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

 

[rampridersrider]

Yeah, every now and then. It would come on on long drives but only like once a month or so. Kinda weird. Then it would just go back off.

 

[spyderturbo007]

You will only get a CEL for the O2 sensor if the fuel trims become maxed out. It is very possible that the O2 sensor is functioning just enough to keep the FT's from maxing, thereby not tripping the CEL.

The best way to find out if your O2 sensor is working, is with a logger. You should see the O2 voltage (sensor 1, bank 1) fluctuating between 0.20v and 0.80v at cruise/idle.

95% of the time people have problems with gas mileage, the O2 sensor is shot. Just remember, it's considered a wear item and should be replaced regularly.

 

[birdii]

i had a mechanic change my timing belt and water pump... ever since the change im getting bad gas mileage.... almost 100miles difference b4 change.... the thing is the timing marks line up perfect (checked by a 2nd mechanic) and the car runs normal.... is it possible that something was installed wrong but the marks line up..... or should i take it to a different mechanic to open up the whole thing up to recheck it.....

thx

 

[97ClipseDSM]

Its possible if these guys arent reputable..they could have damaged something on purpose so you can come back to them so they can charge you another arm and leg to help u with your fuel issues.

Best way is..start with a fuel injector cleaning when you pump your next gas. Check and make sure you dont have any Vaccum leaks..they could have dissconneced a few vaccum lines while doing your timing belt.

 

[Defiant]

Or, your front O2 sensor may have coincidentally tits-up right about the time you had the work done.

Do not use "b4" or "u" on the board.

 

[Old Mitsu Tech]

Its possible if these guys arent reputable..they could have damaged something on purpose so you can come back to them so they can charge you another arm and leg to help YOu with your fuel issues

Comon!!!! What are you? A conspricacy buff? Did you ever stop to think that like Defiant said something else could have gone wrong? I've seen multiple things wrong on older cars. These things are over 5 years old and you expect nothing else to go wrong? I've had fuel pumps go bad while I was working on the brakes. It just decided to die in my shop. Fortunately for me, a simple explanation to my customer usually suffices.

GEEZ



I thought I'd just said something about "u".

 

[twinkiemobile]

I recently drove my car to Phoenix and back. everything went ok. nothing broke. but i noticed that when i got back i'm only geting about 19.5-21mpg on my highway/city drives. I use to get 21-23 mpg on the same city/highway routine. I commute from Fairfield to Sacramento State and drive around town for my girl/work/errands etc. What things should i check out, fuel filter? o2 sensor? exhaust leaks? etc. thanks for any advice you guys can give me.

- Jose

 

[Talesin]

Step 1) Put in your car profile so we know if it's a 1G or 2G at least.

Fuel filter is a possibility. But those don't usually go bad all at once.. the MPG loss is gradual. Sudden mileage degradation tends to be a front O2 sensor (on the 2Gs) going bad.

 

[K_Hill]

Sounds like your O2 sensor is bad. Does it throw a CEL?

 

[twinkiemobile]

Yeah, i'll get to work on my profile tonight. I havent had time due to school. anyway. the CEL has been on for about 3-4 months now. but it came on after i put on my apexi N1 turbo back. when i have to code read at my work and then erased it usually comes back in a matter of days. from my research it looks like if i get a wide band and use it on the second o2 and match it with my dsmlink the CEL will go away. will that get me slightly better gas mileage?
thanks for the advise

- Jose

 

[knighteclipse1]

is it throwing a CEL? It sounds like a o2 sensor to me. keep us posted and let us know what it ends up being.

 

[Talesin]

A CEL is just saying 'something is wrong'. Your rear O2 sensor being missing qualifies as 'wrong' to the ECU, so it popped a code for that. However, that doesn't mean OTHER things haven't gone wrong in the meantime. You can have a CEL for one trouble-code, or ten. Only way to find out is to hook up a logger cable or DTC (diagnostic trouble code) reader... Autozone loans them out with a deposit... if they aren't busy, you can probably avoid the deposit even, just having one of their employees go out with you so you can pull the code.

Get the code checked. It's going to tell you that your rear O2 sensor is gone, and most likely will also tell you that your front O2 sensor is dead. If the front one is dead, replace it. You'll get back your good gas mileage.
I believe that you can tell DSMLink to ignore 'missing/bad rear O2' DTCs outright on the 2G version. The rear one is pretty much just there to make sure your catalytic converter is still working. :b

 

[Defiant]

No profile, no postie.

 

[Spaniel1287]

Ive had my 95 Talon TSI for about a year now. It usually gets about 280 miles to the tank which isnt that bad. Well a while ago it started getting abotu 180 miles to the tank. I took it to the mechanic and they said the front o2 sensor was bad and they replaced it. Well the gas mileage went back to normal. Well recently i been getting 180 miles to the tank again. Took it to the mechanic and they said the front o2 sensor was fine. Why am i getting terrible gas mileage??? Thanks.

 

[blcknspo0ln]

how did they check that the o2 was fine? i'd venture to say it went bad again

 

[red eclipse gsx]

Unless you added anything new to your ride, (like a fuel pump) then I would say to check your fuel filter, do an oil change, and a compression test. You probably already knew this but it is good to start with the basics. Also stick your head inside the engine bay and see if you can smell fuel, there could be a leak??.

 

[Spaniel1287]

Actually that you mention that i did put in a Walrbo 190 Fuel pump. Also the fuel filter has never been changed on the car.

 

[red eclipse gsx]

Then thats your answer, also without the proper tuning with the new pump, you will be using more fuel then what you need. If you have the money, get the DSMLink if your planning on upgrading your car more. An S-AFC would be fine for now, but DSMLink is better.

 

[Spaniel1287]

Yea i planned on getting a SAFC and a datalogger sometime soon. But you think that replacing the fuel filter and getting a SAFC would help me solve my gas mileage problem?

 

[red eclipse gsx]

Definately get a new fuel filter, and an oil change. The S-Afc can wait until you get injectors. You will still be using more fuel then stock though no matter what with the bigger pump.

 

[Spaniel1287]

does the SAFC control the fuel pump?

 

[blcknspo0ln]

the SAFC is just an ECU "tricker" .. it will not control any fuel component in any way. however, getting correct compensation (running leaner in most cases) on the SAFC will net you better gas mileage. but that's not important to you until you get bigger injectors

 

[Spaniel1287]

alright yea. Well im about ready to put on a 14b on my 2g. Then i planned on getting 550s, safc, datalogger after the 14b was on. I guess ill get a new fuel filter for now and then just gotta wait to get bigger injectors and safc.

 

[red eclipse gsx]

The S-AFC does control the injector aspect of your fuel component system. I would not run your car too rich or lean on purpose because its not good for your engine. These cars don't come too rich or too lean from the factory for a reason. Do the simple stuff, like oil change etc. and see where you come out.

 

[Spaniel1287]

alright yea ill change the fuel filter and get the oil changed and then ill let ya know how many miles to the tank im gettin then.