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

 

[knochgoon24]

Optimal gas mileage is supposed to be around 55mph. I see it around 75 or so. 22-23mpg seems about right if you're doing 85-100 half the time. Typical for your driving.

 

[MrBoxx]

Don't drive 85-100mph. Period. It's illegal, unless you're in Germany. And even then, there's no GOOD reason to drive that fast on a public road.

 

[Srt4RacerDave]

When my TPS went bad, I didn't actually notice any difference in performance/ gas mileage, but I did get a CEL because of it. Didn't feel the need to replace it but I did any ways. The TPS is located on the front left side of the throttle body.

 

[tristanmb81]

do you have fuel leak?

 

[jdombrow]

Alright, cars completely stock, except for the K&N air filter, and i did all the free mods.

I also did a boost leak test, seems my 2g bov's holding almost no pressure so i picked up a 1g bov, waiting on the flange to make it work on stock upper intercooler pipes.

gone through 2 tanks of gas this week.. already gonna need to refill. First tank i filled and topped off and i made it to 100 miles before the fuel light came on. I realize there was probably still a gallon or 2 in the tank at that point so i went and refilled and added 10.5 gallons.

this last tank i drove like my dad does never hitting boost shifting around 2500 rpm an driving in 3rd at around 25 and 4th around 35-40 refilled at 115 miles and topped it off again adding another 9.5 gallons

currently at +60 miles and nearing half tank again any suggestions?

I realize with the boost leak from bov would affect it some, the cars setting no DTC's so im assuming the 02 sensors are good?

I also checked my exhaust and its got quite a few leaks, ran seafoam through the bov vacume tube, added seafoam to oil/gas tank planning on running injector cleaner straight through the fuel rail to get a thorough cleaning on my injectors.

I've heard front 02 sensors usually go and kill gas mileage but wouldnt this set a dtc? Also that the cat could be clogged or very restricting. Should i get a test pipe on? Or replace front 02 sensors? trying to keep cost down.

 

[robbo185]

if your bov isnt holding any pressure its a boost leak. Thats probobly the main source of the problem.

Fix all leaks and you shuold be good.

 

[Team Addiction]

x2 on the boost leak issue.
I just had a similar issue and happened to have my wideband on the car at the time. As soon as boost hits it went PIG RICH everytime. Basically pegged off the chart of what my wideband would read and induced rich knock. A must fix to say the least

 

[UGOT8UP]

I'm getting about the same milage on my car, no boost leaks and no DTC for the o2 on the link, only code I have is for the knock sensor.

 

[this_tsi_8urV8]

Do you have any fuel leaks? That mileage is really bad. That's about what some people get running E-85. You o2 might be bad or even your coolant temp sensor.

 

[TofastForU_99]

check your injector seals. i had a 95 TSI and i was going through so much gas and when i turned my heater on it would blow the smell of gas into my car. but i think you would have noticed the smell of gas outside your car if you had this prob just a suggestion.

check your fuel filter especially if its the stock one.

 

[98 Mits GS-T]

x3 on boost leak. And for UGOT8UP, that knock sensor code will affect your gas mileage because it will run rich trying to cool down the cylinders. Something to check out anways.

 

[misphit]

x2 on the 02 sensor and coolant temp sensor. If your temp sensor is off, ecu could be dumping fuel thinking that it needs it. Same goes for the 02.

 

[jdombrow]

x2 on the 02 sensor and coolant temp sensor. If your temp sensor is off, ecu could be dumping fuel thinking that it needs it. Same goes for the 02.

anyone know what a new temp sensor/o2 sensor costs?

I'm getting about the same milage on my car, no boost leaks and no DTC for the o2 on the link, only code I have is for the knock sensor.

x3 on boost leak. And for UGOT8UP, that knock sensor code will affect your gas mileage because it will run rich trying to cool down the cylinders. Something to check out anways.

No codes showing

check your injector seals. i had a 95 TSI and i was going through so much gas and when i turned my heater on it would blow the smell of gas into my car. but i think you would have noticed the smell of gas outside your car if you had this prob just a suggestion.

check your fuel filter especially if its the stock one.

No gas smell, injectors look fine, ill be running a special injector cleaner through the shraeder valve on the fuel rail this week hopefully, along with making sure the ecu's triggering them.

As far as the fuel filter do I need an OEM replacement? or what should it be replaced with? also anyone know cost of the OEM one?

Hopefully will have a chance to run a pressure test on my engine this week. To see how bad it is for being 14 yrs old


Had the car on a hoist about a week back checking for an exhaust leak.. found out after the cat the exhaust was rusted to shit huge leaks after both resonators, and my mufflers not even connected to the exhaust.

Do you have any fuel leaks? That mileage is really bad. That's about what some people get running E-85. You o2 might be bad or even your coolant temp sensor.

The sheet metal protecting my gas tank was completely rusted out ( put my hand through it pushing up ) and is held up by zip ties.

But as far as a Fuel leak I didnt smell any gas unnder it or see anything dripping.

Anyway to test this besides using a chunk of cardboard and letting it sit overnight?

 

[UGOT8UP]

x3 on boost leak. And for UGOT8UP, that knock sensor code will affect your gas mileage because it will run rich trying to cool down the cylinders. Something to check out anways.

I'm not running rich though, I'm stoich crusing and idle with 11.1 WOT, I don't have my WB hooked to the ECU yet so I post log of it, just the link esimate. So I have no clue what my problem is.

 

[misphit]

jdombrow, your 02 sensor will not always throw a code if it's malfunctioning. Would be nice if you had a logger to give you a slightly more accurate indication as to how lean/rich you're running.
I'm also pretty sure you can test the coolant temp sensor with a multimeter to see if it's determining temperature correctly... search around these threads because instructions have to be in here somewhere.

 

[jdombrow]

ill test the 02 sensor and temp sensor tommorow. thanks

 

[lowexpectations]

I love how everyone chimes in with a boost leak is always the problem for everthing.

If it is burning fuel even while out of boost I seriously doubt it is a boost leak causing the excessive fuel consumption.

Faulty O2 sensor would be the first place I looked. Now I am no genius but if it was the fuel filter, I don't think he would be burning too much fuel either. A clogged fuel filter would restrict flow not make it flow more.

If you think the cat is going one indication is the cat will start glowing after it has been driving for a bit.

 

[turboglenn]

I love how everyone chimes in with a boost leak is always the problem for everthing.

If it is burning fuel even while out of boost I seriously doubt it is a boost leak causing the excessive fuel consumption.

Faulty O2 sensor would be the first place I looked. Now I am no genius but if it was the fuel filter, I don't think he would be burning too much fuel either. A clogged fuel filter would restrict flow not make it flow more.

If you think the cat is going one indication is the cat will start glowing after it has been driving for a bit.

One solution...speed density..that way you only measure and add fuel to the air that's making it in the manilfold... you can tune around TB shaft seals and small crap like that, but big boost leaks on speed density may cause some issues... MAF sucks

 

[lowexpectations]

One solution...speed density..that way you only measure and add fuel to the air that's making it in the manilfold... you can tune around TB shaft seals and small crap like that, but big boost leaks on speed density may cause some issues... MAF sucks

I agree with you completly glen. I just get kind of tired of reading when someone posts a problem the first thing someone says is boost leak.

 

[jdombrow]

ill check the sensors today, plan on getting ecmlink this month

 

[jdombrow]

way to test maf?

 

[robbo185]

I agree with you completly glen. I just get kind of tired of reading when someone posts a problem the first thing someone says is boost leak.

dude his first sentence was his bov is leaking........thats a boost leak.

 

[DSMboostin15]

If you find that the sensors are bad, a great place to buy O2 sensors is here: Welcome! They have OEM replacement O2 sensors (NGK, Denso, Bosch) for ~$70. As for the coolant temp sensor, you can pick one up from your local auto parts supplier for ~$30.

 

[90gsx16g]

I get around 15 or so mpg.. and i drive an auto. If i beat on it you can ltierally watch the gas gauge go down. I would say check your fuel pressure and double check for any boost leaks .
- And your positive there are no other aftermarket parts on the car?
-Does it seem like it drives normal? ( assuming you've driven a normal dsm )
-any sputtering or bogging?

 

[wises4057]

hey guys, so I've always owned awd slightly modified dsms(and averaged around 23 mpg), and now I own a 97 gst spyder, and i'm averaging about 21 to 22 miles per galon...1/3 of every tank is highway mileage, and then even the other 2/3s is mostly 45 and 35 mile an hour roads with limited stopping. my question is, is that the kind of mileage I should be expecting from a fwd turbo dsm or should I be getting better? I've changed spark plugs, going to do wires and fuel filter soon. and there are no check engine lights on. car seems to be running fine from what I can tell. oh, and it is completely stock. any advice would be awesome!! thanks