Stacy Thomas

Good afternoon, I have a 2009 maxima 6 cyl and I keep getting p171 and P174 codes. All O2 sensors, MAF, have been changed. Not sure what else is causing it. I have noticed when at a stop the engine kind of loses power alittle and sorta like a misfire. After about 3 to 4 of those the check engine light cones on and I get those codes. Can someone one help with this problem.
Thanks
Stacy

Richard66

Did you thoroughly check for any vacuum leaks, as this will throw those codes? what are you using for an air filter? Have you done any maintenance on the intake or exhaust system lately?

Stacy Thomas

I have not checked for any vacuum leaks. Not sure how to do that. Air filter has been changed by dealer. No maintenence on intake or exhaust system.

Richard66

Vacuum hoses sometimes come loose from their connection points and cause a leak. Look for any small sized hose around the air intake, filter box and the MAF and make sure they are all connected to something. Also if your hearing is keen you could hear a hissing sound.

CorollaULEV

A V6 engine is divided into 2 banks. Cylinder 1,3,5 would be bank 1, cylinder 2,4,6 would be bank 2. Literally/physically, that is both sides of the V. The engine control module monitors air/fuel ratio of each bank with the air-fuel ratio sensor in the exhaust. They are extremely accurate. P0171 indicates lean mixture in bank 1, P0174 indicates lean mixture in bank 2. A lean mixture (too much air OR NOT ENOUGH FUEL) is a significant problem. Air/fuel ratio is EXTREMELY well controlled in modern engines and the goal is 14.7:1 (air:fuel ratio). That is because 14.7:1 gives the cleanest burn with minimal emissions.

You won't get these codes for lean mixture unless the engine control module is having to add an additional 20% of fuel (based upon the readings from the air-fuel ratio sensor) to equal the goal 14.7:1 AFR. That is significant.

The mass airflow sensor inside the intake pipe measures and knows nearly exactly how much air is entering the engine. The engine control module measures the air coming in and knows exactly how much fuel to inject so that the magical 14.7:1 air-fuel ratio is achieved for emissions purpose.

If you are getting a code for a lean mixture on both banks (again, too much air UNMONITORED by the mass airflow sensor or DEFICIENT FUEL DELIVERY), you have a significant problem. As said, if the mass airflow sensor doesn't know the air is getting into the engine (unmonitored air or aka., a vacuum leak), then the engine control module will not know and will not inject the extra fuel needed to achieve the target 14.7:1 air-fuel ratio, thereby leading to a lean mixture. Edit: not true. The control module will inject extra fuel to accommodate the lean mixture, but it is limited by the software programming as well as physical/mechanical issues.

The variables in this problem are both AIR and FUEL. Too much air would cause a lean mixture. Conversely, insufficient fuel would also cause the same problem and codes to appear. A vacuum leak from a hose probably would cause these codes, but on a 2009 vehicle, vacuum hose leaks are not common unless someone has messed around with them. Alternatively and my number one concern would be deficient fuel delivery. I.e., the fuel filter or the fuel pump. If the fuel filter is clogged or the fuel pump is failing, that would not deliver enough fuel, which would net the same effect. This can be checked by measuring fuel pressures, which any competent mechanic can check. I have seen engines blow because of insufficient fuel. I would not drive the vehicle at all until this is fixed unless it is a very short trip to the shop.

Logic: fuel is liquid, when it is injected into the engine, it evaporates and absorbs quite a bit of heat during the combustion process (no different that sweat evaporating to cool the body). Lean mixtures cause engines to run much hotter than they should and leads to eventual overheating, pinging, burnt exhaust valves, etc. not to mention odd idling and loss of power. It's not good at all. Please get this checked out by a good mechanic with a fuel pressure test ASAP.

Thanks,
Travis

black_maxed95

iTrader: (41)

So I just got this similar issue also... I originally had just the p0101 (maf) ses and the car drove fine. I reset the light and about 2 weeks later my ses came back but the car is also now hesitating/stuttering while starting from cold. Now the ses is p0171 p0174 and p0101. I replaced the MAF sensor but that did nothing. This weekend I'm going to try cleaning the throttle body and iacv. I guess if that doesn't solve it, then I will try checking fuel pressure next. Just posting my current issue incase anyone has any ideas.

mjhuffer

Posted on another thread but looking for help.Looking for some advice from the forum. Using the knowledge on this thread, I changed my plugs without changing the plenum gasket. Torqued everything to spec. Drove for about 2,000 miles with no problems but then the car started to stutter at full stop while in drive. Threw codes P0171 and P0174 with an SES light meaning lean bank 1 and 2 (car was getting too much air so was crashing the engine with fuel to compromise). Replaced the plenum gasket but not the throttle gasket. Torqued everything to spec. Cleared the check engine lights. Still getting some slight stuttering at full stops while in drive but not nearly as bad as before. Been a week and problem isn't getting better or worse and no SES light coming back on. Was going to replace TB gasket but the Fel-Pro 61632 from O'Reillys didn't fit (smaller than OEM). Thinking of ordering an OEM replacement but the original one looks to be in great shape. I'm kind of at a loss for what to do next. I also went through the process of replacing all of the small vacuum lines. I would hate to take it into the dealership but it's looking like I might have to so they can see what is going on. Gas mileage is great and no performance lost. Only thing wrong is the slight stutter at full stop while in drive. Any help would be much
appreciated.

UPDATE: I figured I would replace the PCV Valve as my next option. When replacing, the hose from the plenum to the valve had a hole on the backside. Completely missed it when inspecting. $1 worth of hose from Autozone and all is good.

RR5

iTrader: (6)

If you're looking for vacuum leaks around the engine, spray LIGHTLY brake clean at intake joints and listen for a change in idle RPM. Can also monitor the RPM using a Bluetooth ODB2 adapter and the Torque app. For air leaks, I think spraying soapy water over hoses might do the trick but I'd double check this with someone with professional experience. Soapy water into the engine would be bad.

mjhuffer

Thanks for the help. It turned out to be a cracked vacuum hose running from the plenum to the PCV valve on the rear passenger side. completely missed those hoses when inspecting. $1 later and everything is good.