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

 

[bowens1414]

hey kalub thanks the 5lbs of boost s not stock wich you are very right on but im running 5lbs because the numbskull that had the car before wanted to run wastegate boost only. a single vacuum line from charge pipe to wg actuator he deleted the boost controller but luckily im getting one for Christmas. but yes ive heard o2 sensor. a subaru owner told me possible with no tune the fuel pump is giving enough gas for the stock 10-12 lbs of boost hence why im running rich. im trying to see if hes right and a MBC will fix my problem?

no codes either

 

[Jesse2012]

It wouldn't matter how much boost you were running, you only are using enough fuel for that amount. It's not like at 5 pounds you're using the same fuel tables as the stock 12 pounds. I'm in Rochester too btw, you should bring your car to log cabin next year

 

[bowens1414]

ha no shit where about because i parked next to a charcoal eclipse at ridgmont plaza about a month back. and yes when i get my exhaust and fix a couple more stupid things. and yes ill be going with my neighbor hes got a 06 sti makin 420hp at the wheels. haha it was some inspiration to get a nice car.

 

[Kalub]

hey kalub thanks the 5lbs of boost s not stock wich you are very right on but im running 5lbs because the numbskull that had the car before wanted to run wastegate boost only. a single vacuum line from charge pipe to wg actuator he deleted the boost controller but luckily im getting one for Christmas. but yes ive heard o2 sensor. a subaru owner told me possible with no tune the fuel pump is giving enough gas for the stock 10-12 lbs of boost hence why im running rich. im trying to see if hes right and a MBC will fix my problem?

no codes either

I'm saying wastegate pressure on a stock T25 is much higher than 5PSI leading me to believe you have a large boost leak. I would recommend that you check that first.

The Subaru owner has no idea what he is talking about (when it comes to our cars). Ignore him. With the way our cars meter airflow to distribute the fuel to the combustion chamber it's not possible for that situation to occur. The stock "tune" and ECU will handle the fuel delivery based on a few conditions, but mostly how much airflow the MAF meters. If your MAF sees 20lbs/min of airflow it will deliver the correct amount of fuel, however if you're leaking that air before it gets to the combustion chamber then you will undoubtedly run into a rich condition.

That's only one possibility, however If I were a gambling man I would bet money that your problem lies therein.

Purchasing a boost controller to increase your boost will only damage your turbo by overworking it if you have a boost leak. It will also cause you to have a higher IAT and possibly cause more damage.

 

[bowens1414]

ok im also getting new ic piping would this take car of a boost leak. also after this ill be doing a boost leak test to check.

 

[Kalub]

It very well could take care of the boost leak. You won't know until you make a tester and perform the test. Hell, you could have a crack in one of your end tanks, or something like that which would require a new intercooler.

 

[bowens1414]

o wow never thought of that but yes will be doing that boost leak test christmas break.

 

[NjDSMforLife]

Over summer i got a turboback exhasut had custom straight pipes down from local exhaust and megan racing downpipe. The next day after the ehxuast i had check engine light for my o2 sensor before i replaced it i was getting about 13mpg with it 150-170 a tank. i replaced the o2 sensor and no more check engine light but im still getting about same gas milage. Could this be injectors or something else im not to sure all the others forms say it was o2 but i got the new oem one from extreme psi i use 93 at gas stations before i got exhaust i was getting 210 miles a tank note i have only 53687 orginal miles on the car

 

[talon2gbuyer]

could be a plugged up fuel filter, that will ruin mileage if not caught.

 

[keithlag]

i had bad gas milage i changed my tps now it`s back to 22 mpg ish.

 

[BogusSVO]

Check the coolant temp sensor, some ECUs will default to thinking it is -40* and run pig rich.
I am not sure if this is true on a DSM.

Also chec the Vac line to the FPR, if the diaphram in the FPR has a hole, raw gas will be sucked into the intake.

If the vac line is wet or smells of gas, replace the FPR

 

[Tyeler18]

Check the coolant temp sensor, some ECUs will default to thinking it is -40* and run pig rich.
I am not sure if this is true on a DSM.

Also chec the Vac line to the FPR, if the diaphram in the FPR has a hole, raw gas will be sucked into the intake.

If the vac line is wet or smells of gas, replace the FPR

-71F is the default, so you are infact correct. Any boost leaks?

 

[Black-Out]

Check the coolant temp sensor, some ECUs will default to thinking it is -40* and run pig rich.
I am not sure if this is true on a DSM.

Also chec the Vac line to the FPR, if the diaphram in the FPR has a hole, raw gas will be sucked into the intake.

If the vac line is wet or smells of gas, replace the FPR

^^^ This ^^^

And also check to make sure the electric solenoid on the fire wall that runs the FPR is working properly because if it's not passing a vacuum signal it would be the same as removing the vacuum line from the FPR which would effect fuel rail pressure
(causing it to rise) and that would also make it run rich....
Correct me if I'm wrong but doesn't the computer cut the vaccum signal from the FPR when it see's boost go above 2-3psi to get the desired bump in fuel pressure under load?

 

[NjDSMforLife]

Thx guys dsm community is the best I get to work on it this week and by any chance can these problems be making me loose horse power and also I completey forgot to mention that sometimes when I get on the gas at like 40+ seems to be like a misfire

^ if that will narrow it down

Check the coolant temp sensor, some ECUs will default to thinking it is -40* and run pig rich.
I am not sure if this is true on a DSM.

Also chec the Vac line to the FPR, if the diaphram in the FPR has a hole, raw gas will be sucked into the intake.

If the vac line is wet or smells of gas, replace the FPR

just checked my fpr seems to have little crack in it

 

[Eagle96TalonTSi]

Yes its effecting performance. Youre running rich, which is not ideal operating condition for the engine.

Its just some basic maintenance. Go change your oil and filter, run fuel system cleaner with your next tank of gas.

If your FPR has a crack in it the obv replace it. If it doesnt fix it then its still a new part replacing a defective one.

Also, congrats on finding a 95 awd with 50k miles on it. Bravo

GL

 

[cdg7]

Probably already mentioned but i had a boost leak and lost 5mpg

 

[NjDSMforLife]

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check the coolant temp sensor, some ecus will default to thinking it is -40* and run pig rich.
I am not sure if this is true on a dsm.

Also chec the vac line to the fpr, if the diaphram in the fpr has a hole, raw gas will be sucked into the intake.

If the vac line is wet or smells of gas, replace the fpr

there's my crack but the vacuum line doesn't seem to be filled with gas is that just a diaphragm replace

 

[dsmracing92]

Check and make sure you dont have any intake/boost leakes, any vaccum lines, injector seals ect. Change the fuel filter and make sure timing is correct. Tps I dont think would effect it as much but cold start sensor def will. Do a resistance test and you will find your problem is somewhere in there. Good luck to ya!

 

[NjDSMforLife]

Check the coolant temp sensor, some ECUs will default to thinking it is -40* and run pig rich.
I am not sure if this is true on a DSM.

Also chec the Vac line to the FPR, if the diaphram in the FPR has a hole, raw gas will be sucked into the intake.

If the vac line is wet or smells of gas, replace the FPR

thanks man got my gas millage back up to about 180 a tank for coolant temp sensor was not in right spot completye moved fixed it

 

[yayocc393]

I'd definitely get new spark plugs too, simple maintenance but could have an effect on it.

 

[GofaST4life]

Over summer i got a turboback exhasut had custom straight pipes down from local exhaust and megan racing downpipe. The next day after the ehxuast i had check engine light for my o2 sensor before i replaced it i was getting about 13mpg with it 150-170 a tank. i replaced the o2 sensor and no more check engine light but im still getting about same gas milage. Could this be injectors or something else im not to sure all the others forms say it was o2 but i got the new oem one from extreme psi i use 93 at gas stations before i got exhaust i was getting 210 miles a tank note i have only 53687 orginal miles on the car

210 miles to tank is still horrible man. Sounds like the car needs a good work over.

 

[ErikTande]

I have a 1990 GSX and I only get about 19.5 mpg. 106,000 miles, all stock. Plugs and wires are recent. I have a feeling I should be getting better mileage, but I just bought the car so I'm not sure. Does 20 mpg seem low to anyone?

 

[RadDsm]

20 isn't that bad, depending on driving habits.

I recently went from 19mpg to 25mpg just by slowing down on the highway. Went from cruising at 70 to 55-60 (speed limit is 55 here).

Still have the same driving habits in the city.

This is on 560cc injectors and a 16G at 20psi and it sees 20psi every time I drive it.

 

[m240gunner34r]

Wow. I dunno what I'm doing right/wrong. Mine '95 GST is getting 33-34 mpg. 300-350 miles per tank

 

[wmrcs]

I got 22 mpg last week in town and I was stomping on it a ton. I'm stock though HA