1997 Infiniti Q45 P0171

Post by **fontana dan** » Sun Jun 26, 2022 5:40 pm

First i want to say hello and thank you for hosting the factory service manuals on this website. Very hard to find service data anywhere else for this car.
Having code P0171 at idle on my 1997 Q45 4.1L. Fuel trims both bank go +30% while idling or cruising, but trims are normal on acceleration.
WOT enrichment gets both o2s to go rich so I do not think there is a fuel delivery issue.
Did a smoke test, found a leaking upper intake gasket and replaced it. No change, still setting p0171 and running very lean at idle.
Symptoms are similar to the ones described in this thread where the cause was a failing, under-reporting maf. I think i will do another smoke test to make sure i did not miss anything, then try a different MAF to see if that makes a difference. It has already been replaced at some point in the past. The reading is in spec according to the service manual, however on the low end of the acceptable range.
p0171-but-everything-tests-out-ok-havin ... 59342.html

Post by **Q451990** » Sun Jun 26, 2022 9:21 pm

to NICO!

Post by **fontana dan** » Wed Jun 29, 2022 10:29 pm

Did another smoke test. Found smoke leaking out of the BPT diaphragm valve for the EGR. Anyone heard of this thing setting P0171?
I ordered a new one.

OEM p/n: 1474102E00
This is the car.

Post by **VStar650CL** » Thu Jun 30, 2022 4:31 am

On a lot of the older ones the EGR and MAP/baro are rigged up together and the software is funky. I've seen bad cats cause codes for both subsystems, and in some sense a downstream vacuum leak is a downstream vacuum leak regardless of the source. So I think a leaky BPT could certainly cause that code.

Post by **Ludeaem** » Thu Jun 30, 2022 5:47 am

Well, thanks for updating us. Most of the time these posts stay open with no resolve.

Post by **fontana dan** » Thu Jun 30, 2022 1:47 pm

I'll have to update again to say if it actually fixes the problem. I have no codes besides P0171 "Bank 1 system lean"
The way i understand it, if that BPT diaphragm is blown then the EGR valve will not open.
This car has a MAP sensor? The PID is not supported on my scan software, and the engine runs like dog with the MAF unplugged.

Post by **Ludeaem** » Thu Jun 30, 2022 2:10 pm

It has a MAP (which near the firewall on the passenger side) and a MAP/BARO sensor which is in the 'V' below the lower intake manifold

Post by **VStar650CL** » Thu Jun 30, 2022 3:05 pm

fontana dan wrote: ↑
Thu Jun 30, 2022 1:47 pm
I'll have to update again to say if it actually fixes the problem. I have no codes besides P0171 "Bank 1 system lean"
The way i understand it, if that BPT diaphragm is blown then the EGR valve will not open.
This car has a MAP sensor? The PID is not supported on my scan software, and the engine runs like dog with the MAF unplugged.

MAF and MAP are two different things, the former directly measures airflow into the intake (Ma** AirFlow) and the latter measures manifold vacuum (Manifold Absolute Pressure). Many older Nissans also used the MAP as a BAROmeter by diverting the sensor port to either manifold or atmosphere, hence MAP/BARO.

P0171 is a generic lean code, basically saying the ECM has used up all its elbow room in adjusting the alpha but the engine is still lean. When it occurs by itself, the two most common causes for it are downstream vacuum leaks (leaks someplace between the throttle plate and the cylinder head) or bad fuel, particularly fuel with too much ethanol. Upstream leaks between the MAF and throttle plate can also cause P0171, but usually you'll get other codes with it such as P0101.

Post by **fontana dan** » Thu Jun 30, 2022 7:25 pm

Ok thanks for the clarification. I got a bit confused by the MAP/BARO pids i think it might just be my s*** software. This is my first time working on an older nissan also.
Here is a snapshot of data at WOT. Recorded just after replacing upper intake gasket.

Post by **VStar650CL** » Fri Jul 01, 2022 5:16 am

That's a pretty low MAF reading, even a 4-cyl will generally shoot way over 200 g/S at WOT. So if it isn't a MAF issue I'd say something is definitely leaking unmetered air.

Post by **fontana dan** » Sat Jul 02, 2022 4:34 pm

NO change after replacing the BPT valve. Still lean at idle.
The MAF readings are in spec according to the FSM but they do seem low. Should i try a different MAF? Any luck sourcing these from junkyards, or recommended brands?
Data from test drive today.
Idling. O2 sensors are stuck lean.

: q45idle.PNG (33.59 KiB) Viewed 1046 times

WOT@2100rpm.

: q45snap.PNG (37.28 KiB) Viewed 1046 times

It feels like it bogs down some when I punch the gas from a stop. Just does not have much power until it reaches 3k rpm.

Post by **VStar650CL** » Sat Jul 02, 2022 5:04 pm

Have you checked for cracks in the intake boot between the MAF and the Throttle Body? I don't see Q45's often and I don't recall the layout, but most Nissan products can develop cracks on the bottom of the boots that only open up when the engine rocks in the mounts on accel or decel. Sentras and gen1 Muranos are infamous for it, for slightly different mechanical reasons. The symptoms are exactly what you describe, bogging on accel and low MAF readings, along with sometimes stalling or misfiring on decel.

Post by **fontana dan** » Sat Jul 02, 2022 10:01 pm

Thanks for your help vstar. I have inspected and smoke tested the intake boot, no leaks found.
I put the data from my snapshots into this Volumetric Efficiency calculator, and the results are 85-92% efficient.
At this point I don't know how to proceed. Only a few ideas. PCV? EGR valve? Fuel system?
https://www.otctools.com/ve

Post by **3Q Jay** » Tue Jul 05, 2022 11:46 am

Joining the thread late--but not surprised on the BPT--those don't do anything good or bad at idle (unless they are a victim of incorrect vacuum signal).
From your plots, you are definitely maxing the idle STFT toward rich, and that really does mean you are getting unmetered air from somewhere.
Can you elaborate on smoke testing for intake leaks? I'm thinking you might want to try some propane feed (or other flammable) all around the intake system to find the air leak.
As for PCV, you might be on to something. Have you replaced them (two) yet?
I like the 20 holes. Clean look for a (paltry) 16" rim, but no fun to keep clean.....

Post by **MowgliCSM** » Tue Jul 05, 2022 2:42 pm

Start spraying things with brakeleen or starting fluid while it is idling and see if you can get the RPM's to jump. You said you did the the upper intake gasket but didn't do the lowers. Any reason? Might be a good idea to pull it all off, clean the valley, put in new knock sensors and new lower intake gaskets.

Post by **fontana dan** » Thu Jul 07, 2022 12:30 pm

Thanks for the help. Will check next with propane.
If its really that much unmetered air its coming in somewhere sneaky.
Going to look into doing a fuel pressure test. Could the knock sensors be retarding the ignition timing?
I didnt replace the lower intake gaskets because they have showed no signs of leaking so far. I made a smoke machine similar to this. Sealed the intake tube, put 2psi smoke in there and checked for leaks all the way to the maf. No large vacuum leaks to be seen this way.

Post by **fontana dan** » Fri Jul 22, 2022 6:39 pm

Did some highway trips, car pulls good at 100mph, engine light is on. 21.8mpg. Drivability is not much affected by this.
Searched for vac leak with propane feed. Could not hear change in RPM.
Did another smoke test with a real smoke machine. No leaks observed.
Done with that business. Suspect MAF.

Post by **fontana dan** » Sat Jul 23, 2022 9:22 am

I forgot to add that the engine has 21 inches Hg of vacuum at a warm idle.

Post by **3Q Jay** » Sat Jul 23, 2022 6:56 pm

MPG and vac suck sound right for the VH41
Is your STFT still going way rich at idle? how long does it take to do that?
Maybe check (at least bank 1) front O2 and see how often it bounces, and as the STFT compensates. does it keep bouncing, or does it eventually peg lean on the O2?
Did you look at and/or replace the PCV (x2)?

Post by **fontana dan** » Sun Jul 24, 2022 2:21 pm

I got a new PCV valve but i broke it while installing. If pcv valve was leaking, wouldn't there be smoke in the crank case during a smoke test? Or vacuum in the crank case with engine running? I will order 2 more to replace.
The STFT goes way rich as soon as the engine warms up and it goes into close loop. There is no O2 switching at idle. Both bank 1&2 o2 sensors read 0.05v constantly while idling.

Post by **3Q Jay** » Sun Jul 24, 2022 4:50 pm

Well, a couple of thoughts. Have you studied the FSM pages EC-163 to 168?
Does the exhaust start to smell rich after idling for a while? Like--as the STFT calls for more fuel, is it actually getting too much fuel even though the sensor says it's lean?
Have you actually measured the fuel pressure at the rail?
If it appears normal (~38ish psig at idle), what happens to the STFT if you just increase the fuel pressure to ~55 by pinching off the return and/or taking vacuum off the fuel pressure regulator?
The above applies to both banks.
Now, you say both banks read lean at idle, but you don't have a lean code for the right bank, so focusing first on the lefte bank.
Have you ohmed your injectors yet for 1,3,5,7?
You might get lucky and hear one of the injectors not pulsing at idle (or at least sounding different than the others) but I wouldn't count on it.
What is the history of the the front O2 sensors? If unknown, and nothing else seems to be wrong, you might consider replacing it just incase it's dead. (although you might of posted previously that the O2s do bounce repeatedly at a 2000 rpm steady no-load --which would discount bad sensor)
Actually, that gives me another thought--maybe the O2 heater is dead or at least very weak (not enough to throw code for bad heater)? Dead heater would affect idle trim, but would likely not be noticed by the time you hit cruise RPM.

Post by **3Q Jay** » Sun Jul 24, 2022 4:52 pm

oh, and PCV thought was that it would be supplying unmetered make-up air if the ball/spring clogged such that it was always flowing.

Post by **fontana dan** » Sun Jul 24, 2022 8:19 pm

You have a lot of good ideas Jay. I have studied the procedure for P0171 in the FSM but it seems to mostly depend on the Consult scan tool.
I have not noticed a rich odor from the exhaust.
How do you measure fuel pressure at the rail? Is there a service port? Fuel pressure test I saw in FSM was to connect an adaptor with a gauge at the fuel filter. I have not done that. Sometimes the car will have an extended crank on restart after sitting for multiple hours with engine warm.
O2s are original. Interesting theory on the heaters possibly going out. Maybe should test that circuit.
To tell you the truth though, I'm getting sick of chasing this issue. Been testing, testing, and testing with no results or direction.
Not to mention the blown front shocks, failing starter, transmission issues, or possibly the most annoying problem which is the alternator noise feeding through the stereo.

Post by **Ludeaem** » Mon Jul 25, 2022 5:03 am

fontana dan wrote: ↑
Sun Jul 24, 2022 8:19 pm
Sometimes the car will have an extended crank on restart after sitting for multiple hours with engine warm.
I'm getting sick of chasing this issue.

Hah! I know the pain. Though, i've not had my p0171 code in awhile, I do have the extended crank on warm starts after its sat for 30+ minutes. I thought maybe the pump? Is it Fuel Pressure? I am almost certain its the upper or lower gasket as I did the knock sensors. Either way, its not a daily driver for me so I live with it. With the extended crank i just prime the pump on ACC for a few seconds before firing and go on my way.

Post by **3Q Jay** » Mon Jul 25, 2022 5:58 am

fontana dan wrote: ↑
Sun Jul 24, 2022 8:19 pm
You have a lot of good ideas Jay. I have studied the procedure for P0171 in the FSM but it seems to mostly depend on the Consult scan tool.
I have not noticed a rich odor from the exhaust.
How do you measure fuel pressure at the rail? Is there a service port? Fuel pressure test I saw in FSM was to connect an adaptor with a gauge at the fuel filter. I have not done that. Sometimes the car will have an extended crank on restart after sitting for multiple hours with engine warm.
O2s are original. Interesting theory on the heaters possibly going out. Maybe should test that circuit.
To tell you the truth though, I'm getting sick of chasing this issue. Been testing, testing, and testing with no results or direction.
Not to mention the blown front shocks, failing starter, transmission issues, or possibly the most annoying problem which is the alternator noise feeding through the stereo.

Hang in there. Yeah, any 25 yo car will have it's moments of frustration. Then you find the problem and the smiles go for miles.
OK, if it's not actually getting rich, then i'd say the likely suspects are clogged injectors (might show up when fuel pressure is increased), MAF (your idle flow shows within spec, but maybe slightly low)--just be suspicious of a used one until proven good, or the O2s. PCVs i think are more a maintenance item at this point--likely not the root cause.
Yes, fuel pressure is just with a Tee at the inlet near the bank 2 valve cover. Speaking of.....what about your fuel filter? it's just forward of the right rear tire on the FGY33.

Post by **MowgliCSM** » Mon Jul 25, 2022 3:22 pm

Are you running an aftermarket headunit? Pioneers will pick up alternator whine. If it is aftermarket, try grounding the harness to the unit itself.

Post by **fontana dan** » Wed Jul 27, 2022 7:13 pm

Thanks Jay I definitely need to go over the fuel system more in-depth to make progress fixing this issue. I have a new fuel filter to install. More curious to test fuel pressure at idle and leak down.
I added a ground strap between the case of the factory audio head unit and the frame of the car. Still getting a buzz/whining sound in the speakers. Noticed the unit in the car is stamped as remanufactured.

Post by **VStar650CL** » Wed Jul 27, 2022 7:47 pm

You can generally suppress any sort of common mode (power or ground bus) noise just by sticking an ignition condenser like this on the alternator output or any other suspect power circuit. Most Nissan products already have one someplace on the supply for the ignition coils, yours may be worn out.

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1997 Infiniti Q45 P0171

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