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Q: Odd fuel trim pattern cycle causing rough running

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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.

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