Help!!

[Presscott707]

hey hows it going guys my 99 QX4 is getting really bad gas mileage and about a mouth ago before the gas mileage really went down it throw a code for the cat but i dont remember the code. which cats are know to go bad on these? the ones after the headers or the downstream ones?

[Pwnin O'Brien]

There's only one set of catalytic converters on your vehicle, they are mounted directly to the exhaust manifolds. Failed catalytic converters aren't extremely common (but it does happen on older vehicles). What is most common is a failed/malfunctioning O2 sensor. There is a specific DTC code for each O2 sensor (for individual O2 sensor failure) as well as a specific DTC code for each catalytic converter failure (lets you know if there is an issue with catalytic converter itself and not the O2 sensors). A bad upstream O2 sensor can cause horrible gas mileage since it is the sensor which dictates the fuel/air mixture. Your best bet is to check your vehicle for codes and see if there are any still in the system. The previous code will stay in the system for a while (40 drives) so you can pinpoint the issue. If you post up the code we can help you get a better idea as to why you're getting bad gas mileage.

[Presscott707]

i read out the ecu and no codes come up, also no codes in the memory and the obd2 scanner says that everything is working like is should. my cat is a cali spec if that helps any.

[Presscott707]

what sort of mileage do you guys get on one tank with you QX4 because im getting around 230 and that just about all highways miles because i live about 25 miles away from the base.

[alexf20c]

these vehicles don't get good gas mileage. i get about 12-13mpg.

[Pwnin O'Brien]

these vehicles don't get good gas mileage. i get about 12-13mpg.

Ditto.

I'm now getting the best gas mileage since I've owned my Pathfinder (almost 4 years). This is since I removed my roof rack, safari bar and subwoofer. I don't know what my new MPG numbers are yet.

[miamiheat3332]

i have new snow tires on mine, subs & amp, and everything else that comes with the car, add like 5 pounds of other crap like hid ballasts etc. i also have a k&n air filter, and the tornado thing you put in the intake tube, premium gas always, 112xxx miles, and i get like 16ish if it its local and city, and a good day on the highway i pull about 18-18.3mpg with a fully loaded car of 2 kids and 3 adults, without snow tires i could get 19.5mpg, and this isnt driving like a grandma either.

i also got the 19.5mpg with the old tires and fully loaded without my air resonator box mod which probably gives me better mileage now too, but when i did that i got snow tires the next couple of days so i couldnt tell the difference but i bet that helps too.

ps. by the highway driving, i do that local too, but im saying highway driving as when i go to longisland around 150-160ish miles and a little local driving there, then straight back to allentown.
Modified by miamiheat3332 at 7:33 PM 2/16/2010

[sicwitit]

gas milage can depend a lot of different factors, when I put my new tires on, and took a short 20 mile road trip, I noticed just with the tires, fuel milage shot down maybe 3-4 mpg with only that change. my recommendations for you would start with checking plugs, air filter, fuel filter, lubrications flush, alignment, correct size tires properly inflated, and then I would look at the exhaust system if your not throwing any codes. its possible your cats could be partially cloged causign a backup of pressure (kind of like if you had bad cholestral and it was building up in your veins! start with those first, maybe even do a sea foam treatment, you should notice better fuel consumption if you follow these steps.

[alexf20c]

the tornado thing in the intake is worthless. the concept applies only to engines with a carburetor or TBI injection, but even then it's questionable.