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

 

[tsi1991awd]

I might do the fuel filter on Saturday just to make sure its all good. Is there a certain brand that's better than the other or will one from Autozone be OK?

Also, I don't have any type of electronics hooked up. No data loggers or anything like that. I don't have one.

Also, for maintanence, I just did the alternator, radiator/thermostat/hoses on Saturday. Seafoam cleaned through brake booster and gas tank. New valve cover gaskets, even though it still leaks a bit from cracks in the cover. Mobil 1 synthetic 10w30 on the 27th of September with Mobil 1 oil filter. Also ran seafoam in the oil before the change.

I'm thinking it could probably use new spark plugs and wires...not sure about the cap and rotor. But I have no idea when the plugs were done. Wires look like they're in good shape.

Also, what's a BLT?

 

[JayRolla]

The guy getting that mpg is running a lean burn closed loop tune. Meaning instead of the normal 14.7 afr they are running as lean as 16.1 afr during cruise. I dont think its really safe and when I tune that way I see a lot leaner conditions under very light load, like 5% throttle.

 

[Booster-Shot]

Also, what's a BLT?

I believe it stands for bacon lettuce and tomato.






Just kidding, boost leak test.

 

[tsi1991awd]

Yea I thought he was telling me to have anice BLT sandwich, .

Also, I don't have anything to tune the fuel maps. I used to have an Apexi Neo but sold it about a year ago. Never used it anyways.

I have an EVO 3 exhaust manifold on the way. I was gonna do that with a PR O2 housing, 3 inch DP and 3 inch exhaust....but if I just did the manifold for now and the rest later, I'm sure it will be fine.

As far as the boost leak test, might be a stupid question but how is it done?

 

[Booster-Shot]

This thread will be the answer to all your boost leak needs.

http://www.dsmtuners.com/forums/new...se-troubleshoot-boost-leak-tester-merged.html

 

[tsi1991awd]

Thanks bro.

Do you know if the fuel filters at autozone are good, or do I need to buy a Mitsubishi fuel filter or what?

Also, to replace the fuel filter, how do I get the pressure out of the fuel line? Is there a fuel pump fuse I pull? I know on my 240, I believe the ECU fuse was pulled, motor started then it died on its own then the pressure was out of the line, then the filter could be replaced. But I don't know how it is done on this car....

 

[Booster-Shot]

Autozone should be fine, but I wouldnt go with something like Fram. Id buy one made by a better company.

As for the pressure, I think all you do is open your gas cap, disconnect the fuel pump, crack the fuel filter nut/bolt and keep a rag with you.

 

[tsi1991awd]

I'm having my wife go to the Mitsu dealer to order the filter. The guy told me over the phone that they'd have to order it and it would be in on Tuesday. So, we shall see.

Also, how do you disconnect the fuel pump? Is there a harness by the spare tire or what?

 

[DSM1G90]

You'll see a connector at the end of the tank where the pump is underneath that you unplug. Then start the motor and it will die when the pressure is gone.

Just have to watch for those banjo bolts at each end of the filter when you loosen them.

with my turbo, I can easilly hit 28/30 mpg since I do a lot of freeway driving myself to work and back.

Course, there is one BIG diff here: Mine's an FWD where you got an AWD and one's gotta remember that you've got an extra driveline running to the back and if your tires aren't exactly, or at least close to diameter and at the same air pressure, your viscous coupling is engaged and that motor is driving all four wheels on hard pavement - and that could be soaking up your mileage with that extra engine work.

not sure about the cap and rotor. But I have no idea when the plugs were done

Motor uses distributorless igniton, thus no cap/rotor combination. And while you get the filter, get some NGK BPR6ES plugs, gapped to .28

good luck - DSM

 

[tsi1991awd]

You'll see a connector at the end of the tank where the pump is underneath that you unplug. Then start the motor and it will die when the pressure is gone.

Just have to watch for those banjo bolts at each end of the filter when you loosen them.

with my turbo, I can easilly hit 28/30 mpg since I do a lot of freeway driving myself to work and back.

Course, there is one BIG diff here: Mine's an FWD where you got an AWD and one's gotta remember that you've got an extra driveline running to the back and if your tires aren't exactly, or at least close to diameter and at the same air pressure, your viscous coupling is engaged and that motor is driving all four wheels on hard pavement - and that could be soaking up your mileage with that extra engine work.



Motor uses distributorless igniton, thus no cap/rotor combination. And while you get the filter, get some NGK BPR6ES plugs, gapped to .28

good luck - DSM

Can the fuel pump fuse be pulled and get the same result, beside pulling the plug and yadda yadda. That's weird about it being distributorless....never had a car like that. Still learning new things everyday!

On the bolts for the fuel filter, what size are the heads? I have read a few things saying they're hard to get off, BUT most of those are on the 2g....they say the 1g is easier. I haven't been able to find anything about the 1g removal yet. Reason I ask is because I was going to go buy a slotted wrench so it would fit over the fuel line but still grab the bolt without slipping.

 

[AlaskanDsm]

I have a 1991 Talon TSi AWD and I got 28-32 MPG on the highway doing 50-60 MPH and staying off boost as much as possible.

Recent maintenence and modifications:
-stock motor /w Balance shaft removal
-17'' rims and tires
-3'' cat-back exhaust
-BLT (still leaking very slowly from the throttle body)
-Replaced injector seals
-Oil change
-New starter

Known issues:
-2 cracks in exhaust manifold
-semi-blown exhaust manifold gasket (only one corner, ~inch long section)
-transmission needs rebuild (bad syncros and bad internal bearings)
-needs new turbo (stock but has lots of build up and burns a little oil)
-radiator leaks from top right center bolt hole (some mechanic gorilla tightened it too far when installing the new one for previous owner)

As stated before, I think that if you replace the spark plugs, air filter, complete boost leak test and remove the issues, replace the fuel filter, then you should be in pretty decent shape. After all this is done, the other thing you might want to do is get yourself a logger of some kind. Palm pilots are generally cheap on ebay and the cable to make isn't too bad either. Or you can get yourself a serial laptop and use a logger in that fashion. Then you can see if your car is tuned and running properly.

 

[tsi1991awd]

Ill do the plugs this weekend and the filter probably next weekend....hopefully that'll help some.

 

[Laysher91]

+1 to the routine maintenance. Changing all that stuff will have an impact on your fuel economy. I used to get about 28 mpg with a healthy city/highway mix of driving in my turbo fwd Laser.

You can unplug the fuel pump in the trunk. Then cranking the motor over relieves the fuel pressure in the lines but be prepared to get some spillage once you crack open the lines.

I would recommend using a flare wrench instead of an open wrench on the fuel filter fittings so you don't round them off. Also make sure the fuel filter you purchase comes with 2 new crush washers for the banjo bolt on top.

 

[tsi1991awd]

+1 to the routine maintenance. Changing all that stuff will have an impact on your fuel economy. I used to get about 28 mpg with a healthy city/highway mix of driving in my turbo fwd Laser.

You can unplug the fuel pump in the trunk. Then cranking the motor over relieves the fuel pressure in the lines but be prepared to get some spillage once you crack open the lines.

I would recommend using a flare wrench instead of an open wrench on the fuel filter fittings so you don't round them off. Also make sure the fuel filter you purchase comes with 2 new crush washers for the banjo bolt on top.

I meant flare wrench on my post above...I couldn't think of the name at the time. . What size do I need? Also, if it doesn't come with the crush washers, can I reuse the old ones or should I buy new ones at the store?

 

[cavillac]

my old 1g didnt do much better, I replaced the fuel filter and it didnt help, allthough the car had 255,xxx miles on it and I'm almost sure that its the first time it was replaced, nasty brown muck came out of it..

 

[jaysonatticus]

Let us know what the outcome is. I was also getting terrible gas mileage and wasn't getting any codes thrown either. The culprit? bad ISC, bad O2 sensor. Replace those next if your problem still persists.

 

[awdmonster1904]

a good runnin dsm should get 22 mpg highway 19 streets
BUT, not all cars are the same >

 

[Mr Peepers]

A pre-o2 sensor exhaust leak can cause bad gas mileage.

Exhaust happens in pulses and can suck in outside air into the exhaust stream. When this o2 filled air goes past the o2 sensor it will read it as the engine running lean and inject more fuel accordingly.

 

[awdmonster1904]

A pre-o2 sensor exhaust leak can cause bad gas mileage.

Exhaust happens in pulses and can suck in outside air into the exhaust stream. When this o2 filled air goes past the o2 sensor it will read it as the engine running lean and inject more fuel accordingly.

I agree^^^ i had that problem too where the down pipe was loose i was getting bad mpg

 

[tsi1991awd]

I just replaced the spark plugs...it seemed to start up real strong. The car has been sitting from about 6pm until just a few minutes ago, about 10:30pm...it was cold, started nice. The old plugs looked like they were alright. Had a white coating on them, but it didn't look like the electrode had gotten too small or anything. Fuel filter is next...just hope I can get those bolts off.

I also checked the coolant temp sensor...didn't look like anything was wrong with the wiring, but then again it was dark and the flashlight wasn't really helping much. I'll check it tomorrow or this weekend.

I was thinking the O2 sensor might be bad, but I don't know....might replace it as well.

 

[Defiant]

My round trip to work everyday is 70-80 miles. The morning traffic is stop and go at times....this is all hwy miles.

They might be on a highway, but if you're in stop and go commute traffic, that's the same as city 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.

Normal mileage for that use, I'd say. Without the traffic, and cruising at 70 without hills or heavy acceleration, you might make 23.

 

[tsi1991awd]

Well, I guess you're right....I'm going up/down hills constantly. Not HUGE hills, but yea....damn Washington! We will see what happens shortly.

Can anyone tell me what size bolts the fuel filter are and how to change it? I looked at it last night and it looks like there is just one bolt on top and one on bottom, but everything I've read says turn the bolt on top while countering with another wrench right below it. I didn't see anywhere that this would be possible. All I saw was one bolt and then the line right under it....maybe I'm mixing something up here?

 

[jaysonatticus]

Maybe someone else can find the link? I remember seeing a forum on how to change the filter and there were a bunch of pictures. Two bolts, one on top and the dreaded one underneath. Get ready for a world of hurt and lots of cursing. It took me forever to get those stupid things off.

Probably could just reuse the same bolts if you wanted to. I remember the dealership gave me the filter and the bracket was already attached to it

 

[tsi1991awd]

I searched and couldn't find it. Maybe I'll spray some pb blaster on it tomorrow, then do it again sunday...and again monday. Then again tuesday. Hopefully I can change it out tuesday or wednesday. But by that time hopefully it will have penetrated and broke the sucker loose so I won't have TOO much of a problem.

 

[AlaskanDsm]

If you are looking into replacing your O2 sensor, I don't know if this applies to you but when I lived in Fairbanks, Alaska , If your car went in for an I/M or emissions and you got between a certian set number of exhaust output and how rich etc. then you qualified for a new and FREE O2 sensor. Thats how I got mine... But I don't know if anyone else does it.