Author |
Topic: Help in understanding OBD log P0171 code (Read 2951 times) |
|
sontakke
Newbie
I'm a YaBB newbie!
Posts: 4
|
 |
Help in understanding OBD log P0171 code
« on: Sep 9th, 2005, 5:09pm » |
 Quote  Modify
|
After looking at many OBD related sites, I have come to realize that this one offers the most comprehensive and helpful advice on OBD related topics.
I have Nissan Maxima 2000 (77K miles) 3.0L which I had been battling P0420 Catalytic Converter Efficiency Low for a while. But suddenly it has thrown P0171 Fuel Sysem Lean code.
I have a different OBD scanner. I have the log data and have converted it into readable format. I have posted it to anyboard as
Name: sontakke
E-Mail:
Topic: OBD Scan for Vikas Sontakke
Added on date: Sep 09,2005,15:24
The log is for a drive after I reset the code. What surprised me is that even after the reset, LTFT numbers are way high. Most people are saying that I need to replace MAF. This is one item which most Maxima owners end up replacing.
I have already ordered new MAF unit and will be replacing it once I get it.
I would appreciate if some of you experts can take a look at the log file and comment on it. I would like the datafile converted into nice Excel spreadsheet but have not figured out how to change rows of single parameter value into columnar data arranged by different parameters.
What I have noticed ...
LTFT is high for both banks
Sometimes LTFT+STFT is over 40% but car did not throw a code.
Airflow values are in lb/min. Here is the formula from the OBD scanner
MAF sensor Air Flow Rate 0.3 lb/min 2.15 gm/s
The above was the reading at idle which according to the Factory Service Manual is within the acceptable range, albeit at the low end. The range is 2-6 gm/s
Even after driving about 100+ miles after reset, the LTFT are still hovering in +30 range. The performance of the car has not changed much before or after the code. I have noticing following issues for a while
Slight hesitation between 1K-2K immediately after cold start.
Flat spots iin acceleration at around 5K.
I have also noticed very timing advance numbers but I don't know the specification for it. There are couple of negative timing advance numbers in the log. I don't know if the OBD got bad data.
Once again your comments would be greatly appreciated.
Thanks,
- Vikas
|
|
 IP Logged |
|
|
|
Eric_R
Administrator
Cosworth Owner 1998 - 2006 & site co-founder
Posts: 1792
|
|
Re: Help in understanding OBD log P0171 code
« Reply #1 on: Sep 14th, 2005, 11:46pm » |
Quote Modify
|
Vikas,
Sorry it's been a while for an answer.
Yes, P0171 is Bank 1 lean - this should be the driver's side. You mention that the LTFT and STFT have gone way too lean and it hasn't shown a code - but it has, P0171 is the code for this symptom.
Did you also get P0174 - Bank 2 too lean? On our V6s the inlet system is common to both banks, but you engine might have separate inlet systems. A faulty MAF can certainly affect the fuelling and if it is common for your model then its a good idea to change it, disconnect the battery to reset the fuel trims and take the car for a run and check the fuel trims again. The MAF should be reading high rather than low to produce P0171.
Other causes of P0171/4 can be high fuel pressure - caused by a sticky fuel pressure regulator, and faulty/contaminated HO2S sensors that fail to switch.
The driving concerns you have mentioned will often be associated with P0171 - the bank is too lean for proper combustion. Check the car again after changing the MAF and get back to us. If the codes persist we can suggest further checks to make.
|
|
IP Logged |
After a Cosworth, anything else is just ... ordinary
|
|
|
sontakke
Newbie
I'm a YaBB newbie!
Posts: 4
|
|
Re: Help in understanding OBD log P0171 code
« Reply #2 on: Sep 15th, 2005, 1:45pm » |
Quote Modify
|
on Sep 14th, 2005, 11:46pm, Eric_R wrote:Vikas,
Sorry it's been a while for an answer.
Yes, P0171 is Bank 1 lean - this should be the driver's side. You mention that the LTFT and STFT have gone way too lean and it hasn't shown a code - but it has, P0171 is the code for this symptom.
Did you also get P0174 - Bank 2 too lean? On our V6s the inlet system is common to both banks, but you engine might have separate inlet systems. A faulty MAF can certainly affect the fuelling and if it is common for your model then its a good idea to change it, disconnect the battery to reset the fuel trims and take the car for a run and check the fuel trims again. The MAF should be reading high rather than low to produce P0171.
Other causes of P0171/4 can be high fuel pressure - caused by a sticky fuel pressure regulator, and faulty/contaminated HO2S sensors that fail to switch.
The driving concerns you have mentioned will often be associated with P0171 - the bank is too lean for proper combustion. Check the car again after changing the MAF and get back to us. If the codes persist we can suggest further checks to make.
|
|
What I meant was that after I have reset the codes and ran the car for few hundred miles, the code has not yet come back up. However, STFT for both banks are still too high.
For some reason, Nissan will NOT reset STFT values when you reset the code. I know on my Honda reseting the code will reset the STFT to zero.
I am also concerned with the timing advance values. They seemed to quite high but I don't know what is normal. There are some negative advance in the log which does not make much sense.
Please see if my theory is making sense. My MAF must be slowly going bad. Even though the readings are in spec, they are at the very end of the scale. Since MAF is reporting lower air mass, the computer is supplying less fuel. However, the fuel injected for the actual air mass would be then less than the ideal and O2 will read lean. This will cause the computer to shift FT towards richer i.e. positive offsets. Once it reaches certain maximum, it will throw the code. It is a random chance as to which bank will cross the threshold. Obviously, air-leak which leads to unmetered air or low fuel pressure or clogged injector will lead to similar condition. But I would think low fuel pressure or clogged injector would significantly affect the car behaviour at speed.
I am enclosing an extract from the text file at the end of the reply. I hope it is OK. The units are American and thus AirFlow is in lb/min rather than gm/sec and temps are in deg F.
The Nissan spec for MAF is
@idle 2-6 gm/sec eqvt 0.3-0.9 lb/min
2500 7-20 gm/sec eqvt 1.0-3.0 lb/min
As you can see from the spec, there is wide lattitude for the range. This really surprises me. Can somebody explain how ECU can handle such a wide range?
========================================
...
...
RPM," 1600"
Speed," 24"
Timing," -7.0"
AirFlow," 0.4"
Load," 8.6"
STFT1," 0.0"
LTFT1," 32.8"
STFT2," 0.0"
LTFT2," 32.8"
RPM," 1563"
Speed," 24"
Timing," -7.0"
AirFlow," 0.3"
Load," 8.6"
STFT1," 0.0"
LTFT1," 32.8"
STFT2," 0.0"
LTFT2," 32.8"
RPM," 1263"
Speed," 19"
Timing," 16.0"
AirFlow," 0.3"
Load," 11.0"
STFT1," -0.8"
LTFT1," 32.8"
STFT2," -2.4"
LTFT2," 32.8"
RPM," 500"
Speed," 10"
Timing," 15.0"
AirFlow," 0.3"
Load," 15.7"
STFT1," -7.8"
LTFT1," 32.8"
STFT2," -8.6"
LTFT2," 32.8"
RPM," 563"
Speed," 6"
Timing," 19.0"
AirFlow," 0.3"
Load," 25.1"
STFT1," -13.3"
LTFT1," 32.8"
STFT2," -14.8"
LTFT2," 32.8"
RPM," 700"
Speed," 5"
Timing," 16.0"
AirFlow," 0.3"
Load," 16.1"
STFT1," -7.8"
LTFT1," 32.8"
STFT2," -9.4"
LTFT2," 32.8"
RPM," 675"
Speed," 2"
Timing," 14.0"
AirFlow," 0.3"
Load," 17.3"
STFT1," -14.8"
LTFT1," 32.8"
STFT2," -4.7"
LTFT2," 32.8"
RPM," 688"
Speed," 0"
Timing," 10.0"
AirFlow," 0.3"
Load," 15.3"
STFT1," -7.8"
LTFT1," 32.8"
STFT2," -8.6"
LTFT2," 32.8"
RPM," 613"
Speed," 0"
Timing," 14.0"
AirFlow," 0.2"
Load," 17.6"
STFT1," -13.3"
LTFT1," 32.8"
STFT2," -13.3"
LTFT2," 32.8"
RPM," 600"
Speed," 0"
Timing," 15.0"
AirFlow," 0.2"
Load," 17.6"
STFT1," -18.8"
LTFT1," 32.8"
STFT2," -18.0"
LTFT2," 32.8"
RPM," 600"
Speed," 0"
Timing," 15.0"
AirFlow," 0.2"
Load," 17.3"
STFT1," -15.6"
LTFT1," 32.8"
STFT2," -16.4"
LTFT2," 32.8"
RPM," 600"
Speed," 0"
Timing," 15.0"
AirFlow," 0.2"
Load," 17.6"
STFT1," -18.8"
LTFT1," 32.8"
STFT2," -14.8"
LTFT2," 32.8"
RPM," 600"
Speed," 0"
Timing," 16.0"
AirFlow," 0.2"
Load," 16.9"
STFT1," -21.9"
LTFT1," 32.8"
STFT2," -16.4"
LTFT2," 32.8"
RPM," 613"
Speed," 0"
Timing," 16.0"
AirFlow," 0.2"
Load," 16.1"
|
|
IP Logged |
|
|
|
Eric_R
Administrator
Cosworth Owner 1998 - 2006 & site co-founder
Posts: 1792
|
|
Re: Help in understanding OBD log P0171 code
« Reply #3 on: Sep 15th, 2005, 11:39pm » |
Quote Modify
|
Vikas,
Re the DTC, yes, I understand - you meant that it hadn't returned - gotcha. 
The DTC can be expected to return at any time - it just needs all the parameters to be reached in order to be set again. It may also take a while to get a complete Drive Cycle.
I have to say that the MAF does look very lazy - it's reading only 0.4 @ 1600rpm and 24mph, and only 0.3 @ 1563 and 500 rpm. This would drive the LOAD and fuelling too low and cause the LTFT to rise to compensate - exactly as you speculate. I think your reasoning is spot on.
I notice that both banks LTFT are identical, so the fuelling problem affects both banks - it's just that one bank only actually reached the DTC level last time. So it could be either the MAF or the Fuel Pressure Regulator. The LTFT are way high - meaning that the cumulative running is too lean. The STFT are negative, probably to try to get the HO2S sensors to switch. Of the two I would change the MAF first - it's clearly not performing well and is the most likely culprit.
If you disconnect the battery that should clear the LTFT - it does on ours, anyway.
Don't worry too much about the timing advance being all over the place. This has been asked before and I researched this by looking through my readings of my own and other cars - they're all the same. The OBD only catches two in about 40 timing events per second at idle on your V6. The calculation is made for every single cylinder event, taking everything into consideration - the RPM, air volume and temperature, LOAD of the engine, position of the IACV, ancillaries input etc, so the actual advance for one cylinder will vary widely.
HTH and good luck
|
|
IP Logged |
After a Cosworth, anything else is just ... ordinary
|
|
|
sontakke
Newbie
I'm a YaBB newbie!
Posts: 4
|
|
Re: Help in understanding OBD log P0171 code
« Reply #4 on: Oct 24th, 2007, 4:31pm » |
Quote Modify
|
I hate topics with no closures; so here I am, finally reporting that, Yes indeed, MAF was the culprit. New MAF fixed my problems and the LT fuel trim went back to normal.
- Vikas
|
|
IP Logged |
|
|
|
|
Home
Help
Search
Members
Login
Reply
Send Topic
Print