Q: Engine rpm drops at idle every few minutes. Codes for both system too lean and system too rich have been generated.

asked by on January 23, 2018

2001 Infiniti QX4 3.5L V6 with 169000 miles. Engine is surging at idle in park and while in drive. It normally idles in park at around 700rpm, but every 90 seconds or so it will drop to 400-500rpm for a few seconds and then pop back up. OBDII data says that engine is trimming fuel back 25% every time this happens. Codes P0171 and P0174 for system too lean were generated originally. Now codes P0172 and P0175 for system too rich are being generated. All O2 sensors will fluctuate between 0.2 volts and 0.9 volts except for the downstream sensor on bank 1 that will stay at 0.2 volts consistently. I have cleaned the mass air flow sensor with CRC cleaner but it did not have any effect. Curious how to fix this issue without breaking the bank. Thank you.

My car has 169449 miles.
My car has an automatic transmission.

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The possible causes of what you describe include include vacuum leaks and EGR system faults, a faulty idle air control (IAC) valve or circuit, faults in the fuel pump control circuit or fuel pressure regulation, defective spark plug(s) or ignition system components, faulty fuel injector(s) or injector circuit faults, PCV system faults or a faulty ECM (not common; indeed, rare). Often, the application of a real time automotive scanner can find the fault as actual engine operating parameters can be viewed during the "surging". I would recommend that you request a rough operation/rough idle diagnostic and the responding certified mechanic will perform the required diagnostic steps to get this resolved. If you want to attempt the diagnostic yourself, by far the most efficient way to do it is use the diagnostic decision tree that is published in the Shop Manual for the vehicle. That will give you specific instructions to narrow and focus the diagnostic. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.

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