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

 

[AlphaAssault]

what should gas mileage be for our cars? last time i calculated it, i got 26.5 mpg with a mix of 70% highway driving and 30% city stuff, with the a/c on and 300lbs of dead weight (bro and our bags, etc). is that about in the ballpark?

 

[Imback]

It's 90% chance it's the front o2..........

 

[crimsondragon]

Maybe you should actually calculate your mpg instead of saying it feels like 10. I say the same thing but when I calculated I got 22mpg out of it.

 

[videogamemaster]

Maybe you should actually calculate your mpg instead of saying it feels like 10. I say the same thing but when I calculated I got 22mpg out of it.

Well it isnt the same all the time... it ranges from 10-17

 

[knochgoon24]

I get about 17 if I'm on it hard. I can get up to 28 all highway. Normal is about 22. Just search for 'gas mileage' and you'll find all you'd ever need to trouble shoot.

 

[chris9149]

Well it isnt the same all the time... it ranges from 10-17

What kind of driving are you doing?? That will play a big part. WOT is gonna use up that fuel a little bit quicker!

 

[jrohner]

You have a lead foot, you can't tune the car for much better mileage doing that (except unhooking the wastegate actuator rod).

Do you have a logger to see if your o2 is working decent and starts cycling fairly soon after you get going? Any DTC's that could lead you in the right direction? Do you have a good working thermostat (and NOT a low temp one - you should be at 180 degrees or more cruising)?

I guess I get 16-22 on E85, way back when I ran gas I was getting 18-25. I set my wideband to run the car a little leaner, hoping it helps mileage a bit.

 

[videogamemaster]

You have a lead foot, you can't tune the car for much better mileage doing that (except unhooking the wastegate actuator rod).

Do you have a logger to see if your o2 is working decent and starts cycling fairly soon after you get going? Any DTC's that could lead you in the right direction? Do you have a good working thermostat (and NOT a low temp one - you should be at 180 degrees or more cruising)?

I guess I get 16-22 on E85, way back when I ran gas I was getting 18-25. I set my wideband to run the car a little leaner, hoping it helps mileage a bit.

i dont have any logging equipment as my budget is in the negatives right now and what do you mean by DTC?

 

[redline2k6]

diagnostic trouble codes.... do you see a check engine light on?

 

[videogamemaster]

diagnostic trouble codes.... do you see a check engine light on?

No BUT recently it has come on and i take it to checker and all it says is random misfire and when the light comes on the engine feels really sluggish accelerating... but whats weirder is she clears the code and the car works fine again. then it came on again so i took it down there she cleared i and bam it works fine again.

 

[DSM1G90]

What were the codes related to - did they give you any inclination on where to look at..like which sensor is putting out the codes?


There's a lot of issues with sensors that can really cause horrible mileage..the one right now that I can think of is, simply, the temperature sensor that tells how much fuel that the ECU sends to the injectors. If the sensor is telling the ECU that the water temp is only 77*, the ECU will tell the injectors to dump in more fuel to heat up the water-even though the water is at 205* when running.
..and either a stuck open thermostat, or no thermostat is also related to this cooling issue.

If I behave and keep the boost gauge mainly in the negative range during freeway driving at a 65/75mph speed, I can almost hit 30mpg on a good run with my Ply RS Turbo.

 

[JrCRXHF]

I have got 30 before with 60-65mph 2 lane roads. on the expressway 28 going 80mph.

In the winter with letting the car warm up for 10min every monring i get around 22-24mph with mix of city and highway.

I hope with the EMS i will be able to put the car into a lean burn on the highway speeds i have done this with my evo and picked up 1.5-2mpg.

 

[PieEyedPiper]

No BUT recently it has come on and i take it to checker and all it says is random misfire and when the light comes on the engine feels really sluggish accelerating... but whats weirder is she clears the code and the car works fine again. then it came on again so i took it down there she cleared i and bam it works fine again.

P0300 Random Multiple Cylinder Misfire?

 

[clockworktoy]

Uh oh, that doesn't sound too good. But this might actually explain at least partially why your getting such shitty gas millage. On my old engine cruising around 80 on the hwy i got an average of 20mpg. Then i started to drive like my old man and cruise around 65-70 and on my new motor im averaging around 25-28mpg. Oh yeah and id suggest replacing that front o2 like others on here have suggested that helped me a great deal.

 

[D20mobile]

What's your boost set at?

 

[MrBoxx]

How often do you get the Misfire code? What kind of plugs are you using? If you say anything other than NGK BPR6ES, there's the first thing to fix. What kind of plug wires are you using? How long since they've been replaced?

Boost leak testing procedures can be found on the VFAQ site (VFAQ Site - Visual Frequently Answered Questions). You should do a boost leak test every time you alter something in the post-compressor intake tract, whether it be a new turbo, intercooler, throttle body, fuel injectors, etc. The first O2 sensor is the one you have to worry about, and it is located just after the turbo, sticking out of the O2 sensor housing. Changing the O2 sensor precedure is as follows:
1. Unplug O2 sensor from electrical harness.
2. Use O2 sensor socket to unscrew old O2 sensor.
3. Use O2 sensor socket to screw in NEW O2 sensor.
4. Plug new O2 sensor into electrical harness.
5. Drink a congratulatory beer. (optional)

Before you ask, you can buy, or sometimes rent, an O2 sensor socket from almost every auto parts store on the planet.

Out of curiosity, has your car had a 6-bolt swap done to it? Is the MAS (between the filter and the intake pipe) plugged in all the way? Is the TPS sensor (on the front of the throttle body) plugged in all the way?

 

[videogamemaster]

the misfire code doesnt happen very often... the wires are NGK (probably haven't been changed in a long time) but the spark plugs are autolite (just bought them about 2 weeks ago).

I wish my car had a 6-bolt swap the MAS is plugged in all the way. The TPS i will have to locate and check tomorrow.

 

[jrohner]

My cousin had autolite plugs break on him twice in his DSM in a short time; the porcelain inside broke -- lucky it didn't fall off and wreck his engine). When I told my racecar engine builder, he said he's had it happen or heard of that happen with autolites too. Get NGK's, just the normal ones too, not some spendy ones (gimmicks that don't work).

If you don't have a logger, you can hook a volt meter to your oxygen sensor, which allows you to see easily how long it takes to start cycling (get into closed loop mode if it ever does).

 

[NightRider97]

When my GST was completely stock when I first bought it I got about 32-33 mpg (hwy), but it gradually got worse (was down to about 24mpg) and I knew what was wrong even before the CEL flashed because of it. Had a bad Cat and instead of installing a new $600 cat I decided to just put a "test pipe" on it and deal with the smog smell. Best thing I ever did to that car, as it brought my mpg up to 34-35mpg and it stayed there until I started having ignition problems.
But if your car is stock, and you're driving properly on the highway, with a good free flowing exhaust, and assuming your sensors and ignition are up to par you should see close to what i was getting IMO.

The O2 sensors are the first thing to cause poor mileage, and they are supposed to be changed periodically anyway. Change your plugs to the NGK ones listed above and replace your old O2 sensors, then report back.

 

[tsi1991awd]

I just got a 91 Talon TSI AWD...my gas mileage really sucks!! I just replaced the alternator and radiator since those were having problems...I know that doesn't have anything to do with mileage but I'm just saying.

My round trip to work everyday is 70-80 miles. The morning traffic is stop and go at times....this is all hwy miles. I do between 50 and 60 mph, that's all traffic allots anyways. I have to fill up gas every 3 or 4 days! I am only getting between 18 and 20mpg HWY. I don't take my car when city driving, I have my wifes Accord for that.

I barely ever go into boost, trying to preserve as much gas as possible...still doesn't really help at all.

What can I do?? There are no CELs or anything. Idles at 750, temp is good, everything is good. Oh also in the morning, its cold outside...has a hard time starting. Takes 3 or 4 tries then it starts right up. Don't know what that's about....but main thing I'm worried about right now is the gas mileage. Because that's ridiculous for highway driving.

Also, I cleaned the engine out with Seafoam through the brakebooster this past Saturday....improved my gas mileage by like 1mpg.

 

[JayRolla]

That really is not bad at all for an old car. If I remember right thats about what the sticker on the window says when you buy these cars, that they get 21hwy. On e85 I get 14-16 city and on pump I was getting 18mpg.

 

[tsi1991awd]

Is there one or two fuel filters? And where is/are they located? At the fuel tank? I know there were 2 on my 240sx, that's why I ask.

 

[tsi1991awd]

That really is not bad at all for an old car. If I remember right thats about what the sticker on the window says when you buy these cars, that they get 21hwy. On e85 I get 14-16 city and on pump I was getting 18mpg.

Really? Because there was someone on here saying they were getting 25-28mpg hwy. They were running over 20psi as well. Forgot who said it, but that wasn't too long ago.

Will doing exhaust and intake improve gas mileage at all? And will an exhaust leak (due to crack in exhaust manifold) cause bad gas mileage?

 

[No_Skillz]

Fuel filter is located on your firewall right under the battery.

Do you have any logging capability?
What maintenence have you performed recently?
Do you have any mods?

A crack in your exhaust manifold will affect your power, but not so much gas mileage.
I would suggest starting with a BLT.

 

[Booster-Shot]

Replace the filter, replace the spark plugs, clean your air filter, check tire pressure, check for boost leaks, etc.

All cheap things to do which will help with mileage and with performance.

Really? Because there was someone on here saying they were getting 25-28mpg hwy. They were running over 20psi as well. Forgot who said it, but that wasn't too long ago.

Will doing exhaust and intake improve gas mileage at all? And will an exhaust leak (due to crack in exhaust manifold) cause bad gas mileage?

This is most likely because they had an safc. They probably had the car tuned perfectly, which would get the most out of your car. Im guessing you dont have something to alter fuel maps right? If the leak in the manifold is of decent size, then yet it could account for decreased mileage and performance.