hot460

My friend's '99 Maxima with 190K miles is burning a lot of fuel. It has gone from about 25mpg to 11mpg. There is no CEL and the only code that we have gotten is P0325 knock sensor. The car produces a lot of black smoke from unburned fuel. I feel an occasional "studder" driving a low rpm in OD at about 40 mph. This car has run regular 87 octane fuel since new and does not perform any differently on 93 octane. The local (Houston) dealer cannot find the problem since there is no code. Some history - A month ago the car started missing badly and the CEL blinked but did not stay on. It threw a P1320 misfire code so I had new plugs and Nissan coils installed. About 2 years ago I replaced the MAF sensor. Following the coil/plug replacement, the car runs smoother, but started using a huge amount of fuel. I have considered O2 sensors, but again, there is no code indicating they are the problem. Also, I have thought about a stuck injector. I would appreciate any advice to solve this problem.

Maxima 97 SE

You probably have a leaking fuel injector.

Turbobink

iTrader: (2)

Why have you not replaced the KS ... and have you investigated a failed/failing fuel pressure regulator?

Your symptoms (aside from the disappearing DTCs) can also be attributed to failing cam and crank position sensors and/or coolant temp sensor.

hot460

Wasn't aware a triggered KS was an indication of it needing replaced, but will suggest this to the Nissan service department, along with the crank position sensor and coolant temp sensor. The temp gauge is reading about the same as it always has, however. Will also suggest they check the injectors. I was hoping that the Nissan dealership had the right high tech diagnostic equipment to discover the problem, but apparently not. Thanks.

The Wizard

iTrader: (43)

First, pull the spark plugs and see what story they tell you. A visual inspection is a must as it can tell you where you have no fuel, too much fuel etc.

More than likely, you have an injector issue.

dwapenyi

iTrader: (3)

Replace your primary O2 sensors even with no code. They get tired after about 100K miles. When they get tired, they hurt performance and fuel economy.
Diagnose and fix your leaky fuel injector(s)
Lastly, when you replace the KS, after doing all this, you WILL feel a difference between using 87 octane and 93.

You don't have to replace the KS if you plan on running 87 octane forever. You can leave it and the car will be fine. It will not perform at its very best, though.

vqmaxman

The one thing was not mentioned the egr tube if that suckers clogged that will also cause bad mpg and performance check the injectors might be getting to much gas in the chamber,via stuck opened. The one thing that you will have to do as wizard said is to get the plugs out and check them. The ks also causes bad mpg.

max ride 41

iTrader: (4)

i know why i'm getting ****ty mpg, knock sensor and egr codes right now. and, i just had a hanger break off the inside rear wheel well that holds up the muffler. that breaks any further and my new exhaust might be in jeopardy.

dwapenyi

iTrader: (3)

A bad KS and EGR do not cause bad mileage.

You can run with a bad KS forever and get decent gas mileage. Just don't waste your money putting 93 octane in your tank.

EGR mostly affects your tailpipe emissions. With a bad or clogged EGR, your NOx emissions will go up.

max ride 41

iTrader: (4)

trust me man, i've noticed a decline in mpg and those 2 items do affect it. i know it's typically plugs and maf etc allthough the cars chugging along with a clogged egr is affecting my mpg. 2 or 3 miles to every gallon.

vqmaxman

No, the egr and ks do make big difference in the gas mpg your car running in safe mode and the egr is clogged car working harder and hotter to get rid of the extra carbons. It happened to me I was get 8-10 mpg terrible mpg. I fixed it all and now I am happy.

max ride 41

iTrader: (4)

ah, i'll just drive my 07 sonata i just got and save maxy for weekends.

vqmaxman

Lol

hot460

Update on the Maxima:

I had the knock sensor replaced (but not the harness), checked the temp sensor (reading was fine), replaced fuel pressure regulator, checked the plugs for signs of stuck/leaking fuel injector (no sign of wet plug, but all showed effects of running very rich - cleaned & replaced). Still no CEL and only code is knock sensor (even though it was replaced). Car is still getting around 10 - 12 mpg and will smoke like a diesel if you floor it. Runs "jumpy" at low rpm (below 2K) in OD.

Have not replaced O2 sensors but hate to keep throwing parts at this without a diagnosis. Any further ideas will be appreciated.

max ride 41

iTrader: (4)

i am having all the same symptoms as you and i have replaced all my 02 swnsors and my knock sensor as well, i am getting a code for a egr flow malfunction which i believe is causing it to throw a knock sensor code as well. cleared them both and no codes, but i know the egr is still screwed up, i think that's the problem you might be having as well.

hot460

I'm not getting a P0400 code but this could still be the problem. I appreciate it.

JvG

I'm old, and grew up with cars that we had to diagose ourselves. No computer, no trouble codes,
just symptoms to diagnose.

A car that ran like yours back then would be diagnosed as having a sunken float in the carburetor. Which meant over fueling, bad mileage, and black smoke.

Fuel injection replicates carburetion. Instead of a sunken float, you have a broken pintile cap on a fuel injector. The problem is the same. Gasoline leaking where it shouldnt.

Instead of rebuilding a leaky carburetor you need to fix a leaky injector.
The shop which replaced the knock sensor might have removed the uim. Too bad.
They could have installed rebuilt fuel injectors and solved your problem.
I had my injectors rebuilt when I removed my uim. Mine did not leak at the time, but I thought that I might as well service the injectors "while I was in there anyway". My engine runs better than ever.

The oxy sensors should be replaced from time to time. Performace and mileage does improve.

Younger professional parts replacers ("mechanics") are guided by trouble codes. They were taught
What to do, not How To Think Things Through. No cel, no thinking.

The Wizard discovered this diagnosis as a result of having this problem and thinking through it.
He is right. Listen to him. Replace your injectors. All of them. Once one breaks, the others will as well.

General good practice.... service fuel injectors when the uim is off.

max ride 41

iTrader: (4)

mmmm, maybe. i do one at a time as it aint cheap to do all of them at once. but i know which ones i've replaced over time and do the intake clean out and fuel system cleaner in the gas tank when needed.

JvG

I had my injectors cleaned and rebuilt for 18 bucks each at injector RX in Houston. I'm satisfied with their work.

JvG

Servicing the rear injectors requires removing the uim. You could remove the uim anytime an injector fails, or once if all three injectors are changed at once.

I thought that removing the uim sucked. I never wanted to do it again. Replace three injectors while you are in there anyway.

Peace of mind is worth the sixty bucks to service three injectors.

max ride 41

iTrader: (4)

there's no garage around here (pa) that'll do that job that cheap. they're all over-priced rip-off artists. 70 to 120.00 and hour labor, and none specialize in just injector service/rebuilding. they want to send them out to be done and expect you to leave your car all day or more while it's being done.

hot460

The intake manifold wasn't removed to install the knock sensor. My mechanic's figures if there was a leaking injector, that there would be a wet spark plug and that the car would be hard to start, as the fuel pressure would bleed down from the leaking injector. His comments.... I don't doubt that with the mileage this car has that it needs new or rebuilt injectors, it's just that there isn't any hard evidence they are the problem. Appreciate the help.

JvG

Several of us who are familiar with this car disagree with your mechanic. Time will tell who is right.

JvG

The price I quoted, 18 bucks per injector, was what I was charged for ultrasonic cleaning and parts replacement. I'm not including labor. I do my own work.
So three injectors, sixty bucks, including beer.

hot460

I respect your experience so I will see about getting the injectors rebuilt. Does anyone have any recommendations on where to buy a set of rebuilt injectors vs. rebuilding my old ones? I see eBay has a LOT of them listed at a wide range of prices.

JvG

No need for new injectors. Buy only injectors which have been cleaned and have parts replaced. I have seen injectors there which appear to have just been removed from a junk car. Do no buy those.

bumpypickle

iTrader: (1)

My suggestion for you would be to pull oem injectors from a junkyard, have them rebuilt, then either swap them in yourself or pay someone to do it. That way you can still drive your car around w/o having to tow. If your not doing the work yourself. I've never touched an injector so Idk how labor intensive the job is, but i've heard that the fronts are easy and the rears are a PINA. Wizard seemes to be very knowledgable with injectors so I would PM him with advice on where to send them, get injectors ect.

JvG

I used: injector RX in Houston.

hot460

Thanks. They are about an hour from where I live.

njmaxseltd

iTrader: (19)

Get a scanner and look at the actual MAF sensor readings. I'll bet it's reading way to high and causing the ECU to dump fuel. You should also see fuel trims out of wack if that's indeed the case.