O2S voltage keeps between 0.7-0.8 @ idle (P2178)
#1
12-19-2017, 02:23 PM
O2S voltage keeps between 0.7-0.8 @ idle (P2178)
Car: 2010 Mazda 5 (2.3L 4Cyl everything stock).
DTC: P2178 (running rich on bank 1 off idle) - it occurred when the car was traveling around 30-50mph three times and it did not go away until reset. Then it would come back on shortly after reset.
The dealer has replaced the MAF and the front O2 sensor in the past week.
With Torque Lite and ELM327 OBD II connection I noticed the following:
- When idle (~600rpm), the O2 sensor maintains between 0.7 and 0.8 V (normally I would expect ~0.4-0.5), so it's already running rich for some reason
- When rev up steadily, there is a voltage drop from 0.7 to 0.3, then it came back up to 0.7. The voltage then drop below 0.1 after rev passes 3,500rpm
- After letting go the gas pedal and entering idle again, the voltage keeps below 0.1V for 10 seconds before coming back between 0.7 and 0.8.
That 10 second period when the O2S reading is below 0.1V indicated that the exhaust mixture has lots of oxygen but no fuel (really lean). Could this throw the rich code (and why)?
The car has not been stalled and I noticed the fuel economy is pretty bad. The fuelly suggested this model year and trim should have ~25-26 mpg but I am getting less than 20mpg. I can also smell gas (I assume it's gas since it's not coolant sweet) from the tailpipe when the car is idling especially when cold. The idling sound also gave me an impression that it had rough idle (but never stalled).
If the TPS (Throttle Position Sensor) is at fault and caused the air/fuel mixture to run rich at idle, it is hard to explain why it runs extremely lean for 10 seconds after letting go the pedal. Could the butterfly valve got so dirty that retarded the valve movement?
I am going to show the dealer these attached screenshots tomorrow. I am wondering what the next step I could suggest them do (vacuum leak test with smoke? EGR/Purge valve/brake booster? Fuel injector o-ring leak? Fuel pressure regulator). Any comments or suggestions on the next thing to check are highly appreciated and Merry Christmas to you all.
DTC: P2178 (running rich on bank 1 off idle) - it occurred when the car was traveling around 30-50mph three times and it did not go away until reset. Then it would come back on shortly after reset.
The dealer has replaced the MAF and the front O2 sensor in the past week.
With Torque Lite and ELM327 OBD II connection I noticed the following:
- When idle (~600rpm), the O2 sensor maintains between 0.7 and 0.8 V (normally I would expect ~0.4-0.5), so it's already running rich for some reason
- When rev up steadily, there is a voltage drop from 0.7 to 0.3, then it came back up to 0.7. The voltage then drop below 0.1 after rev passes 3,500rpm
- After letting go the gas pedal and entering idle again, the voltage keeps below 0.1V for 10 seconds before coming back between 0.7 and 0.8.
That 10 second period when the O2S reading is below 0.1V indicated that the exhaust mixture has lots of oxygen but no fuel (really lean). Could this throw the rich code (and why)?
The car has not been stalled and I noticed the fuel economy is pretty bad. The fuelly suggested this model year and trim should have ~25-26 mpg but I am getting less than 20mpg. I can also smell gas (I assume it's gas since it's not coolant sweet) from the tailpipe when the car is idling especially when cold. The idling sound also gave me an impression that it had rough idle (but never stalled).
If the TPS (Throttle Position Sensor) is at fault and caused the air/fuel mixture to run rich at idle, it is hard to explain why it runs extremely lean for 10 seconds after letting go the pedal. Could the butterfly valve got so dirty that retarded the valve movement?
I am going to show the dealer these attached screenshots tomorrow. I am wondering what the next step I could suggest them do (vacuum leak test with smoke? EGR/Purge valve/brake booster? Fuel injector o-ring leak? Fuel pressure regulator). Any comments or suggestions on the next thing to check are highly appreciated and Merry Christmas to you all.
Last edited by teetee; 12-19-2017 at 02:36 PM.
#2
12-19-2017, 11:15 PM
OK it does not make sense to have 0v for the oxygen sensor so I am assuming the flatline of the O2S graph below 0.1v indicated that the computer entered into open loop because it lost the O2 sensor signal. Therefore the default fuel rich mode is used. I don't think this is normal though because the signal came back after a few seconds into idle.
I guess my questions right now are:
1. why the idle O2 reading is between 0.7-0.8 when engine is warm (wrong O2 sensor? Faulty coolant sensor)
2. why did the O2 sensor lose signal above 3500rpm
3. why the "running rich off idle" DTC code? If it was triggered when the sudden voltage drop (from 0.7 to 0) occurred, shouldn't it be the code for running too lean instead? Or the P2178 code was the effect of vacuum leak?
I guess my questions right now are:
1. why the idle O2 reading is between 0.7-0.8 when engine is warm (wrong O2 sensor? Faulty coolant sensor)
2. why did the O2 sensor lose signal above 3500rpm
3. why the "running rich off idle" DTC code? If it was triggered when the sudden voltage drop (from 0.7 to 0) occurred, shouldn't it be the code for running too lean instead? Or the P2178 code was the effect of vacuum leak?
#3
12-22-2017, 07:17 AM
DTC P2178
Fuel system too rich at off idle
DETECTION CONDITION
• The PCM monitors short term fuel trim (SHRTFT), long term fuel trim (LONGFT) during closed loop fuel control at off-idle. If the LONGFT and the sum total of these fuel trims exceed preprogrammed criteria. PCM determines that fuel system is too rich at off-idle.
Diagnostic support note
• This is a continuous monitor (Fuel system).
• The MIL illuminates if PCM detects the above malfunctioning condition in two consecutive drive cycles or in one drive cycle while the DTC for the same malfunction has been stored in the PCM.
• PENDING CODE is available if PCM detects the above malfunction conditions during the first drive cycle.
• FREEZE FRAME DATA is available.
• DTC is stored in the PCM memory.
POSSIBLE CAUSE
• Misfire
• A/F sensor deterioration
• A/F sensor heater malfunction
• MAF sensor malfunction
• Pressure regulator (built-in fuel injection pump) malfunction
• Fuel pump malfunction
• EGR valve improper operation
• Variable tumble solenoid valve improper operation
• Purge solenoid valve improper operation
• Purge solenoid valve malfunction (stuck open)
• Purge solenoid hoses improper connection
• PCV valve malfunction
• Variable valve timing control system improper operation
• PCM malfunction
Fuel system too rich at off idle
DETECTION CONDITION
• The PCM monitors short term fuel trim (SHRTFT), long term fuel trim (LONGFT) during closed loop fuel control at off-idle. If the LONGFT and the sum total of these fuel trims exceed preprogrammed criteria. PCM determines that fuel system is too rich at off-idle.
Diagnostic support note
• This is a continuous monitor (Fuel system).
• The MIL illuminates if PCM detects the above malfunctioning condition in two consecutive drive cycles or in one drive cycle while the DTC for the same malfunction has been stored in the PCM.
• PENDING CODE is available if PCM detects the above malfunction conditions during the first drive cycle.
• FREEZE FRAME DATA is available.
• DTC is stored in the PCM memory.
POSSIBLE CAUSE
• Misfire
• A/F sensor deterioration
• A/F sensor heater malfunction
• MAF sensor malfunction
• Pressure regulator (built-in fuel injection pump) malfunction
• Fuel pump malfunction
• EGR valve improper operation
• Variable tumble solenoid valve improper operation
• Purge solenoid valve improper operation
• Purge solenoid valve malfunction (stuck open)
• Purge solenoid hoses improper connection
• PCV valve malfunction
• Variable valve timing control system improper operation
• PCM malfunction
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