dsnowgs4

So I just changed my cars spark plugs, cleaned out my throttle body, reset my ECU, and cleaned/reoiled my K&N Air filter.

I am on my third tank of gas since the tune up and here is what is happening.

1) The car is a rocket ship now. It runs very smoothly and its very powerful. I raced a guy two days ago with an STI and basically was neck and neck with him to 110.

2) I have had 3 tanks of gas go through the car, and I cant get more than 50 miles for a 1/4 tank. I am getting 100MPG for a half tank.

Before you start saying I should ease up on the gas pedal, I have made it a point on the last two tanks to drive normal/conservatively. (STI fun was on the 1st tank of gas.) I also have not changed my driving habits to warrant such a big swing in MPG. Always the same HWY/City mix.

In the past, I normally could get 70-100 miles out of a 1/4 tank, and around 150-200 miles for a half tank. So my mileage has been cut almost in half. I would have thought the plugs/and tune up I did would help with mpg not cut it in half.

Anyone have any ideas?

GS300ToM

iTrader: (32)

i made a thread about this....

dsnowgs4

https://www.clublexus.com/forums/gs-...-is-great.html

Yes but it looks like you got the results I was hoping for. I went the other way. I am Losing Gas Mileage, not gaining more.

Maybe I should try cleaning the MAF sensor.

Infamous 1

iTrader: (20)

Try a fuel line and injector cleaning. It's just an additive that you mix with a full tank of gas.

Kyle Harty

iTrader: (17)

Have you kept track of how many gallons you put in to how many miles you've driven? Reset the trip odo. when you fill up and then next time you fill it divide the miles by gallons?

This would be the only real way to determine what kind of mileage you're getting, I don't trust the 1/4 and 1/2 marks on the gauge to be completely accurate.

It's possible that the lowered mileage is a result of resetting your ECU. IIRC your car will run richer will the ECU relearns your driving habits and will therefore burn more fuel. I'm not sure how long it takes to adjust the curve back to a more sedate level but this is something to consider/keep an eye on.

dsnowgs4

Thanks for the idea I will keep that in mind. I was even thinking about resetting the ECU again.

I agree that the gas guage is not 100% accurate, but I have seen some consistency with it over my 4 years of ownership.

I religiously reset my trip odo everytime I fill up, and like you; I thought on tank 1 & 2 that maybe I did not fill it up all the way, but on tank 3 I topped it off so that gas backfilled into the filler area. This third tank was full and doing the same thing. I mean after 15 miles my guage comes off of "F". I understand I would not see the +5-5% fuel economy with my method, but man the 50% drop is quite noticeable.

I hope your right and the ECU learns that it can ease back on the fuel mixture.

Emil

fuel filter?

GS4_Fiend

iTrader: (22)

I got a GS430. The max I ever got was 23 mpg. That is mostly freeway and partial city. I do 75-80 mph on the freeway.

BCP43002

Do you have any codes? A bad oxygen sensor would decrease MPG. Check you MAF sensor to see if the oil from the air filter was inhaled. What type of air filter - cone(mine) or the flat drop in. I waited 2 days for the oil to dry instead of the 20 minutes the instructions says.

MJImport

iTrader: (1)

+1 to that. I have read this elsewhere on CL, excess oil can cause some nasty side effects...

Kyle Harty

iTrader: (17)

Definitely something strange there! It shouldn't be that big a differance unless something is out of wack! BCP may have hit on something with the oil and MAF, worth checking out.

I hope you can figure out what's going on and get things back to normal!

sleeper408

iTrader: (6)

One of two things. Yes, you should let the ECU re-learn your driving habits. Do NOT reset the ECU again. It will only further delay the "re-learning." I failed the functional part of a smog test once because of this. I used to own an '01 Cobra with hi-flow cats. It failed smog with this on, so I put on the stock crossover pipe (and reset the ECU) and went back to the smog station a few days later. In the few days of driving, I noticed my MPG was substantially lower. It did pass the emissions portion but failed the functional portion because the ECU had been reset so recently. I drive around for another two weeks and used a code scanner to verify that everything was fine with the ECU and no codes were present. At this point, the MPG had gone back up to normal levels (which was not that high considering the amount of mods I had on the car-- but still, back to normal).

The other thing is that sometimes people "over-oil" their K&N filters. This should be only applied modestly after cleaning, not liberally. If you applied too much oil, this may end up at the MAF sensor which would throw off A/F ratios and cause you to run rich, hence why you are suffering such poor gas mileage. You need to remove the air box, filter, and check your MAF sensor.

Rock-a-Lex

iTrader: (4)

...another possibility is that your car previously adapted to the:

Older plugs
Dirtier throttle body
Dirtier air filter
and so on and so on

which did NOT show you great performance at that time and the car did not demand as much fuel to operate where it was at.

Now, you changed parts and cleaned things and free'd up a lot more power. After this and after the re-setting of your ECU it is "learning" the new fuel requirements to operate (with the free'd up power). Simply put, the car is using more fuel to produce more power; that's the unfortunate trade-off to modding and it gets worse ;(. Eventhough you may "test" your efficiency by driving rather conservative the ECU has made it's changes already and will ALWAYS burn more fuel now.

Don't get me wrong you can still try to hit other areas that will guarantee better mpg such as cleaning your MAF sensor with MAF cleaner as what others have suggested.

I am just trying to give you a MUCH different response; it is always good to look at both sides of the coin. GL! Oh, and for the record...I too get 14mpg (65 city/35 hwy)

dsnowgs4

Thanks for all the imput guys.

1) I will have to double check my MAF sensor as I did not clean that. Especially with the newly oiled filter.

2) I have a K&N drop in filter, and yes I probably did not let my filter dry fully and/or possibly over oiled it a bit.

3)Rock-a-Lex I have thought of this, as my car is running very smoothly and fast now. I.e. More power needs more fuel. I guess I was just hoping for the fact that the dirty set up was giving me less power and worse fuel efficiency. I love the way the car feels its just annoying to see the MPG drop so much.

4) BCP43002 -any ideas on how or why my O2 sensor would have gone bad directly after the tune I mentioned. Plus if the O2 sensor was bad, wouldn't the car run like crap and not be strong and powerful feeling? I have not researched the effects of a broken O2 sensor.



Anyway thanks for your help guys.
I will play with it some more and see if the MPG ever retruns/adjusts.

As of this morning on tank 3 I got all the way to 68 miles for my 1/4 tank, it may be adjusting but I am going to clean the MAF sensor anyway.

UDel

If you have a K&N airfilter and have oiled it in the past and recently and possibly over oiled it then it is highly recommended that you clean your MAF sensor as a dirty/oily MAF sensor is most likely your culprit for bad fuel economy. The other most important thing is to keep your tires properly inflated and if you want better fuel economy to keep them inflated a little higher then you the avg but within the tolerances of course. Under inflated tires will kill your fuel economy. Remember to keep your windows up too when driving and make sure there is nothing in the backseat/trunk causing unneeded weight.