Need Help with P0171 System Too Lean

[Mike98]

Apologies for the long post. I wanted to provide enough data to help the experts on this forum diagnose the problem.

My 2006 V70 NA 2.4L M56 has 114,000 miles and in February started reporting a P0171 code (System Too Lean). The engine still had normal power, it idled and ran smoothly. Vida reported code ECM-250A long term fuel trim high limit, this corresponds to the SAE P0171 code.

The PCV system had never been serviced but the glove test was OK last year. I did the glove test in February after P0171 appeared and it failed: glove was deflated at idle right after startup, but after warmup the glove inflated when revving the engine.

In March I removed the intake manifold and found that the original plastic PCV pipe from valve cover to breather box was cracked in multiple places. I replaced the entire PCV system (breather box and all hoses) with Volvo parts from FCP. I checked the engine block oil drain inlet (connects to bottom of breather box) and confirmed that it was not clogged. Also replaced alternator voltage regulator/brushes, all heater and radiator hoses, coolant reservoir, spark plugs (Volvo 3 prong), fuel injector O-rings, fuel rail inlet fitting and O-ring, all vacuum hoses connected to intake manifold.

The removed spark plugs had 30,000 miles of use and the tips on all 5 were consistent: some electrode wear and light tan color on insulators. After reassembly, the PCV glove test is good - constant vacuum when idling and revving.

The car still runs the same, smooth idling and normal power but the P0171 fault returns within 1-2 days of local driving. In addition to Vida, I have an OBDlink SX USB dongle and PC based Obdwiz software. OBDwiz can datalog any SAE OBD parameters supported by the ECU. After logging, the data is available in a spreadsheet file as well as graphically. This week I set up OBDwiz to log these items:

1. SAE 0x06 Short Term Fuel % Trim Bank 1
2. SAE 0x07 Long Term Fuel % Trim Bank 1
3. SAE 0x0D Vehicle Speed (mph)
4. SAE 0x0C Engine RPM
5. SAE 0x34 O2 sensor lambda wide range (current probe) Bank 1 Sensor 1
6. SAE 0x15 02 Voltage Bank 1 Sensor 2
7. SAE 0x10 Mass Airflow rate (lb/min)
8. SAE 0x44 Fuel/Air commanded equivalence ratio
9. SAE 0x0B Intake Manifold Absolute Pressure (inHg)
10. SAE 0x11 Absolute Throttle Position %
11. SAE 0x0A Fuel Rail Pressure (gauge)
12. SAE 0x43 Absolute Load value
13. SAE 0x42 Control Module Voltage

I logged data for a few minutes at idle with engine warmed up, then drove at various speeds on almost-flat roads as well as 70 mph up a mountain (800 ft elevation change over 5 miles).

Initial observations from datalogs:
----------------------------------------------
1. Fuel injection rail pressure is constant 54 psi under all conditions, including the 70 mph mountain climb.
2. O2 sensor 1 lambda stays at 1.0 under all normal driving but jumps to a limit value of 1.3 (very lean) under most throttle-off actions.
3. Under medium-hard acceleration, long term fuel trim consistently exceeds +20%. Is 20% the threshold for a P0171 pending fault ?

Here is datalog info for various operating conditions:

Idle 800 rpm
----------------
Long Term fuel trim: -2.5%
Short Term Fuel Trim: stable around 0%
O2 Sensor 1 Lambda: stable 1.0
O2 sensor 2: 0.6-0.8 volts
Fuel Air Commanded equivalence ratio: 0.98 - 1.0
Absolute Throttle Position: 1%
Intake Manifold Absolute Pressure: 9 inHg
Mass Airflow rate: stable 0.5 lb/minute
Engine rpm (M56 in neutral): 800 rpm

Off Idle 2500 rpm
------------------------
Long Term fuel trim: -2%
Short Term Fuel Trim: stable around 10%
O2 Sensor 1 Lambda: stable 1.0
O2 sensor 2: Toggles between 0.2 - 0.7 volts every few seconds
Fuel Air Commanded equivalence ratio: 0.98 - 1.0
Absolute Throttle Position: 9%
Intake Manifold Absolute Pressure: 8 inHg
Mass Airflow rate: stable 1.0 lb/minute
Engine rpm (M56 in neutral): 2500 rpm

55 mph on flat road using cruise control
--------------------------------------------------
Long Term fuel trim: +15%
Short Term Fuel Trim: stable around 0%
O2 Sensor 1 Lambda: stable 1.0
O2 sensor 2: 0.4 - 0.6 volts
Fuel Air Commanded equivalence ratio: 0.98 - 1.0
Absolute Throttle Position: 15-20%
Intake Manifold Absolute Pressure: 15-20 inHg
Mass Airflow rate: stable 3 lb/minute
Engine rpm (5th gear on M56) : 2300 rpm

70 mph up steep mountain using cruise control
-----------------------------------------------------------
Long Term fuel trim:stable +18%
Short Term Fuel Trim: stable +2%
O2 Sensor 1 Lambda: stable 0.99 - 1.0
O2 sensor 2: stable 0.6 volts
Fuel Air Commanded equivalence ratio: stable 0.99
Absolute Throttle Position: 30%
Intake Manifold Absolute Pressure: 25 inHg
Mass Airflow rate: stable 5 lb/minute
Engine rpm (5th gear on M56) : 2900 rpm

55 mph steep grade down mountain (engine braking in 4th gear)
----------------------------------------------------------------------------------
Long Term fuel trim: stable -2%
Short Term Fuel Trim: stable +5%
O2 Sensor 1 Lambda: stable 1.3 (very lean)
O2 sensor 2: stable 0 volts
Fuel Air Commanded equivalence ratio: stable 0.95
Absolute Throttle Position: 2.5%
Intake Manifold Absolute Pressure: 7 inHg
Mass Airflow rate: Stable 1 lb/minute
Engine rpm (4th gear on M56) : 2700 rpm

Does the data above (especially the 70 mph mountain climb data) point to an item that should be tested or replaced ?

The long term fuel trim seems way too high under many load conditions. The wideband O2 sensor 1 is original. Perhaps the calibration has drifted after 114,000 miles ? If that is true, then the engine would actully be running rich. But the engine runs fine and there are no gas smells from the exhaust. I havent found any intake vacuum leaks.

The battery was disconnected for many days. Should this have reset the Air/Fuel ratio adaptation tables in the ECU ? I have not found any other procedure for resetting the A/F adaptation.

All comments and analysis are appreciated ! I can provide more data, graphs etc and do any suggested tests.

Thanks,

Mike

[scot850]

If the battery has been disconnected completely, I would expect it to reset the trim values. Owners suggest touching disconnected +ve and -ve cables to drain down any residual power in the system. But is the battery has been disconnected for 2 days I'd expect that to happen.

When you throttle off, the engine will cut the fuel flow as it does not need power, and in that case it will show as lean as you will get air but not gas flowing which will give the lean condition. That I'd say is normal.

The long term fuel trin may be rich. This can come from worn O2 sensors, dirty fuel injectors or failing ECT (coolant sensor) reading cool and telling the ECU to richen the fuel input.

I'm sure there are others with an input on this. The color of your old spark plugs would say your your engine is running correctly even after 30k miles which is about change time.

I'd say overall this is a case of analysis paralysis!

Neil.

[Mike98]

Thanks for explaining the lean O2 sensor data when off-throttle.

I assume the engine coolant temperature sensor is working because the ODB parameter for coolant temperature reports correct values during warmup and operation. Also that sensor was replaced last year as part of a new thermostat assembly.

I did another test drive and monitored the OBD parameter for catalytic converter temperature. It showed 1000-1100 F at idle after warmup , 1250-1300 F cruising on 55mph highway, 1300 - 1400 F ascending a mountain at 55mph in 4th gear. I have not found any data on what the OBD reported cat temperatures on a P2 NA engine should be. I did read that temperature would have to go above 1600 F to melt a cat.

If anyone has the capability to read the OBD parameter for cat temperature on their P2 car, I would appreciate knowing the results.

Mike

[Vova585]

I would suspect 2 options here. Option 1-clogged fuel injector(based on the year and millage this car was not driven a lot and such can be detrimental for fuel system especially if ethanol fuel was used. If you can afford to spend $300 I would buy 5 new injectors from fcp and likely your issue will resolve. Alternatively you can look up reputable injector rebuild service who will clean yours and provide you with data of flow pre and post cleaning(I would rather buy new bosch and forget about it). Another alternative-catalytic converter is getting "plugged" and causing restriction in flow, thus lean code. However I would expect P0420 in such case. 3rd option-you are dealing with vacuum leak. I think "vtl" was recently suggesting inexpensive smoke machine which worked great for another member and was affordable($60?). Me being lazy and short on time-replace injectors with new x5, if not better-curse a lot and perform smoke test. Let's hope issue resolves after 1st or 2nd option.

[Mike98]

If the O2 and MAF sensors are accurate, it does look like the injectors are not producing the expected flow rates at medium to high engine loads. The long term fuel trim could be high because it is compensating for deficient injector flow.

Do the catalytic converter temperatures reported by ODB2 seem normal (1000 -1400 F depending on engine load) ?

If the temperatures are normal, I will assume that the air/fuel ratio as reported by the upstream O2 sensor is correct. If the temperatures are higher than normal, maybe the system is running rich and overheating the cat. That could mean the upstream O2 sensor calibration has drifted, because it reports lambda = 1 with only small variations, even when long term fuel trim goes above 20%.

I dont know how the ECU calculates the cat temperature. The upstream and downstream O2 sensors have heating elements but I dont see any mention of a cat temperature sensor in the Volvo wiring diagrams for gas fueled 2006 V70 . Maybe the ECU infers cat temperaure from some other data ?

Thanks,

Mike

[Mike98]

The problem is fixed ! Replacing the front oxygen sensor dropped the long term fuel trim by a few percent but it was still too high under heavy loads. Replacing the MAF sensor dropped the max long term fuel trim to the 5% range. The ECM-250A / P0171 fuel trim fault code (long term fuel trim high limit) no longer occurs.

Mike