sensors were good. I listened to someone on this board and decided to do a O2 sensor and low and behold picked up about 3 MPG. I get about 14 around town and 21ish on the highway. Give it a shot its not very expensive nor hard to do with a O2 socket.
Semper Fi

 

how do you take off the 02 wire? do you just pull it out? never changed the o2 before.

 

Disconnect the connector (just follow the wire up from the sensor and you'll see it) and then unscrew it from the exhaust manifold/header. You will need an oxygen safe sealant on the thread of the new one.

 

Make sure to use an AC delco O2 sensor. The $16 bosch sensors don't work as good for our cars.

 

Does that apply to Bosch heated sensors too?

 

the oil viscosity change wont do anything for your motor at all. youre only changing the thickness of the oil when its NOT at operating temperature, making it a little more thin before it warms up. thats all, once its at operating temp the oil has a 30 weight to it.

 

Ive been to 3 auto parts store all they carry are bosch. were is the ac delco o2 sensors? I don't want to go to a dealer.

 

Geez I envy you. Gas in Ca is nearing $4 per and only 91 octane. Sorry just sniveling

 

There's also the ongoing argument/issue about using a 160 degree thermostat. Many people who change from the factory 195* thermo have found their fuel mileage taking a crap on them. There isn't any firm concensus on this issue so I just wrote in to 'Chevy High Performance' magazine in hopes they'll give us a definitive answer. I did the 160 degree thermostat and had my PROM adjusted for that temp along with the changing of the electric fan settings. Does your water temp guage look like you're running to cold???? Nitro

 

my 90 350 TPI with LT4 HOT cam, one of my bolts on the 3-bolt collector flange broke, exhaust started rattling, clanking slightly. I put it off due to other pressing priorities, but then I noticed my mileage dropped from 16 to 13 all at once. when I replaced the bolt, it went back up to 16. I guess it was throwing off the knock sensor. other than the annoying noise, the engine seemed to run fine, no difference.

 

are you sure you are calculating your mpg's correctly? seems like from the way you worded your post that you think you are getting bad mpg's because you only get 153 miles when you fill up. the correct way to calculate mpg's is divide the number of miles you drove by how many gallons you filled up with. I.E. you drive 153 miles, and fill up with 8.438 gallons that would equal 18.13 MPG. "153 / 8.438 = 18.13"

just want to make sure you are calculating it correctly before you go replacing stuff that is still in good working condition.

 

First of all, how much gas are you putting in there to get 153 miles to a tank. Although the fuel tanks are rated at 15.5 gallons, some members can only put in 12 gallons to a tank (me) while others have put in over 16 gallons.

I've tried full tune up with AC Delco O2 sensor ($75 for the O2 at the dealer) and found no better gas mileage than I did with my 2 month old Bosch O2 that I'd put in there before. It did seem to run a bit smoother though.

Coolant temp. Yep the 160° stats make your mileage go down. Switching from 160 to 180 helped me. And I read about a C4 Vette with the digital mpg gauge getting 1.5 mpg better with the 180 over the 160 and another .5 mpg when he went 195.

About the only thing I can think of is that my GTA is heavy and I'm over 200 lbs as well.

 

Yeah thats why I said to run the AC delco. They're the same at what they do, but I believe the Bosch is a high-res while the computer is set-up for a low res and thus it doesn't run as smooth. Some members declared this is most evident in idle (obviously once it has warmed up to closed loop). Outside of this, I believe it drives the same.

 

I'll add to the poor gas milage issue.....I also have the TPI, 1986 Z28 305.

After reading what you guys are talking about, I may try a new O2 sensor...but I also need some advise/help on changing my thermostat. I believe it is malfunctioning. If I drive down the road with good airflow going thru the radiator, the gauge only reads a tad over 100, but if I sit for a bit with it running it will reach the 220 mark and the fan will kick on and cool it down the the normal 190'ish range. By looking at the thermo housing, it appears I need to disassemble the front portion of the TPI to get to the thermo housing. Am I correct and is it a headache to do. thanx.

 

theres nothing wrong with those temp readings it sounds perfect to me. change your fuel filter and and any ignition parts that could be worn out and scrub down the throttle body.

 

Are you replying to me? I am almost sure that my thermostat is bad in some form or fashion......and I dont have a T'B'I.

 

The throttle body is the piece on the front of the upper plenum where the linkage attaches to the butterflies (see below)

 

Now when you say just above 100, do you mean between the hash and the 220 mark? or do you literally mean a tick above 100. If it's a tick above 100, your thermostat may be stuck open. Look at it optimistically though, it could have stuck closed! You should only have to remove the throttle body to get to the thermostat which is real easy - 4 hoses, two electrical connections, 2-3 cables, and four 10mm bolts. I have the TB coolant delete done, so I only remove two hoses. Consider doing this while you have it off. Also, while the throttle body is off, you can do some good cleaning to it. Just pick up a bottle of "throttle body cleaner" at a parts store and go nuts, get the IAC, the crank vent passages, the vacuum ports, etc. And yes, you have a throttle body with TPI. Thats a picture of an aftermarket throttle body that MP81 posted, however it's almost identical.