P0171 trouble code question

[Fugy]

Hello,

So my P0171 engine code had popped up the other day. Ran a can of royal purple and engine code went away after the tankful of gas but unfortunately came back. Then my car started stuttering when engine is on load in 2nd and 3rd gear which seemed like an ignition problem. I changed the spark plugs and coil packs and it cured the stuttering and the engine code went away without any manual clearance of the code. After about a hundred miles it came back. What's going on!?

I searched and they said it could be a vacuum leak, MAP sensor, ignition issue, O2 sensor, fuel cap issue.. But I was wondering with my scenario is there anyway of narrowing it down? Thanks.

 

[tof]

Lean condition on cylinder bank 1. Could be MAP sensor. Could be air getting into the intake path downstream from the MAP sensor. Could be an issue with one or more fuel injectors. I guess it could even be faulty O2 sensor reading lean when it's not. Easiest thing to check would be vacuum line connections. Be sure to check your PCV hose connection.

Could this be something as simple as a dirty fuel filter?

 

[Fugy]

Thanks for your reply!

I know the fuel filter is clean as I recently have changed it.

If I have a vacuum leak shouldn't my idle be off or not steady cause it idles fine.

I was using a scanner tool and saw that when the car is warned up and I'm cruising on the highway my short term fuel trim on bank 1 goes really high compare to bank 2...

Also, how many O2 sensors do we have for each bank?

 

[jwmelvin]

There are two sensors per bank, one pre-cat and the other post-cat. Only the pre-cat sensor is used for fuel trim. The other is purely for monitoring the condition of the cat.

When you pulled the spark plugs, what did they look like and were there any differences?

Have you ever had the injectors cleaned?

Given that this seems isolated to one bank I would look at things that are particular to that bank. Like the injectors and the O2 sensor.

 

[JW]

There are two sensors per bank, one pre-cat and the other post-cat. Only the pre-cat sensor is used for fuel trim. The other is purely for monitoring the condition of the cat.

'91 to '94, 1 per bank. '95 and upwards 2 per bank

91 NSX Red/Black

 

[jwmelvin]

Yes, my answer only applied to OBD2 cars, as Fugy's appears to be.

 

[tof]

I would like the O2 sensor except I think there is a different code for that. Still it could be a problem with the sensor that doesn't show up to the ecu as a sensor issue.

Checking the plugs is a good idea. I expect one or more to have a white ash kind of coating typical of lean burn. If its just one or two plugs, the problem is likely faulty injector(s). If they are all showing lean in that bank then I like the O2 sensor a lot better. If none of the plugs appear abnormal I don't know what to tell you.

General question: Is it as big a pain to pull the rear bank plugs as it looks to be? I assume the rear bank is bank 1 since piston 1 (in firing order) is in the rear bank.

 

[Larry Bastanza]

I recommend you send out the Fuel Injectors for cleaning. (RC Engineering) You will be amazed at how much better the engine will run with clean injectors.

This code is very intermittent, and I have cured it with the cleaning service, more then once.

HTH,
LarryB

 

[Fugy]

Thanks for all the replies!

Interesting find this morning while driving. I was looking at the short term fuel trims while driving and they fluctuate between -4 to 12 in both banks and the O2 sensor pre-cat in bank 1 would fluctuate between 0.1-0.9 in open loop. Once the O2 warmed up and hit closed loop, the O2 voltage was only at 0.00v or 1.250v. It was stay at each for a few seconds. Associated with that the bank 1 short term fuel trim went all the way up to 45... O2 problem??

Also, would it be cheaper to get the injectors cleaned vs getting new ones? I saw someone on the forum selling it for $300 for the set.

Thanks guys!

 

[Fugy]

I guess it's cheaper to get it cleaned as I saw its $24 per injector.

 

[Fugy]

I would like the O2 sensor except I think there is a different code for that. Still it could be a problem with the sensor that doesn't show up to the ecu as a sensor issue.

Checking the plugs is a good idea. I expect one or more to have a white ash kind of coating typical of lean burn. If its just one or two plugs, the problem is likely faulty injector(s). If they are all showing lean in that bank then I like the O2 sensor a lot better. If none of the plugs appear abnormal I don't know what to tell you.

General question: Is it as big a pain to pull the rear bank plugs as it looks to be? I assume the rear bank is bank 1 since piston 1 (in firing order) is in the rear bank.

To be honest, before I changed all the plugs to new ones, I looked at them and they all relative looked the same in both banks so I'm not quite sure.

The plugs weren't too bad to pull out. I used one of those wobble sockets~

 

[tof]

Yeah, forget what I said about "If they are all showing lean in that bank then I like the O2 sensor a lot better." After I re-engaged my brain that no longer made sense to me. If all the plugs on a bank are showing lean and the O2 sensor is reading lean than the O2 sensor is probably fine.

Now my brain says the problem is the O2 sensor because you say the plugs look normal. To me, the plugs are saying you have a good air/fuel mixture but the O2 sensor is reporting (falsely) a lean condition.

But my smarter brain (my brain on coffee) says do what Larry suggests. First off, Larry is rarely wrong (even when diagnosing issues with nothing more than a few posts on Prime to go bay.) Second, even if the problem is the O2 sensor, cleaning the injectors will be anything but a waste of money as the car will almost certainly run noticeably better.

 

[Fugy]

I think I'll send the injectors in for cleaning despite. Going to change the O2 sensor first to see if it fixes anything.

Any chance that there is a vacuum leak only in the rear bank?

As I mentioned above, after looking at my STFT and my O2 sensor voltage on the scanner, everything is fine until the car runs for about 20mins, then the O2 voltage start dropping and stops fluctuating and the rear bank STFT is stuck around 42.97%. It's the same every time....

 

[Fugy]

It was the O2 sensor (the pre-cat one). It essentially is the air/fuel ratio sensor for bank 1. Now both my short term and long term fuel trims are within normal range. [emoji106]

 

[jwmelvin]

Excellent. Thanks for reporting back.

 

[TURBO2GO]

I'm now getting this same code. 28K miles on an 05. Can the injectors be dirty already?