What is up with my fuel mileage? (Previously entitled, "What the . . . ")

[chrunner09]

So, my '99 I30 just went through HALF a tank in about 3 1/2 days. This is under normal conditions, i rarely drive hard because of gas. I was just wondering what it could be? Just got my oil changed maybe 250 miles ago(full synthetic). Thanks for the help

[DevonD]

half a tank in 3 1/2 days isnt that unsual for me..just usually a tank a week..

But if you think something might be wrong..check your air filter first of all,,fuel filter, o2 sensors.. going above 80-85 kills your fuel economy!!!

[allensteiner]

when's the last time you put new spark plugs in? ngk only!

[chrunner09]

i had new ones put it maybe..... 6-7 months ago. How often am i supposed to change them? and i just reoiled my filter today.

[chrunner09]

oh and i dont know if this has anything to do with it but, when i let off the gas, my car jolts a little bit, enough for me to notice an immediate stop of power to the engine. Spark plugs? Any recommendations if so? Cus last time i had the dealer install them

[lightsout]

you may have over-oiled your filter and caused the MAF(mass air flow sensor) to read the air intake wrong. pull it out and take a look to see if it's clean or oily.

[chrunner09]

well, after i oiled it, the mileage got better.... but not by much. so i know i didnt over-do it. idk, maybe it's just the engine. i have 145k on it

[BigMACKenzie]

Lights out is right, you aren't supposed to used oiled filters on vehicles with MAF's bc as soon as the oil trails back to the sensors it mutilates them.

Edit: With MAF's very close to the air filter element.

[johnnyd6404]

Maybe your fuel filter is clogged. I dont know if you have an inline filter or if it is part of the fuel pump... if it is in the pump here is a how to link to change it. The link if for a 2k2 but it should be relativly the same, if it is part of the pump. If not idk enough about that car to tell you where it is, but that may be the culprit.

http://www.innerbean.com/housecor/fuel_filter.html

[jltibbs]

The fuel filter should be located on or near the firewall on the driver side. Part number is 43178 in Purolator. Only about $10 at a local parts house. That is an inexpensive way to start the process of elimination.

[lightsout]

The fuel filter should be located on or near the firewall on the driver side. Part number is 43178 in Purolator. Only about $10 at a local parts house. That is an inexpensive way to start the process of elimination.

i

is there a second fuel filter b/c mine was in the tank i want to replace the "second filter" if there is a second that i didn't know about.

[MinisterofDOOM]

Yes, there's a second filter. There's the "sock" filter on the fuel pump and also the in-line one near the firewall that jltibbs mentioned.

Though, I'm not sure if 5th gen has the in-line one. I know 4th gen does...-

[allensteiner]

spark plugs - ngk. i'm thinking maybe it's time for a new fuel filter? ever had it replaced?

[chrunner09]

not while i've ever owned the car. i've had it for about 1 year now

[allensteiner]

git-r-done who knows what kind of cheap fuel the previous owner was running. also get it hooked up to a good scanning tool - i like snapon's scan - it'll let you know if you're running rich and it doesn't have to throw a code.

[chrunner09]

where would i get something like that?

[snwbrdr435]

autozone or the likes usually has that sort of stuff

[Q45tech]

"HALF a tank in about 3 1/2 days"

Normally one measure in miles per gallon or KM per liter not days or weeks.

One doesn't trust gas gauge but logs refill gallons in a notebook so you can average to 2 decimal points - MPG.

[reknab]

I have a couple of Maxima's an '86 and a 2001. I always record my miles per tank on my gas receipts. I do this for tax purposes and to monitor gas mileage. When things are good the cars get about 21 to 22 city, I have manual trans. On few occasions I have noticed the gas mileage degrade to 17 to 19 mpgs. The solution on the 86 was to replace the "Oxygen Sensor". On the '02 the "Vehicle was running rich" so they replaced the MAF and reprogramed the ECM whatever that means. I had about 76k miles at the time. In both cases the milege returned to normal. With todays prices you want to watch this stuff.

[nj1sh]

is it necessary to oil filters after you clean them?