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.

Car idle is rough Inspection $79.99 - $99.99 Get a Quote

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.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing


Experienced Mechanic?

Earn up to $70/hr

What others are asking

How does the front passenger power window switch work?

Your car is equipped with power windows in the front and rear (unless you drive a coupe, in which case you only have front windows anyway). The passenger side power window switch offers the ability to do three things: ...

Keyless remote broken

The system will need to be checked to see if the drivers door lock is having an issue or the remote is the cause. The remote may have a bad button for the drivers door since it is the one...

what should I do to prepare my car for the winter. It is currently in Puerto Rico but will be moving to Rhode Island.

Your best bet is to use the recommended fluids in the owners manual for normal use. The tires should be good for all seasons. Replace your coolant with a 50/50 mix. Otherwise, just service your vehicle as normal. There should...

Battery replaced, radio stopped working

Hello. This frequently happens after a battery replacement on this car. When this car loses battery power the programming in the computer can become corrupted and this occurs. Some of the time it will come back on its own, but...

Replaced all engine mounts but steering vibration still there. When at a stop with the gear on D or R, and not P or N

As you may know, this is a very common issue with the Volvo XC90's and is heavily debated in terms of what causes it and what the correct fix is. In my experience, I have seen this corrected by replacing...

Cupped tires

Hi there: Cupping on a tire is one of the more difficult items to diagnose; due to the multiple causes of cupping. In most cases, cupping is caused by suspension components like struts that are worn out; causing the tire...

I hear a whining noise as my vehicle increases speed.

These types of noises can generally come from a faulted cv axle or badly worn wheel bearing on the vehicle. A cv axle with torn grease boots may make more noise during tight cornering and a worn wheel bearing will...

No star no crank

Hi There, Depending on whether or not the vehicle will turn over will determine what the next step should be. If the engine does not turn over, I would recommend checking all connections related to the battery and check to...

One click in car won’t start Volvo S60 2.4t

Considering that all those parts were just replaced I would recommend retracing your steps to determine if any loose connections exist anywhere. If the battery connections are loose this can cause the vehicle to make a click sound but not...

Related articles

A Buyer’s Guide to the 2012 Infiniti EX
If If you’re in search of sport meets practicality, the EX may just be your perfect match. With the sedan-like driving of its G37 sibling combined with utility functionality and plenty of power under the hood, this is one SUV...
A Buyer’s Guide to the 2012 Infiniti G
The The 2012 Infiniti G is a sporty luxury sedan that has the looks, the power and speed, and the price that’s right for many buyers. It offers great handling, and aesthetics that will turn heads. Key Features The vehicle...
A Buyer’s Guide to the 2012 Infiniti G37
The The 2012 Infiniti G37 is a sporty vehicle that looks as great as it performs. It’s a head turning vehicle that could be the perfect entry point to the luxury market for many people. Key Features The driver’s seat...