Skip the auto shop - Our mechanics make house calls

Q: Odd fuel trim pattern cycle causing rough running

asked by on

I just rebuilt the bottom end of my Frontier's motor. New bearings, seals, oil pump, water pump, etc. The wiring, sensors, and distributor are all the same. New hoses on everything, new plugs, wires, gaskets, etc. There are no trouble codes/no check engine light. There are no vacuum or exhaust leaks. Upon starting, it runs smoothly for a few seconds, then, over a period of about 20 seconds, progressively runs worse and worse. Then instantly starts running smoothly again. I got a scan tool with live data. The fuel trim in bank one progressively drops to -25 over the 20 second period. The fuel trim in bank 2 progressively climbs to +25. When the fuel loop opens, both fuel trim banks snap back to 0, and the engine runs smoothly. Then the cycle repeats. I tested all injectors, and they all have a consistent spray pattern. What's going on!?

My car has 170000 miles.
My car has an automatic transmission.

Hello and thank you for contacting Your Mechanic. What seems to happening with your engine Bank 1 is going rich and Bank 2 is going lean and repeating itself. Now, for this to repeat itself, there is probably a problem with 3 possible issues. The first is either the mass air flow sensor (MAF) is dirty or is not working. I would check the MAF and clean it out. Then check the O2 sensors to see if they are heating up. If the O2 sensors are not heating up correctly, they will make the computer default the fuel system. Eventually the engine light will come on for codes P0171 and P0172. The third thing to check is the spark plugs. I know you said that you replaced the spark plugs, however, if you were to install aftermarket spark plugs, they may not fire correctly. It is always recommended to replace the spark plugs with OEM parts. If you replace the MAF, and replace the O2 sensors and still have the same problem, then I recommend having one of our certified automotive technicians from YourMechanic come to your home or office to inspect, diagnose, and repair the issue.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!

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

Get a quote

What others are asking

Q: Transmission control module

This may be a result of low transmission fluid or potentially a faulty transmission control module. A faulty transmission control module may cause a delay in acceleration or erratic shifting of gears. The transmission control module has various sensors such...

Q: drive shaft

They could have damaged a u-joint while pressing on the bearing, improperly installed the bearing, or improperly re-installed the drive shaft. To check this I would raise the rear axle off the ground and have an assistant drive the truck...

Q: My rpm needle moves down a little then up when im at a red light.

Many early 2000's Jaguars, including the X-Type and X-Type (R) had issues with faulty O2 sensors, air fuel sensors, and ignition sensors that would cause similar symptoms to what you're experiencing. However, an idle control valve that is dirty could...

Related articles

How Do Power Car Windows Increase Passenger Safety?
Power windows are responsible for approximately 2,000 emergency room visits every year. When a power window closes, it exerts enough force to bruise or break bones, crush fingers, or restrict an airway. Though...
How Long Does a Distributor O Ring Last?
The distributor is part of the ignition system in your vehicle and its purpose is to route high voltage from the ignition coil to the spark plug. The spark plug then...
How to Avoid Back Pain in a Car
If you have back problems, sitting in a car for an extended period of time can be excruciating. Even without back problems, you could experience discomfort and soreness from...