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

 

[MichaelSuch]

Change the fuel filter for sure and a bad o2 sensor will give you bad gas mileage... My '92 GS-T was making around 28-29 MPG with mostly highway driving with only a K&N air filter on it any nothing else. Change the fuel filter and o2 sensor and see where that takes you...

 

[wises4057]

yah, o2 is probably going to be my last thing. do you know how to get the charcoal canaster off underneath the battery bracket...I tried changing the fuel filter earlier this winter(was like 18 degrees outside) and gave up trying to get that off...it basically seems like you have to get the charcoal canaster off in order to get the battery bracket off. I've changed filters on these before, but never had to deal with a charcoal canaster on any of them(were all removed already)... thanks!

 

[kenamond]

21-22mpg with mostly city driving is normal. If you get on the gas much, 18mpg or less is typical. Highway mileage of 30mpg is typical. Boost leaks can make you run rich, too.

 

[wises4057]

well it's not really city driving...I mean it's really 40-60 mph the whole way, with mayb 2 stops in a 9 mile drive. the one full highway tank(65 the whole tank) that I've done of driving got me about 26-27 mpg.

 

[AlaskanDsm]

the O2 sensor was the culprit for a lot of loss in my mileage. But once I replaced it I saw an extra 5MPG. Also, even though you said you had stock, I noticed that witha larger turbo, increasing turbo spool, You seem to also notice an increase in gas mileage up to a point. On my car I keep off the gas as much as possible and shift under 2400 RPM as thats where the 14b spools for me.

 

[jrohner]

Probably your driving habits more than anything. 22mpg is decent unless you're on the highway all the time without stops. Plus if your other car had exhaust and a different air filter, that could have helped the mileage some. Also, the mileage is always a little lower when it's colder outside.

 

[wises4057]

yah, I dont get on it very much. i would think it'd be the o2 sensor, but no check engine light. on my 95 awd, the check engine light came on as soon as my o2 went bad and my fuel mileage dropped...would think the 97 would through a check engine light right away as well. but it is a dsm, so I could be wrong! ya, I know fuel milage is down when you have cold weather, but my awd has been getting better mileage then my fwd, so i thought I would just check.

 

[roadtrip_69]

After reading these entries I feel kinda special. My 99 gst spyder gets about 27 mpg highway and about 22-23 city. Which works out to about 360 miles a tank if i want to push it. And i have the stock turbo maxed out. Along with a few additional mods, like intake and exhaust. Though I try not to use it as much as i can restrain myself. I have read that these cars stock SHOULD get close to 30 mpg on the highway. Seems like that is a far fetched estimate by Mitsubishi if these forums are accruate .

 

[garrek11]

ok i read all the threads pretty much and you guys give the same answer for this question. why am i getting such horrible gas milage? i get maybe 150 to a full tank around town. i know i have a bad boost leak throu my BOV and i need a new intake system but that shouldnt make my gas milage that bad should it?

Mods (TurboXS BOV AKA POS!, AFPR, 255 fuel pump and 3in downpipe)

 

[Lofer]

You should put a stock bov back on if you cant recirc that one or the flange leaking where it was welded on i would recirc your bov then do a boost leak test.

 

[booostedGSX]

Boost leaks will kill gas mileage. I agree on putting the stock BOV back on. They're a great BOV and will get rid of that boost leak you've got. Try that and check back.

 

[juanpac0]

Also check your timing.

 

[canada4g63]

turboxs bov aren't pos you probably don't have an shims in it. It is opening to soon, and what turbo?

 

[EagleTalon1993]

Are you running with a thermostat, If not that can zap some of your MPG. I would also check the simple stuff and make sure there are no fuel leaks from the tank all the way up. My return line was leaking slightly not enough to smell it when stopped or while moving but leaking none the less.

 

[snowy91]

Not trying to hijack the thread but going of the OP, the first thing for gas mileage should be a new intake and a boost leak check?

 

[juanpac0]

Not trying to hijack the thread but going of the OP, the first thing for gas mileage should be a new intake and a boost leak check?

Check your air filter, change your plugs to NGK BPR6ES, check plug wires for damage, replace if necessary, change oil if you don't know the last time it's been changed, do a boost leak test, and check your timing. If you still have issues do a compression + leakdown test.

 

[zhjiln]

wow... 150 for full tank is just HORRIBLE.. something is not right... i fill up at 250 miles...thats 12 gallons used for me...

and just like everyone says.... bad O2 ... boost leaks, easy with that right foot.... MAF/MAS is bad... bad plugs...

 

[rush2252]

Probably the same reason Ive gone from 245 to 180 miles per tank when swapping to the 16g. You need a tune. I see alot of bolt ons with no tune. Believe it or not if you do the math buying "LINK" will actually pay for itself in just a few months of gas savings depending on your daily commute Im already saving up.

I would almost gurantee with new plugs/wires/fuel filter/injector cleaning etc your still going to see bad MPG's until somebody tunes it. Trust me on this. My car runs Great minus the gas mileage

 

[snakeyes711]

Check your air filter, change your plugs to NGK BPR6ES, check plug wires for damage, replace if necessary, change oil if you don't know the last time it's been changed, do a boost leak test, and check your timing. If you still have issues do a compression + leakdown test.

Explain why for the bpr6 plugs? I replaced mine not too long ago but did bpr7's.

I just checked my mileage with it being 18.5 around town. Also, I did a LOT of playing while I was checking it too.

 

[rush2252]

6 is the heat range. NGK uses a assbackwards heat ranging sequence. Normal plug manufactures use a larger number for a hotter plug NGK does it the opposite. A 6 is a slightly hotter plug than a 7.

 

[snakeyes711]

Right I understand plug heat range nomenclature . My question is why would he want a hotter plug?

 

[juanpac0]

Search the spark plug faq thread, with a 16g there's a 99% chance that you aren't going to need a colder plug than the bpr6es. The faq has heat ranges explained pretty thoroughly, I'd link you but I'm on my phone.

 

[PIMking]

I get around 190 miles to a take just driving around town. I will get 190 on a half a tank if I do some highway driving.

I got lots of mods and no real tune.

 

[rush2252]

I get around 190 miles to a take just driving around town. I will get 190 on a half a tank if I do some highway driving.

I got lots of mods and no real tune.

Id donate my left nut to get 190 to a half tank. Im also auto which kills the MPGs but 13 sucks

 

[garrek11]

hwo do you check timing on these cars without a scan tool?