Bad Gas Mileage [Merged 9-6]

[99GST]

I am still getting some pretty bad gas mileage.

I am only getting about 19.7 mpg with mostly city driving shifting at 4k.
I have already replaced my front O2 sensor and am looking to replace the fuel filter.

I have a 99gst with about 55k miles on it. Should my fuel filter already be bad?

What else should I check or change? I have NKG plugs and Accel wires are on the way.

Thanks,
Nathan

 

[reardrv]

Actually I get awesome gas mileage,































when I drive my girlfriends Saturn .

 

[tweek814]

im getting about 9mpg

i can seriously watch my gas gauge move on my drive to work (about 10 minutes).

bone stock, i dont know wtf the problem is. no leaks, running really rich. will see after a tune up?

 

[talon187]

I would stop drving it. if your running that rich i bet its not all geting burnned and its in the oil. sounds like somthings holding the injectors open check the ecu and wires around the injector plugs

 

[wises4057]

I would say it's time to change out your O2 sensor

 

[cgmtboarder]

hey my car is getting pretty bad gas mileage as well. havent actually tested it but im thinkin around the 12 mpg area. I have full exhaust, ported manifold and upper intercooler piping. so nothing major. what could be the cause? i have a check engine light but its so dim you cant see it during the day. not sure if it means anything or not. still need to check it

 

[Spoolin4Ever]

im getting about 9mpg

i can seriously watch my gas gauge move on my drive to work (about 10 minutes).

bone stock, i dont know wtf the problem is. no leaks, running really rich. will see after a tune up?

I would stop drving it. if your running that rich i bet its not all geting burnned and its in the oil. sounds like somthings holding the injectors open check the ecu and wires around the injector plugs

Yeah; check your oil for the smell of RAW gas...if it smells like fresh raw, unburned gas, then change your oil ASAP before you wash you bearings out. Could be a stuck injector or injector seal leaking.

Did the car ever sit for a while?

Yeah on the ECU inspection ASAP.

Report back on those 2 things please..

hey my car is getting pretty bad gas mileage as well. havent actually tested it but im thinkin around the 12 mpg area. I have full exhaust, ported manifold and upper intercooler piping. so nothing major. what could be the cause? i have a check engine light but its so dim you cant see it during the day. not sure if it means anything or not. still need to check it

You MUST:

1. Found out what the CEL is for by pulling the code with a datalogger.

2. Remove and inspect ECU open it up and smell it to see if the capacitors are leaking.

3. Report back this info.

 

[pdub42]

last tank of gas I went 405 miles on 10.7 gal. 37.8mpg in my eclipse NT almost all highway of coarse but didn't know they got that good mpg's until I finally got it running a few months ago.

 

[tweek814]

Yeah; check your oil for the smell of RAW gas...if it smells like fresh raw, unburned gas, then change your oil ASAP before you wash you bearings out. Could be a stuck injector or injector seal leaking.

Did the car ever sit for a while?

Yeah on the ECU inspection ASAP.

Report back on those 2 things please..

car sat for at least a month before i bought it, probably longer.

changed plugs, changed oil, thought it had a hint of gas smell to it, not overwhelming, injector wires look good, havent checked ecu yet. car is not running due to alternator/weather atm.

 

[jwahl16]

i get 11 mpgs right now, I'm running really rich (the exhaust smells terrible). I'm changing my front o2 sensor this week, could it be anything else?

I have a code for p0505 - idle air control (if i remember correctly)

 

[wises4057]

if it's 11mpg, and rich...I would bet that it's the front o2. can't tell you much on that check engine light though

 

[jwahl16]

that's what i thought, and thanks for the quick reply!

 

[shadoof77]

Yea im only getting roughly 240 miles to the tank. with only your basic bolt on mods dont to the car... not sure if its the awd taken up much of that gas...

 

[cgmtboarder]

if you have the code for idle air control valve then thats why your getting terrible gas mileage. when that sensor goes bad it runs horrible and makes for bad gas mileage. fix that and you shouldnt have any problems

 

[tweek814]

i changed plugs, front o2 sensor, and flushed my coolant. my gas mileage has almost doubled to 17-20mpg. ill take that, especially for an awd/auto trans in freezing balls winter!

 

[Daylightdies]

if you have the code for idle air control valve then thats why your getting terrible gas mileage. when that sensor goes bad it runs horrible and makes for bad gas mileage. fix that and you shouldnt have any problems

Thats what I have right now :/ I need to fix it...

 

[cgmtboarder]

well havent been able to check the check engine light. you cant see it at day, only at night. i think it may have somethin to do with the altinator. i checked the ecu and it did smell a little funny. and one of the capacitors is leaking slightly underneath. not sure if its enough to cause problems though

 

[kingcoyote]

I'm currently getting around 11mpg, an improvement from around 8 when I picked up the car. I've changed the front O2, the plugs, the air filter, and the wires. I've ran seafoam through the gas.

I tried tapping on my MAF to see if the idle changed but my idle isn't completely stable so I wasn't able to tell if the variances in idle speed were due to tapping on the MAF. After about an hour of freeway driving my idle goes up to about 1500.

I'll check out the ISC and see if its bad since thats been mentioned, but with these particular sensors is there anything else I should take a look at ? I haven't had time to borrow an air compressor and check for boost leaks, but theres no way that boost leaks alone would cause this sort of mileage, right ?

 

[97GSTurbo97]

I have a fuel trim malfunction error in my 97 GST.... the haynes manual says could be my o2 sensors or injectors... im not getting horrible mileage but my car acts like its bogging down when im going up hill.... im leanin more towards o2 sensors rather than injectors. anyone have any advice?

 

[GnarlyGst]

Im gettin 300 highway and city miles

 

[disillusions]

Yes yes, I know this question is ALL over the forum; however, there aren't really any decisive answers since everybody's situation changes from post to post. So here goes the story:

I'm currently driving a 95 GSX A/T. The only performance mods done are a 1G BOV and some polyurethane bushings. She recently had a full tune up done, which got rid of a plethora of previous issues like bogging and doing a "Magic Fingers" impersonation during acceleration. I drive about 2/3 highway and 1/3 city, usually only accelerate using 3-4k RPM, and only partake in some spirited driving every now and then. One issue I have is that my tranny has a possible rolled seal, which is causing hesitation during acceleration; but only till about 2,500k. Now with letting the car heat up in the mornings since its been freezing, I get about 10-11 mpg.WTF

I did do research on other posts, and it seems like theres a lot of people getting a good 20-25mpg on even decently modified cars, so I'm completely at a loss. Normally I'd have thought the tranny was causing the issue, but even after a rebuild 3 years ago it still got horrible gas mileage.

HELP

 

[9O_tsi]

I have the same problem, im getting on average around 14mpg. Try replacing your o2 sensor, this might help a little. I'll be doing this to my car as soon as it warms up.

 

[DSMTyson]

yeah i was getting about 12mpg before i switched my o2 sensor to a better, used one(mine had lots of race gas ran through it). Now I'm averaging 15-16mpg, but i'm also on 22psi and get deep into the boost ~4-5 times everytime i drive.
And i've got a high stall converter too so that really doesn't help with MPG either.
When i stay out of boost for the most part i get about 18mpg city, 23hwy. give or take.

 

[disillusions]

I have the same problem, im getting on average around 14mpg. Try replacing your o2 sensor, this might help a little. I'll be doing this to my car as soon as it warms up.

Sounds good, I'll put that next on my list. I was thinking about heading to this shop around me that boasts a 900whp 3000GT still getting 25mpg. They sound like they'd be able to help achieve a nice 20mpg.

yeah i was getting about 12mpg before i switched my o2 sensor to a better, used one(mine had lots of race gas ran through it). Now I'm averaging 15-16mpg, but i'm also on 22psi and get deep into the boost ~4-5 times everytime i drive.
And i've got a high stall converter too so that really doesn't help with MPG either.
When i stay out of boost for the most part i get about 18mpg city, 23hwy. give or take.

See for me its hard to tell how much boost I'm actually getting since I've yet to install an aftermarket gauge. If I go by the stock piece(which we all know is a joke) im almost at "full" boost at 3-4k.

So do you think it's better to opt for a used o2 or a quality new piece? I highly doubt mine was exposed to anything other than 93 octane, but the previous owner did seem to not understand how to maintain a car.

 

[turbo tim1]

Get and a/f gauge when you get your boost gauge, this will tell you what your a/f ratio is. If your mileage is that bad you are running rich, also check your ignition timing to see if it is with in spec.
And I agree a bad 02 can cause poor mileage.

 

[99gst_racer]

disillusions - Fill out your vehicle profile so that we can see what mods your car has. It's helps greatly with threads like this.


Any fuel mods? What are you tuning with?