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

 

[LookDadNoBdyKit]

I still cant figure this out. My last tank was about 120miles. O2's are cycling. Plugs are new. Ari says wires are pretty new. It is NOT my driving habits. I drive pretty slowly and out of boost most of the time. My coolant temp sensor is reading normal temps and all the other sensors are also reading the right values.

2g mas/660s/ngk bp7es's/ngk wires/50 trim

Fuel trims are a little weird. My MED trims are fine, stay at 95%. HI settings are around 88%, a little rich, but I dont think that should have an effect on low rpm street driving, which is usually how I drive. My low trims are acting really weird. The other day I checked them on the logger and they were at 99%. Today I checked them and they were at 88%. I leanedit up on the low settings at 1000rpm and it went to 105%. I richened it up a click and then it went to 101%. This is where it gets weird. I let the car idle and just watched the logger. All the sudden it goes down to 89% again. So I lean it out a click again, and drive aroudn the block with low rpm's in 4th gear. I check again and its at 101 again. Its like the low fuel trim is going up and down for some reason.

What could explain that? The idle surges a tiny bit, barley noticeable. More like a little pulsing. The lowest I can get my car to idle at is at around 800RPM on the dash tach, however the Logger & AFC tell me its idling at 1300 or so. I cant get it any lower by adjusting the biss. I did a boost leak test a while ago and didnt hear any leaks anywhere. Everytime I drive and let off, even if im not in boost and just normally driving, the rpms will drop really low and the car will stall unless I blip the throttle. This could be just caused by venting the type s but im not sure. The BOV is as tight as I can get it without having it surge. I dont have any problems at all boosting, but it is possible theres another tiny boost leak somewhere I guess. I will do another test and see if I can find anything. Vac lines are eliminated though, and the ones that are there all look fine. The IC piping is also tightly clamped so I dont think thats an issue.

Any other ideas as to what this could be?

 

[beat90tsi]

First I would say to get your low trim up, try maxing it at 139, then go from there.
Maybe try swapping out the maf if that doesn't help.

That RPM discrepancy may point to the ECU. See if you can swap that out as well.

 

[Prodigyman]

Just got my 97 GSX and got a BIGG problem IMHO. I tank a car full (about 43 Liters) with 94 octane and drive for about 150 miles and my gas gage shows me that I have 20% gas left? Is that normal for 97 GSX? Because this is JUST insane amount of gas this car eats or there’re something wrong with it? I mean I cant go 2-3 days without tanking the car full? And with current gas prices especially for premium (91) or supreme (94) which cost way too much. I don’t want a fuel economy car but V8 don’t eat as much gas as this… and car just have an intake on it ATM and everything else is stock for now.

Any suggestions?

 

[paranoidTSi]

I'd say either your gauge is off track (common), you are running rich, or one of your O2 Sensors are defective.

My gauge says I'm empty when I have about 1/4th of a tank left, I just usually wait for the sensor to go off on the gauge cluster before I go to a pump.

 

[Prodigyman]

I'd say either your gauge is off track (common), you are running rich, or one of your O2 Sensors are defective.

My gauge says I'm empty when I have about 1/4th of a tank left, I just usually wait for the sensor to go off on the gauge cluster before I go to a pump.

Hmm interesting! Why would it run rich? It’s a stock car for the time being. Well I will drive it until I literally run out of gas and see what is going on.

Here are my calculations for stock and what I have now for fuel efficiency

Last time it showed empty tank and it took me 42 liters to full her up

Stock
22 miles for 1 US Gallon City
29 miles for 1 US Gallon Highway

34 km for 3.7 liters City

9.1 km for 1-Liter City
12.1 km for 1-Liter Highway

Current 97 GSX Stock (with intake and muffler)
208 Miles for 42 Liters
332 KM for 42 Liters

7.9 Km for 1 Liters City
29.23 Miles for 1 US Gallon

What do you guys think?


PS: I noticed this car doesn’t have CAT but a straight pipe could that effect anything fuel wise?

 

[marcmsj]

If your O2 sensor is bad and not sending a signal to the ECU, it will think it is running lean and increase the fuel, hence making it run richer then it has to.... so i would start off by checking your O2 sensors. it would usually trip a CEL light but not always.

your guage might not be working properly aswell. fill the tank all the way again and calculate your km/l or mpg.

 

[Prodigyman]

your guage might not be working properly aswell. fill the tank all the way again and calculate your km/l or mpg.

Ya check the above post!

its 7.9 KM per Litre... supose to be 9.1 KM litre and 12.1 KM/litre on high way... and its defenetly not

but thank you for your suggestions guys! I will check my O2.

 

[AleksLukic]

Well I have no major mods at all, only think I've done recently was replace my end clutches. But I noticed that my car, yes it runs much much better then before, But I'm burning fuel like crazy. I don't drive crazy, or floor it at all. I've been driving the last 3 days like a grandma just to see the situation and i notice that fuel burns like crazy regardless. I filled up 3/4 of a tank yeterday and now I'm down to a quarter. What can be possibly draining that much fuel, I've never guzzled this much this quick before. Any ideas what this can be, or any way corrected to better my fuel by even alittle? Thanks

 

[andymoraitis]

Even though you're mostly stock, there's a few things you can do to track down what's causing you to run so rich. Check for these symptoms and they should point you in the right direction.

1. Pull a couple of your plugs. If they're carbon fouled and black you're definitely rich.
2. Check your tailpipe after you start the car. Look for wet sooty discharge on the ground.
3. Look in your rear view mirror when you punch it. Black clouds are a giveaway.
4. Consider if the car feels sluggish under heavy acceleration.

If any of these things are present, your upstream O2 sensor may be going bad and needs to be replaced. In general, when these fail, they will make you run pig rich and burn fuel like it's going out of style. Also, if you have a cat, verify that it's not clogged by removing it and looking through it. If you hold it up and can't see light on the other side, this means it's toast. You may also want to have a local shop like Autozone check for CEL's in case yours isn't working.

If it's not the cat or the O2 sensor, check for leaks in the fuel lines as a last resort.

Good luck!

Andy

 

[AleksLukic]

Sweet, thanks for your help. Well I know for sure I'm runnign rich, you can smell it hardcore at idle, and when i punch it black smoke comes out the rear. I'll replace the o2 sensor, that may be it since it was never changed it seems since the car first rolled off the assembly line back in 96. Thanks for your help. If anyone else has suggestions I'd be more then interested to hear them.

 

[WICKEDTEEN84]

i know alot of people claim they get bad gas mileage. im getting sick and tired of it. i have totally stock fuel system. im running about 10 mpg. my motor boggs like crazy all the time. has anyone discovered the reason for this bad gas mileage

 

[CyberEye]

i know alot of people claim they get bad gas mileage. im getting sick and tired of it. i have totally stock fuel system. im running about 10 mpg. my motor boggs like crazy all the time. has anyone discovered the reason for this bad gas mileage

Replace your O2 sensor, plugs, wires,air filter, fuel filter.

 

[HighPsi92GST]

anyone discovered why we get bad gas mileage?

because there is something wrong with your car. I dont get bad gas mileage at all

 

[DSMSpyder99]

u got more problems then bad gas mileage if ur motor bogs Also your stock fuel pump could be taking a dump on u.

 

[DSM MAN]

Im getting pretty bad mileage too. How much of a difference would plugs,wires, and a filter make? Like an extra 2-3mpg? Or more? Just curious I do plan on replacing those soon anyway. But im getting like 17mpg max. The fuel filter is like a year old but the plugs, air filter, and wires are real bad.

 

[paranoidTSi]

DSM MAN, you named what your gas mileage problem is. If your air filter is not getting enough air into the system then you will have more gas than you will have air, if you aren't getting good sparks, some of that fuel won't be ignited..thus wasted.

WICKEDTEEN, the "bogging" could be bad plugs and wires..and if you are getting 10mpg the chances are at LEAST one of your O2 Sensors are out or both.

 

[tstkl]

yea, if your not getting good gas mileage, its probably your car, or your driving habits.. i get 20mpg, all city pretty much, and my town is the hilliest place i know of... there are less flat places than hills, and some places have like 60 degree inclines... its crazy... but i keep it in high gears and downshift at the last second if nessisary..(and yes i know that the tps is more important than rpms, thats why i never go over about 40% tps.... except taht one time i wanted to see what my datalogger would read at wot,.. which was 99.6%...isnt that a little high?)

 

[CyberEye]

yea, if your not getting good gas mileage, its probably your car, or your driving habits.. i get 20mpg, all city pretty much, and my town is the hilliest place i know of... there are less flat places than hills, and some places have like 60 degree inclines... its crazy... but i keep it in high gears and downshift at the last second if nessisary..(and yes i know that the tps is more important than rpms, thats why i never go over about 40% tps.... except taht one time i wanted to see what my datalogger would read at wot,.. which was 99.6%...isnt that a little high?)

if by TPS you mean throttle position sensor, then at WOT you should be at 100%, wide open throttle: throttle position sensor, it's not that hard to figure out.

 

[tstkl]

most people dont see 100% because 100% would be an absence of the plate it self... 100%=no restriction... the throttle plate doesnt just disappear... and also i know im not the only one who sees sub 100% numbers, and i believe its rare for people to see 100% wot, from what ive heard/seen...

 

[WICKEDTEEN84]

my o2 sensors have been change, accel hp plugs , 9.3mm spark wires, airfilter, all maintanace have been done

 

[Catbox_95]

I'd say that with all your mods, any one of a thousand things could be affecting it. valve timing off, for example, is what it sounds like to me SINCE you said that your O2 sensors have been replaced. I know I don't have your turbo, but I've got tons of mods, 300 hp and I average between 50/50 city and highway about 26.5 mpg if I keep my foot light.

 

[CyberEye]

most people dont see 100% because 100% would be an absence of the plate it self... 100%=no restriction... the throttle plate doesnt just disappear... and also i know im not the only one who sees sub 100% numbers, and i believe its rare for people to see 100% wot, from what ive heard/seen...

I see 100%

 

[treefrog1112]

i get 16mpg city. New air filter, newer plugs and wires. fuel filter is god knows how old

 

[truwarrior]

Do you have an exhaust leak? Maybe it's throwing your 02 off and it's running richer. Maybe you have a intake leak, that will freak out your 02 sensor too.

What's your vac at idel and what rpm is the reading took at.

 

[DSM MAN]

Thats another thing I forgot to mention, my exhaust manifold has 3 pretty good sized cracks in it. Im looking for a 2g if someone wants to sell me one hehe.