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

 

[redturbo66]

Originally posted by DSMSpyder99
300-310 miles on a full tank with 75% highway and 25% city. I use to get worse but then i got my new plugs and wires and i gained about another 25 miles in a tank.

which plugs and wires did you buy? I'm only getting about 230mpg right now. I know I drive hard but that's ridiculous.

 

[10minutecar]

Wouldn't the 02 sensors throw a code if they went bad?
Shouldn't the CEL come on if they go bad?

What are the best plugs and wires to buy?

 

[redturbo66]

Originally posted by 10minutecar
Wouldn't the 02 sensors throw a code if they went bad?
Shouldn't the CEL come on if they go bad?

What are the best plugs and wires to buy?

get NGK bpr6es plugs with magnecor wires.....you can't go wrong!

 

[WSU99GSX]

I have read a ton of posts, saying that Accel is the better wire set, something about Magnacor being really bad about customer service, and way expensive.

 

[TheTSiGuy]

I had magnecore's.

terrible wires imho, didnt last very long at all.

I've got some stockers in there now. much better

 

[redturbo66]

no problems here......

 

[10minutecar]

Wouldn't the CEL come on if the 02 sensors were bad?

I think we need to clean the injectors, new fuel filter, new spark plugs and wires and then maybe 02 sensors.

Think that would improve the mileage?

 

[LightningGSX]

The CEL should come on, but it doesn't always do so.

There are so many other things that can influence gas mileage too:

Alignment
Tire width
Tire diameter
Tire pressure
Friction(wheel and other bearings, hanging calipers, etc)
Viscosity of fluids(engine oil, gear lubes, etc)
Aerodynamic drag (Aftermarket Body kits, Spoilers, etc)
Quality and additives of the gas itself

probably 20 more I can't think of

 

[Sephroth57]

yeah i get about 200 miles to a tank... thats not really great but guess what, i can AWD burnout and smoke a ton of cars around here. you cant have everything...

 

[10minutecar]

This guy I know who is really good with DSM's is going to help us check the injectors with a stethiscope to see if they are working right.

He says that the fourth injector sometimes dumps fuel at the wrong time, has anybody else heard of this?

 

[RuBiCaNT5X]

I've noticed my gas mileage is particularly piss poor... There are a number of things I have done to fix the problem with no real resolution. Here is a list of the possible items that could cause it if they were malfunctioning or dirty... Right now I am getting about 16 - 17 mpg and that's just not acceptable..

Old Fuel Filter
Dirty Fuel Injectors
Bad O2 Sensors
Malfunctioning ECU
Big Exhaust Leak
Bad Tuning (running pig rich)
Lots of Boost (heavy foot on the gas)
Old Spark Plugs


I have changed/checked the following:

New Fuel Filter
New Fuel Injectors
1000 miles on Spark Plugs
New 95 ECU
No more than 30% throttle
No visible signs of exhaust leak(s)

Any ideas? Thanks for the suggestions in advance..

 

[midnght]

Do you vent or recirculate?

Are these town or highway miles you are comparing? If both how much time do you spend on either (EX: 80/20 highway-city)?

I am just trying to get a full run down. I would think that a majority of most of the guys miles are spent in the city and hitting boost alot. I do it, just the nature of the beast.

I get right around 220-240 on a tank before I refill. Not always down to the "E" but low enough. I spend about (70city-30highway).

 

[RuBiCaNT5X]

Originally posted by midnght
Do you vent or recirculate?

Are these town or highway miles you are comparing? If both how much time do you spend on either (EX: 80/20 highway-city)?

I am just trying to get a full run down. I would think that a majority of most of the guys miles are spent in the city and hitting boost alot. I do it, just the nature of the beast.

I get right around 220-240 on a tank before I refill. Not always down to the "E" but low enough. I spend about (70city-30highway).

That's the problem, I don't hammer on the boost and throttle, but my highway vs city driving is around 50/50 or 60/40.

 

[Defiant]

Originally posted by RuBiCaNT5X
Old Fuel Filter

Nope.

Dirty Fuel Injectors

Don't look now, but there's no such thing. Injectors run nice, fresh gasoline, jim-jammy full of detergent, surfactants, and -oh yeah- gasoline. Ever clean something with gasoline?

Bad O2 Sensors

Nope.

Malfunctioning ECU

Most likely candidate.

Big Exhaust Leak

If you can still hear the radio, the leak's not that bad.

Bad Tuning (running pig rich)

Well, have you been tuning it? Or is this just back to the Bad ECU idea.

Lots of Boost (heavy foot on the gas)

Yup. The amount you can push down mileage with the pedal is rather astounding.

Old Spark Plugs

You'll usually have rough-running issues with the arc welders they use in modern ignitions, if the plugs are that bad.

 

[RuBiCaNT5X]

I hate to disagree but fuel injectors can get dirty... I had a used set of Denso's get cleaned and flow balanced and after cleaning, they did flow better.. I just got a new EPROM 95 ECU with injector compensation from DSMChips.com. The injectors and ECU cancel each other out. The ECU had the new Denso injectors compensated for.

 

[gs-t_96]

I've heard low tire pressure can give you bad gass milage.

 

[c5chris]

Check your intake couplings. I had a leak in my UICP coupling that made my gas milage go down to 14mpg. It was running too rich.

A bad O2 sensor is also a culprit, as well as deflated tires and/or bad shocks.

 

[RuBiCaNT5X]

Originally posted by c5chris
Check your intake couplings. I had a leak in my UICP coupling that made my gas milage go down to 14mpg. It was running too rich.

A bad O2 sensor is also a culprit, as well as deflated tires and/or bad shocks.

I'll have to do an intake leak test this coming weekend when I am getting my bumpers repainted. I didn't think of that, thanks!

 

[eclipsed95]

Faulty 02 sensors can be a problem. My 2G's gas mileage improved after replacing them. Hope you find it. I get better mpg than you with my foot in the gas a lot

 

[RuBiCaNT5X]

In my logger, it doesn't show anything out of the ordinary in terms of going bad, but who knows... I might just end up changing them to rule it out...

 

[c5chris]

As the other fellow DSMer said, it might be something outside of your tranmission.

Check the ff:
1. O2 sensor (a bad one would give you crappy milage)
2. LEAKS! (intake and exhaust). I had a leak in my UICP coupling and did not even know it. I used to have 14mpg before I found this out.
3. Bad shocks
4. Bad alignment

Rubicant has opened a thread regarding this matter, maybe your answer is there. Goodluck.

 

[james13]

Since i have been running my bov to the atmoshpere..my car has been running rich between shifts.

what do i need to repair this?

i have heard to change o2 sensors (which one?)
and also change my spark plugs and wires..

any adivce is greatly appreciated


-james

 

[C2autoinc]

Venting to the atmospehere does cause it to run rich between shifts because air that has already been calculated by the MAF is now no longer present in the system. Replacing the 02 sensor/plugs/wires isn't going to help. To fix the problem you either need to recirculate into the intake, or if u reall want to vent to the atmosphere if your goin for that "fast n Furious" sound o0o o0o you can reposition the MAF which isn't too cheap, do a serach for that on google and see what ou come up with.

 

[damax03]

the only way to fix a rich condition that is caused by venting is to stop venting, end of story

 

[james13]

well i knew that and i forgot to post..i am switching my setup to a greddy that can be recirculated

-james