2001 Nissan Maxima 95K miles

I too am getting a P0171 code. Car is running fine except for a very slight hesitation for the first minute after a cold start. Recently started getting even more milage than ever (car has gotten apx 360 miles per tank for the last 6 years... now it's getting about 420 miles per tank). Maybe I shouldn't complain, but I know mixture that's too lean can damage the engine.

Here's the information thus far:

Short Term Trims B1 & B2 go up to apx +/-15% while driving.. they vary between +/-5% while idling in neutral

Long Term Trims B1 & B2 are fairly seady at apx +33%

MAF reads apx 2.5 gm/s at idle (600 RPM) and apx 8 gm/s at 2500 RPM

MAF goes up to 20 - 30 gm/s when at acclerating at highway speeds

I plotted MAF against engine RPM... they tracked pretty closely

Ran a can of fuel injector cleaner through the gas tank

Cleanned MAF sensor with MAF cleaner (didn't do anything)

Replaced B1S1 O2 sensor (didn't do anything)

Check vacuum at fuel pressure regulator (18")

No detectable vacuum or exhaust leaks

What is causing the trims to be so high? Does anyone have the ability to monitor their MAF reading while idling and under load.. I'm thinking the MAF is just reading somewhat low when under load causing the system to run lean...

Any suggestions?

I'll run this through ASIST at work in the morning and see what I can come up with.

A few other data points.

#1 Checked throttle position sensor by plotting with OBDII data logger (looks good)

#2 Checked Speed sensor with data logger (looks good)

#3 Check Air Inlet temp and Engine coolant temp sensor ... All good.

It always throws the code when driving at 50 to 60mph (RPM at apx 2250 to 2400 RPM) when the car is fully warmed up (got this data from the freeze frame data)

I know what the normal range for the MAF is while idling and at 2500RPM in neutral (from the FSM), however, does anyone know what the normal range for the MAF is when driving under heavy load or at highway speeds (like 50mph). If someone has a OBDII data logger, I'd appreciate you checking your MAF sensor while driving

ThanksSteve

It sounds like you have clogged injectors...Replace them all as a set. We have had a lot of problems with injectors going bad in the Northeast and I have been told it's the fuel we have been getting. Injector cleaner won't fix this problem. Good luck.

isnt there a ecm reprogram for po171 code? check maxima.org. alot of stuff there

There is a reprogram available for the 2001 Maxima but it has to do with the O2 sensors. I would have to know the ECM P/N in order to know if that vehicle is eligible for the reprogram. The only way to know that, is to connect the vehicle to CONSULT.

I'm guessing that the MAF is tracking airflow, but it's just reading low which is causing the fuel trims to be high (trying to compensate). I say this because at idle and at 2500 RPM, the MAF readings are in spec, however, thay are at the very low end of spec.

I would think injectors clogged enough to cause this problem would cause the engine to run rough or get less milage (not higher mileage). Also, the fact that both fuel trims track very closely leads me to believe it's not clogged injectors (bank 1 and 2 injectors would need to be clogged almost exactly the same to get the same elevated trims on both banks)

What will really help me out is knowing what the MAF reading should be under load at higher speeds (like 50mph)... knowing the long term trims at that speed would also be helpfull

Steve

When you have clogged injectors you usually don't feel it running rough. Some rare occasions you can feel something. You would get better gas mileage because you are using less fuel hence the lean condition....I just replaced a set on a car today for the same issue you have.....You're fuel trims for both banks can be equally lean.

Fuel pump pressure/volume? Do a driving test with gauge attached idle, cruise and WOT at 6,000 rpm.

Unfortunately, I don't have a fuel pressure gauge to be able to measure the fuel pressure and volume. Also, after looking at the FSM, cleaning/changing the injectors looked like a fairly involved procedure (not something for the backyard mechanic)

That's why I wanted to verify that MAF is good prior to looking towards the injectors

I measured the MAF on a Nissan Villager with a 6 cyclinder engine (not exactly the same engine but close)... the MAF readings were 2 to 3 times higher than mine.

Also, I verified no leaks in the intake by covering the intake to the MAF with a piece of plastic wrap... the engine died of course... and it held a vacuum after the engine quit... you could hear the his when I pulled the plastic away.

Well, I decided to shotgun the MAF sensor, so I ordered one today ($83 + shipping) from Southpoint Nissanthe, the place I've read so much about here)... Martin took the order and was great.. I should have be end of next week... I'll let ya'll know how I make out.

PS... the MEL light went out today... however, the LT trims are still very high... I'm sure it'll go back on as soon as I get back up on the highway.

One final thing... I went to the local Nissan dealer and they tried to sell me a MAF for just under $500... I told them to double check that they had the correct part number because I knew it should only be about $100... they said they had the correct part number and that it was $500..hmmmmmmmmmmm... makes you wonder.

Yeah, walk-up dealership sales are hugely increased.

OK... the answer is in!!!!

The new MAF sensor arrived ($83 plus shipping from Southpoint Nissan)... Took 10minutes to install (I installed the entire sensor, I did not try to save a few minutes by swapping out just the element as some folks have done, there was a note inside the package that specifically said to replace the entire sensor, it said you cannot just swap the elements... I wonder why, the housings looked exactly the same).

The airflow readings with the MAF sensors are about 50% to 200% higher than the old sensor (the biggest difference between the new and old sensor occurs at the higher RPMs, 2500+).

I reset the long term fuel trims and have since put 180 miles on the car over three trips. The long term trims for both bank 1 and 2 have gone from +33% to apx -10%. The short term trims are now near zero most of the time and never go much above or below +/-5% (this tells me that the system has completed adjusting to the new sensor).

Summary diagnosis... the MAF sensor was failing "soft"... i.e. it was improperly reading low, but it not low enough to throw a MAF code. This caused the fuel trims to run high throwing the 0171 (and an occasional 0174 code).

I was about to reply to your post actually.

We see the infamous P0171 in the 2000-2001 Maximas, VERY common. You can go looking for air intake leaks, exhaust leaks, test for bad O2 sensors all day and find nothing. It just comes down to a pre-maturely failing MAF that is failing, but not enough to trip the ECM.

Actually, I just had a 2001 Maxima GXE with about 88,000 on the odometer come in today with this code. I threw in the new MAF and did an ECM reprogram to the new updated P/N and Wah-la, no more problems. I however, had a little more to go on then you. I had air fuel alphas sitting at around 115% at idle, borderline to me, looked a little phishy. Also, while at idle, the MAF voltage was about 1.5V, about 0.2-0.4V lower then what it should be at idle. So the ECU is seeing less air than what is actually coming into the engine, thus not giving enough fuel, thus causing the Fuel System Lean Bank 1 code.

But congrats on your fix.

OK... it's been 4 years an probably about 60K miles... and guess what... the car is starting to hestitate again when initially starting to drive on cold days. Hesitation goes away pretty quickly when car warms up.

Measured the MAF sensor at idle.. .it was apx 2.9 g/s. If I remember right, when the sensor failed 4 years ago, I was reading low 2's before replacing it and about mid 4's after replacing it. So I think the same thing is happening again. The sensor is reporting less air than is actually going through it which causes the computer to send less fuel to the engine.

I've got a new MAF on order.