I am at wits end and considering selling the car. I am getting 12 city and 16.6 highway. I used to get 23.5 highway and I was using regular gas. I cant seem to find anything wrong. Should I put in new o2 sensors even though I am not getting any code? Also the maf is only 6 months old and the car seems to be running smooth. Any other sensors I should look at?

 

I am so... confused what does that have to do with 02 sensors if the light didn't come on then o2 sensors is working fine. Do you maybe have a leak on your gasline? You should check that out..

 

How many miles is on the ride...

 

My '95 used to throw codes for the O2 sensor without the check engine light coming on...

 

Millage would help... Although I will bet it probably high. Regular tune up stuff done recently... Plugs, wires, Coils etc. Fuel pump/filter??? Air filter? did you buy an ebay MAF or OEM??

For fuel consupmtion to change that much there has to be BIG factors involved. Are you the only driver??? And how are you calculating your MPG???

 

New oem maf and new knock sensor (or sim it). Quit using 87.

 

142000 miles. The maf is oem. The coils and plugss are new oem (about 8 months old.)
No gasoline leak. I will check the air filter. I calculate my milage by filling up setting the trip odometer to 0 and then when I refill i calculate miles by amount of gas.
Should I change the knock sensor even without any codes?

142,000 miles with a new oem maf (about 6 months) and new oem plugs and coils. I will check the air filter. Should I change the knock sensor without any codes?
I am calculating correctly by using the trip odometer to get number of miles between fill ups.

 

im in to hear this. I have an 00' with 99k and it get downright terrible gas mileage

 

Hmmm my auto got like 20-22 and after the swap I get like 25-28. Mantience is the key, fuel usage is the most annoying thing in a vehicle. Been trying to fix my work truck for months.

 

You need to use 91.
Ethanol-Free is best.
You can have a bad O2 sensor without ever getting a code.
You should look at the pre-cats, they're known to break down and plug up. You can have a mechanic do an exhaust back pressure test and I'd recommend replacing the O2 sensors while you are in there anyways.

Knock sensor is possible.
Injector harness is possible.

How are you calculating your mileage? Most people do it wrong.

 

It could very well be a dirty intake system. Try using the seafoam spray and clean all inside the TB and intake system, that should help.

 

Also vacuum leaks can cause this and normally go un detected

 

Hmm I'm getting between 19 and 22 mpg. But I'm glad this thread has a ton of info on what to do mpg starts decreasing.

 

Did you check your TPS reading? Both of my A33's read 0.0% at idle. Both have decent FC. If its partially open, you're burning unecessary fuel.

 

I had a very similar issue with my 03 maxima, although a different engine... took my car to Nissan twice, and had consumer affairs nissan corp pay me back... sold the car however on the graph, my pre cats were going w no CEL for over six months, nissan said it wouldnt make such a mpg drop so I sold the car as if it were fine, but go to Nissan for $120 and get a diagnosis test.... and if nothing call Nissan consumer affairs and speak with Chelsea, I'm from NY so I'm not sure if shes the regional manager for NY locations, however she was the highest one I could speak to in the conmsumer affas department, I was refunded both of my diagnosis' fee's, so dont give up without a fight, when I sold my maxima, literally hysterical wasnt the word, my whole family was like omg..... as in to see me like that. But now I've got my i35 and average 18 around town, instead of the 8-10 I used to..... Also use 93, or atleast 89... I prefer sunoco, but with my I i use whatever and its all the same.... and try fuel system cleaners....... try cleaning your MAF, and throttle body... text me at 6318890749, I went through it all at the same time as going into college and losing my job, so if I couldnt save my own car, I'll try and help another

 

What is tps

What swap are you talking about?

 

Throttle Position Sensor

 

He swapped his auto trans for a 6mt recently.

 

http://forums.nycmaximas.org/showthread.php?t=65203


Posted from Maxima.org App for Android

 

Just read your thread and I see you gave up and dumped the car. I will look into the different suggestions given here and then if I dont have any luck i might also dump it.

 

That was also after 6 months of diagnosing.... losing my job, and being told my cats were on their way out w no fund to replace them.... dont give up so quick if you love the car... and I sold it got a 06 sonata LX V6, hated it, got rid of it after 5 weeks, then now an I35, so i came back.... dont be so quick to give up, Nissan consumer affairs will work w u as I told u in a previous post.

 

so not true. O2 sensors get old but often dont go "bad", they start to give readings that are further and further from being correct but the ECU cant sense anything wrong with the O2. Its called drift.

vacuum leak would cause lean, not rich. If it was bad enough to cause bad mileage (assuming that ever happens) he would notice the whistling and poor running.

my suggestion is to stop driving in the city

 

A vacuum leak causes extra air, correct, lean, then the computer corrects for that lean condition by adding fuel. But it does this as a global change. A vacuum leak can only be present at very high vacuum, so changing the entire fuel map, say, 5%, will add 5% everywhere. Even where there isn't a vacuum leak.

The computer will then see a rich condition because it doesn't need the extra fuel everywhere, so it corrects again, it will go back and forth, causing poor efficiency, rough idle, etc, the computer corrects rough idle by adding fuel. The computer defaults to "rich" almost always because it's safer. As such a vacuum leak will always decrease fuel mileage

 

thats weird because when Ive had a wideband going and a vacuum leak it was lean, then lean, and then more lean. Maybe if its a bad enough leak you could end up rich all the time, but good luck actually driving it anywhere anyway with a bad vacuum leak. I welcome you to do your own wideband test if you wish.

 

What were your fuel trims at during this time? The computer can only correct up to 25% so if you had a big enough leak and enough mods that it was pulling in around 25% more air than it did off the showroom floor, the computer won't be able to correct for that.

 

small leak, it was probably a hose off the vacuum port just behind the throttle body if I had to guess (I forget that one when I put the UIM back on way more often than I would care to admit)

 

A completely disconnected hose is a pretty massive leak lol. Even though it's a small hose

 

its like a 3/16" hose. I would bet it doesnt get more than 10% of the air at idle through that hole and basically nothing at part/full throttle (open loop) and open loop is the part that matters. Closed loop the computer will always correct to 14.7 unless you have a real problem. Thats how my car still gets 26 MPG mixed but pulls richer than 10:1 under full throttle and runs super rich during warm up (read: open loop)

 

I second the faulty primary oxygen sensors. I had a car with HORRIBLE mileage and no check engine light. Replace all primary o2 sensors and car running good again with descent mileage.

 

should i put on foulers on the o2s also or just replace them? primary is bank one correct?

 

You can't trick the primaries unless you want it to run worse.

 

I would like to get oem or ones that are as good. What brand is the oem and does anyone know if the store brands are reallly oem? For example I bought coils from advance auto (the rear ones) which where rebranded but they they were oems from japan.

 

just get bosch. Theyre reasonably priced and have been making O2 sensors since they first came out. Whatever you get, you can either buy OEM style wiring with the OEM connector or save a bunch of money and wire up the correct universal ones yourself

 

That's a vacuum hose, that is completely disconnected, it's a pretty massive vacuum leak.
It's physical size is small, but it's WAY bigger than the ever-so-slight paper thin gap that could cause a vacuum leak from a bad intake gasket.

At -20 PSI or more of vacuum, that little hose will suck a LOT of air in, way more than you think.

Also, your thought that it will matter more in open loop is incorrect. Engines burn the most fuel while they are in closed loop, that is where an engine operates 90% of the time.

The only reason a vacuum leak is a big deal in open loop under throttle/WOT is because it WILL run lean and could blow up the engine if it's bad enough.

If you are running 10:1 or lower on a N/A car, you're doing it wrong, and I can pretty much assure you that you are not getting 26 MPG, unless you are looking at the dash-display when you're coming up with that

Either way, this is a silly thing to go back and forth about. I applaud you if you are getting that good of mileage running your car excessively rich. Hope you have deleted the cats dude.

I agree that O2 sensors could easily be at fault here

 

unless your engine targets 14.7:1 or leaner during open loop you are burning more fuel. Unless of course you are talking about the absolute amount of time while driving, in which case you are totally missing the point

haha, its not tuned for that (its not tuned at all anymore), im pretty sure the pressure regulator is fubar so during open loop while the computer is ignoring our narrow band O2 sensors its just following the normal injector duty cycle maps even though the fuel pressure is running wild. During closed loop it runs fine because with the O2 feedback the ECU scales back the injectors and gets to 14.7 without issue.

derp derp I accelerate slowly, egg on pedal style so that I dont go open loop unless absolutely necessary and cruise 70 on the freeway, mileage calculated by trip mileage/gallons and its over many dozens of tanks. Trip computer reads about 30.5 MPG FWIW.

Are you a Nissan/other manufacturer tech by any chance?

 

Same here.

Our ECU's only correct if they see a problem, and their vision is only post MAF and if it's before the MAF on the TB side it will never get corrected.

O2's as we all know are only used at part throttle/low load situations. I mean really low loads. And usually aren't being used (listened to) after 1600 RPM (open loop/closed loop swithcover and very load load/throttle). If a lot of your driving is done at or near this RPM, and low loads, such as highway driving, then you will see an increase of MPG.

I have no primary O2's and usually get 27 MPG or so on the highway and 24 mixed.

But, one time I spent nearly an entire tank in the city and barely mustered 17MPG, which I'm guessing most of the time I was at or below the 1600 RPM threshold and not using too much throttle.

 

My point was that the bulk of the engine's operation is done in closed loop. As such, the bulk of the fuel used by the engine will be consumed in closed loop.

I'm not saying the engine 'uses more fuel' in closed loop, just confusion in wording/interpretation.

That's crazy good mileage, I'm very jealous.
Wonder how bad it would plummet if you drove it 'normally' with the fuel issue. Do you have an aftermarket RFS or something that is messed up?

Just a good old fashioned self declared keyboard super-genius.

 

Its still not terrible, like 24 mpg. Surely if I spent a lot of time winding it out it would be horrific, but even with normal operation driving that way makes it terrible. Traffic sucks here so im either going 70 on the freeway or might as well be in the city, I would call it a 50% freeway speed/50% city style split. The fuel system is all stock, ive gone through the whole system except the stock pressure damper so that little bastage is next. Was considering going to an aftermarket return system since finding a stock regulator for sale is next to impossible but I might finally have a line on one.

 

So you think it's the in-tank regulator? Never heard of them failing, I'd be tempted to put a fuel pressure gauge on it before you bother. Fuel dampers, however, I've heard of faults with, but they shouldn't ever cause that issue. That's odd.
Just brainstorming, who knows.

Like you said though, who cares, if you are getting that mileage, ef it. As long as you don't have stock precats on there, they will plug up and break down and cause issues especially with that much fuel getting pitched at them.

I wish I could get that mileage in my DE-K. These things are not great on fuel, and mine is worse, but I've never bothered to chase it down. I average about 21 MPG and I 80% highway

 

its basically the only constant through the time its been this way. The car has at times had different injectors, coils, spark plugs, ECUs, intakes, headers, and other things but through all those different changes the super rich open loop has always been there. its a strange thing