The fuel economy on my 99 max GLE has suddenly dropped off quite substantially. I was getting about 525 km per tank until about 2 months ago, now I'm only getting 400 km / tank...a pretty major drop. My driving habbits have not changed in that time. I have been using premium gas since I bought the car 1 1/2 years ago, and I am still using premium now. This drop in fuel economy seemed to happen after the following work was done:

- replaced bosch plugs with NGK Plats
- replaced all 6 ignition coils
- cleaned throttle body
- cleaned iacv

There are no other symptoms other than the drop in fuel economy. I was almost positive that my knock sensor had died, but I just checked the ecu for error codes and all I got was 0505 (meaning no errors). Could it still be the KS, even though it's not giving any codes, or should I be looking at some other cause?

Note: The reason the above work was done was because the accelleration was very jerky (only when I would floor it though, under normal accelleration it was OK). That problem is now resolved but has been replaced by very bad fuel economy.

Any ideas?

Thanks,
David

 

Winter > j00.

 

Do you guys use winter formulations of gasoline up there? ie. Ethanol added? Also, winter use of heat/defogger/defroster takes away a lot of your mileage. I typically lose 15-20%.

 

You should probably change your fuel filter and check/clean/replace the air filter. Next time, don't use Bosch, stick with NGK plats or copper, copper gives better spark but less life.

 

fuel filter - i would think. keep it simple for now.....good luck

 

Winter time means cold engine, cold engines need more fuel to start and run until they reach operating temperature. You always burn more gas during the winter time. Go on a long trip, I bet your mileage will be normal. Your every day driving, start, stop, start, stop ect. on cold winter days is whats eating up your fuel.

 

CEL 0505 doesn't mean u don't have a KS problem.

http://forums.maxima.org/showpost.ph...3&postcount=42

i'm pretty sure u have "partially working KS" right now. cuz speaking from my previous experience, i was getting low gas mileage with ghost KS. everything else was checked/cleaned/replcaed. when i install the new KS my gas mileage came right back up.

hopefully stevtec's info will help u

 

Thanks for the input everyone. This is my second winter with this vehicle though, and I know my milage was much, much better last winter. The air filter was changed at the time the other work was done, but perhaps the garage used a poor quality filter? The gas filter was not changed, so I will replace it. Regarding the ethanol gas, it is available but most folks just burn regular gasoline and it is never really an issue.

 

Get the 1996 300ZX fuel filter if you can find it. It only cost me $4 more than the Maxima one, and has a much bigger diameter, which means more time between replacements.

 

wait who says using the heat in your car uses more gas!?!?!?!

 

primary oxygen sensor?


my moms 02 went from 23 mpg to 8 mpg and it was her oxygen sensor

 

OK, my bad. Clearly a mistake.

 

The knock sensor sends a signal to the ECU, and if the ECU decides the engine is knocking, it will retard the timing. It should also set the KS code at the same time, although I'm not sure offhand how much/how long it has to run at retarded timing to post the code. So I doubt it's your knock sensor.

I would look also at your engine coolant temp sensor. They're only $20 to replace and not too hard to change, even in the dead of winter. If it's reading wrong (but not way out of range) your car will run rich all the time instead of during warmup. If you're real ****, you can remove it and test it in cold/hot water with a multimeter and thermometer, but since they do fail from time to time simply replacing it makes more sense IMO.

Also check tire pressures. Cold temps can drop pressure by several PSI, and that can make a difference in fuel economy. I kinda doubt this is the problem, but it's still a good reminder.

Another possibility is something adding significant friction to the drivetrain - like a sticking parking brake cable, or bad bearings somewhere. Put the car in N on a flat surface, release the e-brake, and try to push the car a few feet. You should be able to roll the car without much force (yes, it has a lot of mass, but if you get it moving it shouldn't just stop when you stop pushing). The e-brake cable is a common problem.

Dave

 

You know thats a great point. If the O2 is working but in reality not sending the ECU 100% correct information it will cause performance & economy issues. It probably wount set a trouble code either.
It might be worth replacing both of them.

 

Not only do your tires gradually lose air, but air pressure drops as the temp drops. I noticed my mpg was down, realized I hadn't checked the pressure in a while, and found the front tires at 26 psi. That'll hurt mpg for sure.

 

winter gas has my mileage dropping by around 2-3 mpg

 

yea winter gas in chicago normally drops me down 3mpg on average as well.