Timm80

Alright, so I bought my '97 maxima back in July of 2011. For the first month that I was driving it, I was manually calculating my fuel economy, I did it three different times. Each time I averaged 26 mpg, which is what it should be. I live in the suburbs, not a lot of stop and go traffic. I drive my car conservatively, I never let my engine rev above 3k, I only go 5 over the speed limit, I coast when I can, and I check my tires once a week to make sure proper pressure is maintained.
Then after the first month of driving my car, I noticed my fuel economy start to decrease for some reason, I was getting noticeably less miles per tank. I averaged it again and it had decreased to 22 mpg. I didn't think much of it, I figured I was just involved in more stop and go traffic then normal. So 2 months go by and I notice I keep getting less and less miles per tank. I averaged it again, and it had dropped to 20 mpg. At this point I'm getting kind of annoyed. It wasn't making a big dent in my wallet, but it told me there was definitely something wrong with my car. My dads a mechanic, and I'm pretty auto literate myself, so whenever I have a little problem with my car I like to get it fixed asap so it doesn't become something bigger down the road.
I jacked up my car I crawled underneath to inspect all the fuel lines and smell for fuel... nothing. I looked around and on top of my fuel tank, figured maybe there was a leak up top and fuel was leaking out while my car was moving... nothing. Followed the fuel lines to the engine bay and up to the top of the engine to see if maybe the fuel rails had a leak... nothing.
I decided to buy one of those neon chemical kits that you put in your fuel system and then use a black light to see if it leaked out anywhere. I used it all in one tank over the period of a week, then waited till night, jacked my car up, and took a look in all the same spots I described before... absolutely nothing.
The next day I unscrewed all my spark plugs to see if they were guzzied up... they all look great. My valve cover gaskets were leaking, so I decided to change those, and in order to do that I had to take my intake manifold off, and I was able to get a good look at the EGR tube, so I cleaned that out, as well as the throttle body plate, both sides. I checked all my emissions controls while my car was running to see if there was possibly a vacumn leak around them... they're all in good shape. I put a container of fuel injector cleaner in my gas tank as well and used a whole tank over a period of a week... none of this solved anything. Theres some other stuff I know I tried, just can't remember it right now. I had tried everything I could think of to solve this, and nothing had worked, so I figured I'd just give it some more time to see if it would solve itself, miracles are possible right?
Over the last few months I've noticed that my fuel economy has been slowly decreasing because I'm getting less and less miles per tank. I averaged it the other day, and I calculated 18 mpg... I am legitimately pissed now. Of course I asked my dad to see if he had any suggestions to offer that I hadn't already tried, and everything he suggested I have already tried. So after trying absolutely everything I can think of... can anyone think of something that I have missed? Anyone have this type of problem and successfully solved it? Could it be some sort of problem with my fuel pump? My car hasn't shown any symptoms of that while I've been driving it, but I suppose its still a possibility. Any and all suggestions are appreciated.
By the way, I understand there may be some enthusiasts on here who will give me crap for babying my car and driving it efficiently... rest assured, I only plan on driving it this way until I get the funds for some substantial upgrades. One day soon, its going to make for a very nice boosted track car ... thanks in advance for any suggestions guys.

Gismo-T

iTrader: (38)

Ever change the knock sensor?

Amerikaner83

iTrader: (17)

Fuel filter maybe? You mentioned you had cleaned the throttle body, what about the MAF? Winter blend gas does rob a few MPG, but not as much of a difference as you seem to be seeing. Are you using premium or cheap-azz 87 octane?

CallMeThatOneGuy

iTrader: (2)

At least do the seafoam treatment. Possibly clogged cats or something like that? there's a whole lot of things that can eat at your gas milage

njmaxseltd

iTrader: (19)

Your dad is a mechanic and nobody has put a scanner on the obdII port to check for stored errors or check running parameters? Really? Looking for fuel leaks because of poor mpg?


Do some basic trouble shooting, check for stored error codes, check o2 sensors and MAF signal as well as ects readings. As was mentioned you may have a bad knock sensor.

dwapenyi

iTrader: (3)

Change your front O2 sensor. You're welcome.

DW

Timm80

I have never changed the knock sensor, the ECM hasn't logged a trouble code from it. I have cleaned the MAF sensor and fuel filter, sorry forgot to mention that. I use 87 octane, I've never had a problem with it. I was considering the sea-foam treatment, I have been reading reviews of it and it seems to be a great product. I was replacing my oil pan gasket a couple days ago, and as I was taking the exhaust off I noticed how old and rusty the cat was... the bolts attaching it to the manifolds were basically nonexistent they were so rusty. I imagine its the original cat, because I have service records for the car going back to 30k miles and non of them list cat replacement. I would actually love to replace the cat on my car because I could just plug a resistor into the o2 sensor connector and run a straight pipe... more power better fuel economy... plus I've heard that 90's cats have 400-500 dollars worth of platinum in them at today's prices ... if any of you guys know where I can pick up an o2 sensor resistor and cat straight pipe replacement piece that would be great

Timm80

My dad is a mechanic for UPS, so I'm not sure if he owns an advanced scanner or if his works place supplies him with one... if his work place supplies him with one he definitely wouldn't be able to bring it home and use it. I'll make sure and ask him. He owns a really basic scanner that I've tried, only reads trouble codes when the computer throws them up. Thanks for the knock sensor suggestion.

Timm80

Sensor... or sensors?

Kuhn_man

iTrader: (8)

all three of my O2 sensors dont work, and i get 28 mpgs on the highway..explain that

Kuhn_man

iTrader: (8)

theres your biggest problem...use premium fuel and change your knock sensor, when you use 87 your working the h*ll out of your knock sensor because our cars are ment to run on 93 and 89 AT LOWEST so your knock sensor is going bad from age and working so hard

Timm80

Actually funny enough, that less then it should be. The fuel economy ratings on cars back then were very underestimated, its not 29 mpg. I took a road trip with my friend who owns a mint '96 max with only 70k on it. As soon as he hit 70 mph he activated the cruise control and we didn't fill up for over 400 miles later. We averaged his fuel economy and we calculated 32 mpg. Gotta love Maximas

Kuhn_man

iTrader: (8)

not true the cost to get all of the platinum out of the cat, would cost more than what they would get out of it, i have no idea why some places give you a bunch of money for them but for some reason they do...sometimes.

Kuhn_man

iTrader: (8)

oh i know, most ive gotten out of mine was 29 but a they get great fuel milage if you know how to drive it right lol

CallMeThatOneGuy

iTrader: (2)

I only get like 20 mpg city... I just have to clean out all of the crud from my engine amd replace that pesky front o2 sensor.


as far as running 87, when I did that I lost a fuel injector. Never again will I do that. I run 91 no ethanol or 93. It WILL pay off.

Kuhn_man

iTrader: (8)

i had an injector go out on me, went threw a half a tank in 75 miles thing broke at wide open

but 20 mpg in the city isnt that bad, it depends how much stop and go you have and how heavy of a foot you got

Timm80

fffffffuuuuuuggggggg I really don't wanna pay for premium in my car. Its not that nice haha. Replacing the knock sensor is a total pita, so I'm going to replace my o2 sensors first. If that doesn't solve it, I'll replace my cat. If that doesn't solve it then I'll replace my knock sensor... and if I have to, use premium haha. Thanks for all the suggestions. I'll make sure and let you all know when I solve the problem.

ChrisMan287

iTrader: (34)

Start running premium and then throw a resistor in your KS.

I don't see why people make such a big deal about the extra cost of premium. What's it like $3.50 extra per fill up?

Kuhn_man

iTrader: (8)

takin the long way around for an easy fix, your gonna end up spending so much more money and time doing it that way and for the gas. spend the extra 10 cents a gallon its way better for your car, it is more than worth it

Kuhn_man

iTrader: (8)

right? and it more than worth it to do so,

Timm80

Alright alright, I'll take everyone's word for it and start using premium... I guess the bit of extra power will be worth it as well. Where did you pick up your resistor from? I've looked all over the internet and then looked at reviews for the ones I found, and I can't seem to find any good ones

dwapenyi

iTrader: (3)

Premium's not going to get his mileage back. Remember, he was getting great mileage before, and we can safely assume it was with the same grade gas he was using.....87 octane.

Timm80, like I said before, change JUST the front O2 sensor, the one right there in the exhaust manifold when you pop open the hood. O2 sensors have a tendency of getting tired and NOT failing. In other words not throwing codes, no check engine light, etc.

And again, YOU ARE WELCOME

DW

njmaxseltd

iTrader: (19)

Throwing parts at it for no apparent reason isn't going to solve your problem.

dwapenyi

iTrader: (3)

Since all your O2s have failed the ECM is running on pre-programmed settings. Those settings happen to be very good.

Think about it...on a slightly different note, when your MAF fails, the car still runs. It wont go over 2500 rpms, but it still runs.

There are lots of fail-safes built into the ECM.

DW

dwapenyi

iTrader: (3)

That is very true, BUT, when you have a situation where all else has failed, logic would dictate exploring the unlikely. O2 sensor "tiredness" seems to be a relatively new thing given that cars have only recently (like in the past 30 years) become computerized.

As Mr. Spock would say with one eyebrow cocked........"most....fascinating."

By the way, I experienced this 1st hand. My friend's I30 was acting funny. I gave it a total tune up, plugs, air filter, oil change etc.....all the standard stuff. His car still acted strange. NO CHECK ENGINE LIGHT. Well, he did have a check engine light for EGR, but that was unrelated.

Reading thru the FSM I figured that the ECM only reads the front O2 sensor for correct air fuel ratio, and his O2 may have been tired. I changed it and his car became a bat out of hell.


I fixed the EGR, too....much later tho EGR only affect emissions, not gas mileage.




DW

ChrisMan287

iTrader: (34)

All you need is a 470K ohm resistor.

Timm80

Yes, I have been using 87 octane the entire time. Oh ok, I thought by front sensor you meant the front two, the rear being the one behind the cat. I'm pretty sure thats the first part I'll end up replacing. I'm going to borrow one of my friends good scanners though and analyze my ecm to see if I can troubleshoot it that way. I have a pretty good hunch your right about the o2 sensor though.

Timm80

Throwing parts at it for no apparent reason would be if I hadn't tried anything else and the first thing I did was start replacing parts... obviously from everything I've listed on this thread, this isn't the first thing I'm trying to solve this problem. It definitely doesn't hurt to at least remove the o2 sesnors, inspect them and possibly replace them. Besides... if I find thats not the solution to my problem, I can always return the part and replace it with the old one.

CallMeThatOneGuy

iTrader: (2)

the fact that he used 87 octane and had good mpg doesnt mean that using 87 didn't cause his milage to deteriorate. If he had used premium to start then maybe the whole apparent "chain of problems" wouldnt have happened

dwapenyi

iTrader: (3)

I'm pretty sure you cant return O2 sensors, but if you know yours are old, replace them.

Replace just 1, the one in the front exhaust manifold. You might need to replace others because the your ECM may eventually "see" the difference between the fresh "fast" new one and the older slow old ones, but you'll cross that bridge when you get to it.

DW

Timm80

Yeah I bet your right. It would make sense that using 87 octane has slowly worn out my knock sensor since this has been happening over a significant period of time. I'm going to hook up a high quality scanner to my computer and analyze my cars vitals before replacing anything though.

get_max_out

it might be your o2 sensor or stop using 87 and use 91 cause that cheap gas burn like water

Kuhn_man

iTrader: (8)

we never said premium is going to fix all his problems but using 87 could cause alot more issues for his car because its not designed to run off of it

Kuhn_man

iTrader: (8)

whats the rear O2 sensor for than?

Kuhn_man

iTrader: (8)

what happens is when the knock sensor goes bad the computer majorly retards the timing to prevent engine knock and that kills fuel milage i.e a safe mode

GGENIUS

iTrader: (1)

BLAM.gif

dwapenyi

iTrader: (3)

This just speculation but I believe that the 4th gen ECM relies primarily on the front O2 to get proper air/fuel readings. The rear O2 is there to tell the ECU that the rear 3 cylinders are pretty-much-running-within-specification. It's like the ECU reads the front O2 with a magnifying glass but the rear O2 with a casual glance.

My speculation only.

My friend's I30 is still running great about a year and a half later with no check engine light and only the front O2 replaced.

DW

dwapenyi

iTrader: (3)

Timm80, another thing you should do is add a grounding kit. Or, clean the original ones. Cleaning is simple, but the effect only lasts a short time. You can clean the grounds by simply unscrewing and re-screwing the bolts in. This will dislodge some rust and clean up the contact for a short time.

Do a search on the org to find all the ground points. One of the main ones is the -ve battery cable. That cables grounds to the inner fender and then to the engine. Unscrew and re-screw those points in and see if your car acts a little better.

If you do notice a difference in your car's behavior, then you need to do a proper ground cleaning, or add another, like this;

http://www.vqpower.com/v2/news.php?readmore=136

You can make your own ground kit by buying some battery cables, too.

DW

Kuhn_man

iTrader: (8)

i could see that

Timm80

Alright, would it be necessary to replace the knock sensor then? I talked to my dad about it and he said hes never heard of a knock sensor needing to be replaced because of using the wrong octane fuel, but I've heard multiple people tell me it needs to be replaced.