vvslavavv

Hey everyone,

I'm getting pretty horrible gas mileage right now, less than 17MPG (measured by filling up, reseting trip ... driving ... filling up again). This is with 50% highway/50% city. I've read the stickies abt the gas mileage, and here is everything i've done up to now! My max has 115K miles.

1. changed fuel-filter, air-filter, spark-plugs (NGK plats)
2. cleaned throttle body
3. the CEL was never on, however, i was getting the KS ghost code (the knock sensor ohmed out to abt 14MOhms), so I replaced the knock sensor, and reset the ECU. I'm now not getting any codes ( ecu reads 0505 ).
4. Tire pressure is normal
5. Oxygen sensor - since CEL isn't on, and i'm not getting any codes, I assume they are ok.
6. Fuel system - ran injector cleaner for a few tanks of gas (before I changed fuel filter/spark plugs)

I'm a bit at a loss at what else could be the problem, and at today's gas prices, this is getting extremely annoying. Does anyone have any more suggestions?

I feel like the engine isn't running as cleanly as it used to ... but I can't describe the sound. But i think i can hear like RPMs are dropping just a tiny bit every few seconds (when idling). However, the tach needle sits dead on 600 w/ A/C off, and abt 900 w/ A/C on (i think that is normal).

Thanks

TurTLe*

iTrader: (11)

you mention A/C... right now i average 20 mpg. but if i used AC i'm sure my MPG will drop a couple.

Well you changed all the parts you can. maybe a leaky injector or the way you drive and factor in AC usage.

vvslavavv

Is there anyway to test whether an injector is leaky? I thought leaky injectors would throw a code?

sky jumper

what mpg were you getting? have you checked if your brakes are dragging? check for excess carbon on one or more plugs, that might indicate too rich or leaky injector. you didn't put 50wt oil in did you?

JwaxMax99

iTrader: (14)

Warm up engine, then turn the engine off, with a fuel pressure gauge installed, fuel system should hold pressure for 3 or 4 hours. If the pressure drops off 30 min. to say 2 hours or so, you may have a leaky injector.

Motorhead52

What grade of gas are you using? Cheap gas, as well as gas with that ethanol trash, will make your mileage go down the toilet.

vvslavavv

Before doing all of this ... i think it was around 21 MPG. I'll check spark plugs for carbon, but the ones I took out didn't look that bad. I know i have to change front brakes ... so maybe they are slightly dragging (i'll check it). I used 5w-30 oil.

oh yeah i also changed my transmission fluid to MT-90

vvslavavv

I'm using 92 octane gas. no particular brand, if thats what you are asking

DR-Max

iTrader: (3)

My car was giving me poor mpg a few weeks go. I cleaned the throttle body and what a difference!

CRiME

iTrader: (21)

I think i probably have the worst mileage of all. i filled up with amoco 93. I'm now at 1/4 tank, and i only have 180mi on my trip odo. all mostly conservative city driving, no AC. changed fuel filter, plugs - NGK copper gapped to .044, air filter, KS, cleaned TB, tires are 35psi all around. i also ran chevron techron through my system twice, and i have some fresh mobil1 5w30 in there. needless to say, i'm stumped! but have no choice but to deal with it. anyone think a seafoam treatment via the brake booster line would help? how are you guys pulling 300+ ? that's impressive
when I was reinstalling my ks, I didn't use a torque wrench. If i overtightened it, could it in any way adversely impact my mpg?

njmaxseltd

iTrader: (19)

You know what they say when you assume....

Your O2's signals tell the ECU how your cars a/f mixture is. The sensor might be working, but really not giving the ECU the correct information. In other words, it's lying. The ECU doesn't know this, so it things everything is OK.
Realistically, your probably running way to rich, which is why your burning a lot of fuel.

Replace your upstream O2 sensors and your gas mileage should get much better.

Chris99SE

iTrader: (1)

How would gas with ethanol decrease mileage?

vvslavavv

Does anyone in my area have one of those ODB-II scanners that connects to a palm/laptop ... i guess i could figure out if i'm running too rich with it.

99grnmaxgxe

iTrader: (11)

Autozone should be able to check codes for free. I'm with NJMax, I would change the two front O2 sensors.

ChasMan

Ethanol has less power per gallon in it than regular gasoline. So, the more Ethanol in the fuel, the less mileage you get out of it.

Sort of like Wood and Coal, if you burn 50 pounds of coal, you get a lot more heat than 50 pounds of wood.

You can get High Octane gas that burns less efficiently than low Octane gas with enough Ethanol in it :-)

Chris99SE

iTrader: (1)

The gas I have seen only has 10% ethanol. I filled up a couple of times since the ethanol gas got to the area and have not noticed any decrease in mileage, I get consistent 24-25 mpg or 400 miles a tank.

azn_s3nsation87

iTrader: (1)

I get 350 miles before the orange turn on, is that bad or good?

jman7777

when i turn my key in the ignition my car starts up, is that good or bad?

wikidminds

thanks for reading the stickies by the way then posting.

i agree with changing ur upstream o2 sensors. change em and see if u notice any improvements.

also....do u have a heavy foot on the gas pedal?......

darklegend06

iTrader: (11)

Thats good. anything above 300 I think is good. I average about 300+.

maximas are supposed to have platinum spark plugs not copper. NOT sure but might make a difference in your gas milage.

Zog

iTrader: (1)

Maybe I'm lucky, but I got 27.xx the other day on a 370 mile trip. Stopped 3 times and sat in traffic for about 30 minutes. Still had about a 1/4 tank left too. I'm getting ready to do most of the maintenance listed in this thread too....

Bollwerk

iTrader: (1)

My 99 max (cali spec) gets about 20-22mpg city and 25-27mpg highway. (as a point of reference)

KCMaxima98

iTrader: (4)

I want to see evidence that oxygen sensors that aren't throwing codes need to be replaced. No code=leave it alone in my view. Although if I was getting 17 mpg consistently, I would probably try it just to see if it helped. That is bad mileage. I have the same miles on my car as you do and I get 22-24 mpg.

Bobo

You are telling him that crappy gas mileage is good. The worst I have ever had since day 1 is about 350 miles to a tank when commuting. I routinely get in excess of 500 miles on straight highway driving and have had 550 or slightly more a handful of times.

vvslavavv

I would say that ~95% of the time i'm very light on the gas pedal.

However, I was stuck in traffic today (downhill), and while rolling down the hill at very low speed (not pressing the brake pedal of course) .. i could hear faint scratching, so it just might be my brakes. Will have to take a look at it either this weekend or next. I need to replace front pads (and maybe rotors) anyway

99black5spd

iTrader: (1)

maybe you have a bad catalytic convertor.

BumbleB80

I did about 370 mi this past week on a tank. However, I usually get about 75% of that mileage from highway driving. I think once when I did a tank on local I got no more than 250 on 93 octane gas. I should really clean my TB, since I have over 140k.

TurTLe*

iTrader: (11)

you guys right now are only throwing parts at it hopeing to fix something that you dont know its wrong. I don't think you should do any of that, untill you know for a fact. Because o2s and Cat's are very expensive. I mean its ok to throw in parts if its like $10-20 items but thoses are far from it.

Best thing to do is check your fuel trims if you can get a OBD scanner with these features.

but if you wanted to throw parts at it. i would also look at the MAF sensor as well.

Mr Willis

You can't be serious.
I'm jealous!
I get 330 miles per tank and it pisses me off frankly.

kil2board

did anyone else pick up on this!?

your tire pressure should be 32psi cold in the front and 29psi in the rear.

try doing this and check your MPG after that. I know it sounds stupid but tire pressure can adversley effect your performance(tightness of steering) and MPG if they are over or under inflated...

i do feel you though on the whole ****ty mileage thing. i'm getting 18mpg right now. I just did an oil change Mobil1 10w30 & just ordered a knock sensor cause' mine is dead .

i'm really at the end of my rope too. if you find out anything, pass it on for us fellow org'rs that have this problem.

Taxvictim

Good idea to check the exhaust system (cat converter) as previously stated.

Do you have a 5 speed? Could your clutch be slipping? Maybe your RPM's are too high because of clutch slippage.

Or...

On my first '96 I-30t, the gear sensor in the VLSD 5 spd. got screwed up, so my speedometer was always reading lower than my actual speed. That means I was also clocking fewer miles per tank on the odometer, but actually driving the same number of miles. Then the sensor became totally shot and my speedo quit altogether. I think I put 10k miles on the car that did not show up on the odo before I finally got it fixed. Anyway, the low mileage was my first indicator that something was wrong. Before I realized it was the sensor, I got an expensive tune up that did no good.

To check this, drive your car ten miles on a highway with mileage markers, and make sure your odo shows ten miles.

vvslavavv

How do you check the cat converter?

CRiME

iTrader: (21)

thank you for the info, i didn't know this. I was under the assumption that lower PSI = more comfort, at the expense of a few MPG. my rims and tires are the 17" 350z rims with the bridgestone potenzas. does that make a difference? or should I still adjust my tires to the pressures you recommended? thanks again

Sounbwoy

iTrader: (2)

The best way is to have a tech check the temps. With the car running, the OUTLET temp of the cat should always be HIGHER than the INLET temp (by a good amount too). If both temps are the same or the outlet temp is lower than the inlet temp, you have a clogged cat.