Recently bought an 03 maxima and my gas mileage is horrible.
So far I've changed my O2 sensors, fuel filter, and cleaned my MAF.

The car performs just fine however I'm getting around 12-14 mpg city and highway, at one point I got 21 mpg on the highway cruising around 90, I'm not sure whats wrong. I don't think its the MAF since I dont have any idle or performance issues but I'm not sure.

I've tried driving extra slow and gentle in the city yet I still get <18 mpg.
I'm thinking about changing out the plugs and hoping that helps. Any advice?

 

cleaned your trunk? slow down! cruise @55 and then tell us what you get lol. more tire pressure, remove subwoofer box if you have one, change plugs, what kind of air intake you running? Make sure it isn't clogged a tad bit(it can happen)... I doubt besides the plugs its mechanical, but you never know, but before you start spending money, get all the cheap, simple stuff outta the way.

 

As far as changing the plugs. How many miles on the car? Need more info.

 

are you putting in 93 octane ? try throwing in some gumout fuel system cleaner, make sure your air filter is clean, and that should help

 

I changed the air filter about 2 months ago when I got the car, it has around 80k miles, I ran an entire can of Seafoam through a tank of gas as well. I'm not sure if I should just invest in an maf and hope it helps

 

What engine speed are you at when at 70mph?

 

just gonna throw it out there
are u driving in 3rd gear or "D"

 

Exactly how are you getting your MPG values?

 

I calculate it based on every time i fill up and the reading from the computer on the dash. And yea I'm driving in D, I know at 80 mph I'm a little below 3k

 

Just out of curiosity, what does the MPG state on the dash? It's usually incorrect, but by how much and in what direction can indicate an issue related to STFT's.

 

Right now it says 9.2 mpg.

I got another p0158 which is the rear most o2 sensor, I replaced it about a month ago with a universal walker one, while I originally thought this was bringing my mpg down, I read the secondary o2's don't do anything.

However, on another note I think I read somewhere the MAF could also cause a 0158 by creating the improper fuel mixture but I don't know how true that is.

 

Even though the computer trip is not 100% accurate, but 9.2 mpg is pretty low. Try cleaning it by following this link http://forums.maxima.org/5th-generat...10-w-pics.html

Be careful cleaning it though, good luck.

 

Cleaned MAF twice using that procedure, used entire can of cleaner

 

Also, for some reason, when I turn the car to ignition, at night I can hear the pump constantly running as if the line isn't pressurizing. I figured it was the fuel filter preventing the line from pressurizing so I replaced that but it didn't help.

 

Check the gap on your spark plugs and make sure they are correct.

 

A clogged catalytic converter could be causing low gas mileage.

 

Wow this is rough since you drive at high speeds me too but as long as i get about 210 miles off a full tank of 30% highway and 70% city I'm good!!

 

You should still be getting like 350 miles...

The MAF will definitely kill mpg.

 

Please make note that if you are using gas with Ethanol in it, it'll lower your MPG. Also higher octane gas (like 91/93 that's labeled as "premium") will NOT give you better gas mileage. In fact, it wont even do anything useful for our cars except for break the bank. Unless you have a custom tune for high octane, then it's not helping. Save the $$ and use regular.

 

I get 400+ out of a tank in my '06 SE-R, usually, and only use about 15-16gallons of fuel to do so. I still want to take it on a road trip and see what kind of gas mileage I can get out of it.

 

Have you checked the basics?

1. Tire pressure?
2. Wheel alignment? How is your tire tread wearing?
3. Check your air filter again, just to be sure.
4. Parking brake released all the way? Parking brake cable okay?
5. Rear caliper seizing/dragging? Is one wheel much hotter than all the others after a drive?

If you're lead-footed that sort of mileage isn't unreasonable, but if you're driving normally (which on my car still out-accelerates traffic from a red light), and the above are okay, you should also be having other issues like sluggish acceleration, etc. Do you have any?

Fuel type will make a 10% difference in mileage on my car but generally does not cause driveability issues (this is why it says "supreme fuel recommended" on the sticker inside your gas door, and in your manual).

 

I checked all of the above, the car is like a rocket when I first start driving but after it warms up its gets pretty sluggish

 

That's most likely the knock sensor and it explains the poor mpg. It's running in safe mode. I guess it could be another sensor, though. There's been a few threads on this symptom.
When the engine is cold it ignores the sensors running in open loop.

 

Is there a way to check if the knock sensor is okay?

 

Yes, it's very easy.
Go to radioshack and buy a pack of 470K ohm resistors. Unplug the connector that goes to the KS. You can't miss the wire coming from the middle of the block (inside the V) on the driver's side. Now, where the pins would normally go in the connector is where the resistor leads are getting shoved in. Fold the leads on the end so they're doubled up. It just makes them fatter so they stick in the connector solid. Put some tape over the top to make sure it stays in. Go for a test drive. If your power stays after the engine is warmed, congratulations.

If not, I'd go scan the codes. Perhaps the KS signal is not making it back to the ECU. It does not trip a CEL.
I think a couple people on here had a break in the KS harness wire.

And if still nothing, bang your head on the hood.

 

And if still nothing, bang your head on the hood.

Always a good last resort! LOL!

 

The knock sensor test doesnt sound very scientific lol, I'm gonna try the MAF voltage tests first and see whats up

 

It's not. It's about simulating a working KS. That's why it's very effective. Open loop uses the MAF, so that test would be pointless, imo.

 

easy fix.

1) unscrew radiator cap
2) back car up
3) drive new car underneath
4) replace radiator cap
5) done. fixed.

 

I'm super confused, I filled her up today, it was 150 miles for 14 gallons since I filled up last friday. On my way home I set the cruise to 59 mph and the ecu's on dash mpg reading was around 30-33 mpg for 20 miles.

Once I got off the highway I drove maybe a quarter mile and it dropped to about 28 by the time I got home which is way way way more than the 10-11 mpg I got on the last tank.

Perhaps its just because I drove only 20 miles home but what would be a cause say 28 mpg on the highway but only 12-14 mpg city(with moderate acceleration) and around 16-17 with very light acceleration

 

KS will do that, too. Cruising on the hwy doesn't need a lot of power (low throttle). The timing also seems to retard more at higher throttle. Anytime you need power, it's dumping in the fuel like normal, but you're not going anywhere. You're describing exactly what mine did.

 

Interesting I'm starting to think you really are right, sometimes after the car has really warmed up when I accelerate I feel like traction kicks in because the rpm dips when it should be higher even though traction wasn't kicking in.

Someone is selling a new knock sensor for 60 on the classifieds I'm about to just get it and give it a go. Does the entire intake manifold have to come off? I might just do plugs and knock sensor while I'm up in there.

 

IM does not need to come off. A small ratchet can do it with patience.

It takes 30 secs to put in a resistor and costs almost nothing. No point wasting money if it's not the issue.

 

Will try resistor tomorrow

 

I find similar mileage on my 2000, and formerly had a 95 5spd SE as well and it did pretty well too- remember being able to make it to Knoxville on under a tank (~400 miles).

I had noticed a drop in MPG recently, but I think my mileage will improve on the 2000 now that I fixed the connection (the semi square tube with the collar) in the air intake circuit

 

I had noticed a drop in MPG recently, but I think my mileage will improve on the 2000 now that I fixed the connection (the semi square tube with the collar) in the air intake circuit

Are you referring to the sleeve going into the filter frame? I had that tie wrapped so it does not slide back.

 

Good luck getting that fixed... I guess I'm lucky I never get below 420 or so miles on a full tank.

 

So uh can anyone hook me up with a picture of the knock sensor location and/or the harness? I kinda failed at finding it today

 

I can probably get some pics tomorrow.

 

See right in the middle of the engine block between the 2 heads? The only wire connected to the round sensor in the middle. That is the knock sensor. This picture is a 350 or G35 but you get the idea.. Under your intake manifold.

http://media.photobucket.com/image/v...nsor014310.jpg