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 again 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 these items: 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:
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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
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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 is 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. Also O2 Sensor 2 seems to indicate that Air/Fuel ratio is correct. 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