Poor gas mileage on the Xterra.

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I'm seriously at a loss here.

I drive the same distance, the same driving habit every day. When I initially got my X, it had 32k on it and was averaging about 260 and the best was 280 to a tank(280 was on a long highway trip).

It has gone downhill since then. I am getting 210-220 miles to a tank, same commute. I changed the PCV valve, Fuel Filter, Air Filter(K&N, to Nissan, to K&N again), reset the ECU, flushed and changed all fluids to synthetic and NADA. It keeps getting worse.

I bought NGK G-Power(platinum) plugs, gapped them to .39 per single tip recommendation. I had them at .44 and it was pinging all over the place(not change in fuel consumption) so re-gapped to .39. Wires were also changed, MAF cleaned, TB cleaned and now am out of ideas.

The truck only has 41,543 miles on it now, and can't stand getting 11mpg's on a mostly highway commute. The truck feels strong, good power and acceleration but no idea whats up with it. Can a distributor go bad with only 41k on it and it being 5 years old? Anything else I can check, can the dealer do adjustments etc? I can't afford to keep dumping gas into it like it is because I am tryin to get a house and the budget will be even tighter. I love this thing!

I will also mention, the Xterra is at stock ride height, had a new pair of tires put on in August. Around October is when I noticed a really big mileage drop and its been going down since then. I was reading that it could be timing but I am not sure how to adjust it as the instructions weren't very well written.

The dealer is my last option and one that I really don't want to take as I know I will be overcharged and have a possibility of it not being corrected. I wouldn't find out until a few tanks of gas either :|
 
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New tires? Stock size? Did you change tire size? Are all 4 tires new and the same size? Does your truck have an auto 4wd mode? Over inflate them 3 psi and make sure they are all matched in pressure see what happens. Also, is your trans shifting all the way into OD on the freeway? What are your rpms at 65mph?

Are you running MMO in the fuel? If so, stop using it and fill up with Shell 94 or BP 91 and a bottle of techron.
 
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Tires are the same size as stock ones were. 265/70R16. All 4 were replaced with the same tire. Also I tried inflating them over what was recommended(35/40) I tried 42-45 and no improvement and the ride just got worse, bumps in the road were very harsh for the butt dyno.

Truck is an auto with a manual 4x4 mode(shift lever for 4wd). Trans is fine, all fluids in the truck were switched to Synthetic back in April/May. When I'm on the highway I am just cruising in low rpm range with an auto, i want to say between 2200-3k rpms? I'll have to check after work. I typically just move along at 60ish mph but don't recall it revving high.
 
Originally Posted By: mpvue
winter fuel.



Never in my driving history has it caused me to lose that much mileage. We are talking a good 60 miles or more out of a tank of gas.

I actually did try driving around without MMO and it didn't change much if at all. This was done over 4 tankful's of gas since October(i haven't changed my sig). I only recently just started to use it again because of how cold its been getting.

I tried super for a tankful and mileage went up slightly, hardly worth the extra cost of it. But even the super/high octane here has E10%.
 
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Make sure the thermostat is opening at the proper temperature (or just replace it) if the ECM (brain) thinks the engine is cold, it will enrich the fuel mixture and kill the gas mileage.
 
How can I make sure the thermostat is working correctly? The truck heats up quick and nice and warm in the cabin? This is my first vehicle I've actually tried to maintain myself so its a learning experience.
 
Wider tires can decrease the gas mileage. If you changed to a taller tire you would actually be going farther than the odometer indicates (set for the original tire size).

Sounds like a combination of colder weather and winter gasoline blends, both will reduce your mileage. Since you are in NJ you may also have the mileage penalty of Reformulated gasoline usually about 2-3%.
 
I assume you have checked to see if your brake calipers are sticking?

The thermostat can be checked by putting it in water and bringing to a boil and watching it with a candy thermomter to see what temp it starts to open. Also, you can do a rough check by starting engine with rad cap off and watching to see when the coolant starts to flow.

Winter fuel economy will be 10-15% less at any rate
 
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Vehicle when braking is straight(hands off the wheel in parking lot. Accelerating it is also straight). I will try to have it jacked up this weekend to check ease of movement.


I will try opening the radiator cap and monitor that. Getting to the T-Stat on the Xterra involves a lot of work. I'll have to use a shop for that(buddy has a shop) but really didnt have anymore suggestions as to what it could be.
 
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Originally Posted By: shanneba
Wider tires can decrease the gas mileage. If you changed to a taller tire you would actually be going farther than the odometer indicates (set for the original tire size).

Sounds like a combination of colder weather and winter gasoline blends, both will reduce your mileage. Since you are in NJ you may also have the mileage penalty of Reformulated gasoline usually about 2-3%.



Tires are the same size as the OEM's were. Just chose a different brand. OEM were General Grabber AW 265/70R16. I switched to Kumho Road Venture APT's 265/70R16(more Highway Tread/All Weather, whereas the Generals were more AllTerrain/All Weather).
 
Check to see if a rear caliper is hanging up slightly. Other than that I would say winter fuel also. These things are pigs. You are probably doing plenty of extended idling in this cold weather also.

If you want good gas mileage then buy an economy car. If you are trying to buy a house and can't afford gas (at these "modest" prices), then dump the car before it really costs you.

My saturn dropped from 30mpg to 25-26mpg just from winter gas, and a very little bit more of extended idling. MMO didn't affect the gas mileage at all.

Your truck is probably good for 15-16mpg on a good day. 11-12mpg doesn't sound that bad of a drop.

My old f150 would get 13mpg during the winter, and about 17-18 during summer.
 
I don't idle the truck. In the morning its basically 15-20 seconds and off I go, i'm on city roads for like all of 4 minutes and highway for 20 minutes or so at 60mph, no racing no natta, just basically cruising along and letting it climb slowly to speed.

Could timing cause poor mileage as well? Just information I am reading. [censored] my 94 Wrangler got better mileage than the Xterra and it was neglected!
 
the computer controls all the timing. it constantly adjusts based on throttle position, temp, etc.
if your brakes were hanging up, they would get red hot. jack it up and check if you like, but I don't think its that. I'm still voting for winter gas. my VUE would drop from 27 to 24 or so w/ winter stuff (my new car I only just got, don't know how much difference it'll be).
 
What is the actual drop in mpg? If it's over 3 mpg I would say it's something in combination with winter gas.

Clark
 
Hmm i'll have to see about the brakes(touch etc) or temp gauge. I do know that the computer is always adjusting, and with the Xterra I actually disconnected the battery for 24 hours for it to reset the computer so it can relearn my driving habits from the previous owner. This did nothing in terms of improvements.

I did read though here about adjusting timing. Remember i'm still learning and all help is appreciated. I just have lots of questions.

http://www.automotiveforums.com/vbulletin/showthread.php?t=2610


This is why I was asking about timing(grasping at straws here).
 
Anies - Changing the plug gap a few thousands made your car ping? It advanced the timing by changing the plug gap??
There is no logical connection, and I have never experienced this.

Cold weather will make gas mileage drop. Fuel injected cars rely on the coolant and air temperature sensors. This causes richness for longer period, in winter. All fluids and greases are thicker and tougher to churn. Winter gas is often credited with poor performance - maybe it's true sometimes.

New tires add another element. If they are physically larger in diameter, your odometer will read the same, but you will actually be going farther. You are getting better mileage than you think.

One more thing: Even if you are not throwing a check engine light/code, you O2 [the one in FRONT of the cat] may be sluggish or otherwise faulty. AT over 100k, this is a very real possibility.
 
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Oxygen sensor(s) could possibly have gone lazy prematurely. They won't throw a code if they're not "bad" as I found out in my Civic. I went from 29 highway to nearly 33-34 just by changing these out (after 150k, mind you), with summer gas.
 
Is there anyway to test an O2 sensor? I changed the gap from .44 per NGK's recommendation to .39 per Nissan's recommendation. At .44 it was pinging, and at .39 it fine an purr's. I dont think changing the gap changed timing, i am just unsure if it could be timing as the link above mentioned him adjusting it and fixing his mileage issue.
 
If a code is not thrown, the O2 sensors are tough to test, because they are not totally bad.
I don't know of any for sure reliable test for marginal O2 sensors - only crude tests.

As to knocking/pinging occurring by changing the plug gap a few thousands:
You might as well say it knocked/pinged because you locked the doors. It makes no sense.
 
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