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

 

[EagleTalonTim]

i just filled up my tank 150 miles ago, and it was almost empty when I filled it up again. I am running the stock set up 13g on my 93 TSI talon Automatic. Everything is stock except for the brand new K&N air filter with a chopped can.

There are no boost leaks except for the bubbling in the oil pan (can't figure that one out.....searched already), no exhaust leaks, injectors are stock white tops, no fuel leaks, gas cap seals just fine, no CEL, timing is 5 degrees from TDC, spark plugs are NGK's gapped at .32, fresh oil and filter, new plug wires, new timing belt & tensioner, I did not run the A/C at all, boost gauge reads 9 psi at WOT and -18 at idle, and the compression is at 155 across the board. The turbo has no shaft play and spins very freely. I drove the car normal staying out of boost for 90% of the time.

For some reason, if I crank the car and put it in to drive or reverse then give it a little gas and let up, it will die. If the car is already running and I have driven it for a few seconds, it will not die but idle around 600 rpm's. The transmission is freshly rebuilt with fresh fluid. I adjusted the BISS screw to idle the car at 900 RPM's and it still does it. I also tried 1,000 RPM's and it still does it. All vacuum lines are in their correct places and have no leaks. While accelerating, I can hear the turbo spool up when it is supposed too. I am not sure if you are supposed to hear that with stock piping, but I started hearing that when I chopped the air can and put in the K&N filter. I did not mess with the MAF honeycombs. I did take out the silencer though.

Please help! I am really stumped

 

[brodiedehass]

Your oil is bubbling in your oil pan during a boost leak test? Never heard of that happening. Only way it could theoretically get pressurized air would be thru the turbo oil return.
Did you gas mileage reduce directly after the K&N install?

 

[McFox]

O2 sensors could be the culprit. (If not the bubbling oil)

When was the last time they were changed/how many miles on the car?

 

[EagleTalonTim]

I am not sure about the O2 sensor. It does not look that old. I just bought the car a few months ago and had to rebuild the head. The bad gas mileage was from the first time i drove it, not from the K&N. Could this be a bad turbo?

 

[brodiedehass]

It could be many things, ECT (Engine Coolant Sensor), O2 Sensor, etc. It would be best if you could give us a MPG rating buy dividing your mileage driven since last full tank by the gallons it took to top it off.

Did you ever drive it before the head rebuild and know the previous fuel mileage?

 

[EagleTalonTim]

The MPG is 13.98. I was not able to drive the car before because I bought it with 2 bent Intake Valves. The head was pressure tested and it was fine. I have noticed that the engine temp sits a very very small amount lower than the middle mark unlike my N/T talon which sits a noticeable amount under the half way mark. I put a new radiator and thermostat in while rebuilding the head. Could that be a bad sensor, or is it really getting that hot. The under hood engine temp gets alot hotter than i am used to from my N/T talon.

 

[EagleTalonTim]

I have also noticed that when I start the car in the morning, it idles at 800 or 900 even when the motor is cold unlike my N/T that idles higher when it is cold.

What is the name of the coolant sensor that has one vertical and one horizontal male connectors? I know the one right next to that with one wire is what controls the temp gauge. Are they both called coolant temperature sensors?

 

[95TalonOwner]

My bet is on the o2 sensor or the MAF.

 

[EagleTalonTim]

I just ordered an 02 sensor from Oreilly's and it will be in tuesday. Hopefully that is my problem. I will be double checking for boost leaks as soon as it clears up outside.

 

[Littlebeeper]

when i didn't plug in the temp sensor to the computer the car threw a CEL (i belive there is 1 sensor for the guage and 1 for the computer). a data logger would answer a lot. my money is on the O2 sensor and maybe the temp sensor if the car doesn't idle high when cold

 

[sYnOnYx]

So ever since ive had the car ive replaced :

Trans rebuild + new tq converter
new front 02 sensor
plugs & wires
used a can of seafoam on it

i have a SAFC and its at like -20 at some points and it STILL runs rich as hell !

and im only getting about 15mpg and about 200-220 miles per tank.

what else could be wrong ? to cause a 100 mile per tank loss ?

i mostly freeway drive and hardly get on it.

 

[turboglenn]

is there a bigger turbo, fuel pump, injectors and some boost turned up to go with that AFC? If so that's probably a good part of it right there. if the car is completely stock, take the AFC off or set it to ) al the way across the board.

 

[sYnOnYx]

well its got about 200 per tank even before the afc, and its negative across the board meaning less fuel. so i should get better milage. only boost leak im aware of as of a week ago is a very tiny one at the throttle body. otherwise its good.

i forgot to also mention , the tire pressure is correct. and air filter is new aswell.

 

[kenamond]

If you have stock injectors, your trims should be around 0, right? Only reason for SAFC is either to fine tune stock injectors (small trims) or hide air from the ECU for larger injectors.

 

[viprez586]

Just a little FYI. Oxygen sensors rarely go bad, what happens is they get clogged or dirty. You can use rubbing alcohol to clean them and they should work properly.

 

[Defiant]

Miles-per-tank is brainlessness. Set your trip odometer and determine your miles per gallon. It's the only measure with any accuracy, and you can usually count on at least a 2MPG variance between tanks.

 

[dsmotors09]

Well, I just bought my 96 talon tsi awd (2-3 months ago), and I've been getting about 14 mpg. My check engine light came on, and yesterday I took it to Toyota (Only because most my family works there) So they decoded it for free. It appeared to be Idle Air Control (IAC), and Cylinder 1 misfire. Any suggestions?

 

[matthewdesigns]

A bad IAC usually has the effect of creating idle surge, as when the car won't idle steady. Is your's doing that?

Cylinder 1 misfire could be anything from the ECU to the plugs. Replace the plugs with NGK BPR6ES gapped to .028, and put on some new wires as a start. The car probably needs them anyway. If that doesn't solve the misfire, then you'll need to go farther up the line, like the coil pack and potentially the ECU.

Bad mileage can also be caused by a bad front o2 sensor. Might try replacing that as well.

 

[dsmotors09]

Yeah, it has that rolling strong idle. They said I had the cheapest plugs, and they said I would be better off getting factory ones from Mitsu. I also asked about the gas mileage and if it would be the o2, they said probably not? I was thinking about going with the 8's on the plugs instead of factory. What else could it probably be?

 

[dsmotors09]

Oh yeah they also said, if the check engine light comes back I would probably be better off replacing the IAC motor.

 

[matthewdesigns]

You do not need to go with 8's until you are running some serious boost and generating a lot of heat in the cylinder. I have 7's on my 20psi daily driver and they are just barely needed on my car. Just get 6's and you'll be good. They are proven to be the best plugs for these cars in stock/near stock configuration. And don't waste your money on iridium, split tip, etc.

The idle issue then is most certainly the ISC. New ones are pretty expensive (hundred$). You can usually find a used one in the classifieds here or on dsmsource.com for less that $100. Don't forget to replace the o-ring gasket at the same time or you'll have a giant boost leak there.

The o2 sensor can definitely throw your mileage in the gutter. It's what determines the amount of unburned fuel that's left in your exhaust, and that value is one of the things which signals the ECU to adjust the amount of fuel being injected. It's not the only thing that can affect mileage, but a common one. Another common one is keeping your right foot planted all the time. When I drive like that my mileage drops to about 16-18 mpg.

 

[sprfunk]

NGK BPR6ES gapped to .028-.032 is the way to go unless your car has a ton of mods. Then people suggest 7's. I would try this first because it is cheap and it cant hurt. It might end up saving you a few bucks.

 

[dsmotors09]

Which heat range do you suggest and where is the best place to buy them? Also, I heard the bosch o2's are worthless?

 

[mitsuclipsegsx]

Also replace the fuel filter

 

[matthewdesigns]

Which heat range do you suggest and where is the best place to buy them? Also, I heard the bosch o2's are worthless?

The numbers 6.7.8. etc. are the heat range indicators. Get the 6's.

The Bosch sensors are not a plug n play. You have to manually wire them into the harness. This is why people say they are crap. AFAIK, they work as well as an OEM sensor.