mysterious fuel consumption problem

[Jakob]

hey there,

i have a 1995 nissan s14 with factory swapped notch top sr20det.
i own that car since summer 06. when i drove it back home i got around 25mpg!
it's a european model and used to have EGR fitted, which i removed as i replaced the blown turbocharger with a rebuild unit.

for almost 4 years now i have a big problem with that car when it comes to fuel consumption.
i get around 10mpg no matter what i do.
over the years i researched and looked for answers and replaced several things without results. it stays at 10mpg

the strange thing is that my car pulls great power deilvery is good and the car is fun to drive when i ignore the fuel gauge :ddog:

it doesnt smell like fuel inside the car or outside the car, i dont have any stains under the car or on the underbody around the gas tank or anything. the enginebay doesnt smell like fuel either.

the spark plugs look like this:
http://farm5.static.flickr.com/4047/4464252049_237b0bba7c_o.jpg

and here's a list of things i replaced:
fuel lines
fuel filter
engine temperature sensor
o2 sensor
spark plugs
maf
knock sensor
throttle position sensor
fuel pressure regulator
fuel pump

and i've checked the following things:
IC piping for leaks - i have none
pressure tested the tanke for leaks - none
fuel tank o ring seal - is fine because it didnt leak during the pressure test
fuel pressure is fine too
timing is right
injector o-rings are fine
injectors are fine
no exhaust leaks

well that's it so far. i'm basically done with my knowledge and logic to diagnose my problem.
what kills me the most is that my car runs and pulls fine, the plugs look ok (i compared them with another s14 with around 20mpg and they look almost identic!!) and i do not smell any fuel or have any stains.

if you need further information or pictures or anything else to help me diagnose this problem i will provide them.

please help me zilvia :sadwavey:

[nathanong87]

4 years at 10mpg...u must really love driving this car =(

any gauge readings? have you ever been tuned?

[Jakob]

it has never been tuned it's completely stock.

i will hopefully get a wideband from my buddy today and check out what it says.

ya i like that car a lot :) last year i wasnt driving it at all because i broke my transmission but i was able to save up enough money to buy nice wheels

[Jakob]

got an aem uego wideband hooked up to it today the results are:

cruising afr: 14.5-15.5

full throttle afr: 12.1-11.0

idle afr: 14.7-15.1

lol perfectly fine, i mean almost every sensor and what not is new and everything is sealed and properly adjusted

means my fuel doesn't get lost in my engine thats what i already knew looking at those sparkplugs haha!!

my theory is that i lose fuel somewhere during driving because i can't explain it myself. i RARELY go full throttle and if i do then its for one gear at most to overtake a car. so its not a user error i think.
but i dont smell anything nor do i have leaks under the car or does it smell like fuel near my gas tank.

[Jakob]

bump :D

i'm going to pull the tank this weekend to check for any cracks and stuff.
i doubt that i will find anything but we'll see.

[steve shadows]

I would check out your VTC,

Make sure the VTC is functioning 100% correctly.

If this is not working it could affect your cars driveabililty down low.

I would also get a vacuum leak tester (the one that uses a smoke machine) so you can find out for sure if you have any leaks,

Make sure your engine doesnt have any exhaust leaks, make sure your o-rings on your injectors are perfect and make sure that your voltage going to your fuel pump is not overamped. You should check your fuel pressure under load as well as on idle. Make a longer fuel line and tap it into your pressure line and run it with a gauge into your car (just temporarily because it's not safe to have fuel coming into the cabin) then make some pulls and watch what your fuel pressure does under load, it should increase 1:1 with boost.

I recommend using stock FPR unless you are going for over 600 WHP

[Jakob]

thank you for the help steve, i will try that this week.

[spooled240]

check your alignment, wheel bearings, brakes or anything that will cause your car to have a lot of rolling resistance. I was tinkering with my front toe and I noticed that I was giving more throttle at times when I shouldn't be.

so yeah, check your tire wear for feathering (jagged tread) bad bearings or sticky brakes.

[Jakob]

I would check out your VTC,

Make sure the VTC is functioning 100% correctly.

If this is not working it could affect your cars driveabililty down low.

I would also get a vacuum leak tester (the one that uses a smoke machine) so you can find out for sure if you have any leaks,

Make sure your engine doesnt have any exhaust leaks, make sure your o-rings on your injectors are perfect and make sure that your voltage going to your fuel pump is not overamped. You should check your fuel pressure under load as well as on idle. Make a longer fuel line and tap it into your pressure line and run it with a gauge into your car (just temporarily because it's not safe to have fuel coming into the cabin) then make some pulls and watch what your fuel pressure does under load, it should increase 1:1 with boost.

I recommend using stock FPR unless you are going for over 600 WHP
fpr is a new one and its the 3rd one i tried already.
my car's performance is great, i have absolutely zero problems with performance.
vtc is working properly too i have a consult cable and datascan where i can monitor all the stuff the engine does.
o rings are fine too, i pulled the fuel rail with everything connected and turned the ingition on (didnt start engine) and i have no leaks

check your alignment, wheel bearings, brakes or anything that will cause your car to have a lot of rolling resistance. I was tinkering with my front toe and I noticed that I was giving more throttle at times when I shouldn't be.

so yeah, check your tire wear for feathering (jagged tread) bad bearings or sticky brakes.
breaks are fine too, bearings are different ones and i dont think that my alignment is the reason for it because i have this car for a while now and i always had that problem, alignment or not.
thanks for the help :)


well i got a new ecu and i tried if anything improved and its nothing.
:rl:

[R_G]

is your fuel gauge working properly? Do you have any leaks in your gas tank>?

[Jakob]

ya fuel gauge works too. i drove my tank completely empty a couple of times because i couldn't believe it either.
i honestly dont know if i have leaks in my gas tank. i think if i had leaks i would smell gas all the time right? and would have wet spots under the car etc.
but i dont.

i will see if i pass emissions this month and if i do i know its not getting lost in the engine.

[Sycobob]

How are you determining your fuel mileage? If you are using the the amount of gas on your receipt when you fill up and the mileage on your trip/odo have you made sure your gauges are counting right? Going off spools suggestion, did you check that the e brake isn't stuck too? Do you sleep drive? lol

Do you notice the normal patterns in your fuel mileage? (ie gets a bit better on highway trips, or worse when you drive hard)

I'm a newb so sorry if any suggestions are stupid, just figured I'd give it a shot.

[Jakob]

How are you determining your fuel mileage? If you are using the the amount of gas on your receipt when you fill up and the mileage on your trip/odo have you made sure your gauges are counting right? Going off spools suggestion, did you check that the e brake isn't stuck too? Do you sleep drive? lol

Do you notice the normal patterns in your fuel mileage? (ie gets a bit better on highway trips, or worse when you drive hard)

I'm a newb so sorry if any suggestions are stupid, just figured I'd give it a shot.
newb or not doesn't matter, i am always glad for people helping me because everyone has different opinions and views :)

i always set my odometer (my gauge cluster is also new/different lol) to zero when i put fuel in my car and then i drive it until the gas light goes on.

yea you see i dont see a difference between highway trips and regular trips, but i do notice a big increase in fuel consumption during full throttle.
it always comes down to 20liter per 100km (i'm european haha) doesn't matter if it's highway or city driving.
hell i even took a bunch of buddies for a ride so they can have a look at it because i thought i might be imagining things, but sadly i am not :doh:

my e brake is fine, because my rotors aren't blue yet and i dont smell brakes when i drive long distances haha
i passed the emission check in 2008 with no trouble at all which doesnt make sense to me because of how low my mileage is i should have A LOT of unburned fuel in my exhaust gas :dead:

i will try and build an experimental tank with a spare fuel pump i have and route some lines directly to the fuel rail so i can run m car without using my stock fuel system, except the fual rail which isn't leaked because i checked it.

that should at least give me a clue where to continue searching for my problem.
i will update this thread with results, whether it is the engine or the tank haha


thanks everyone for the help!

[Sycobob]

How many RPMs are you at when you're in 5th gear doing 45mph? You could try sitting a laptop in the passenger seat and taping a cheap webcam to the bottom of the car, look for any mysterious leaks that you haven't been able to find before lol. Good luck.