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Q: Engine tapping noise related to short term fuel trim but only on cold start up.

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I have an OBD reader. When engine is cold and at first start up. It run and sounds perfect. The obd reader .stft is showing 0.0. O2s haven't started registering yet. As o2s start stft starts to move. Bouncing from -02 back to 0. Engine still sounds great. Now the stft starts to move into the positive and the tap or tic starts. Starts very faint. Then builds in sound. As the stft increase so does the sound +10.00 on bank 1. 2 to 3 on back 2. Stft falls back to 2 or 3 tic disappears. Then it comes back. Now if it's in the middle of a tic tic tic. I turn the engine off and start it right back. No tic. Until the stft starts to move again from 0.0 back to+5+8+10 on back 1. Back 2 stays around +3+4. Advance fluctuations from +5 ° to 10°. Once engine warms to 170. The tic is gone. Stft on both banks is now bouncing -2 ±0 +2. Engine runs like a top. Quiet enough to hear the injectors ticking away. New cam, crank, knock map sensors and O2s. No codes ever. Thanks

My car has 198000 miles.
My car has an automatic transmission.

A: Has a mechanic's stethoscope been used ...

Has a mechanic's stethoscope been used to track down the specific location of the noise? There are several solenoid valves in the engine bay that do not run all the time and may be pretty loud once they start, such as an evaporative emissions purge valve and the like. The high fuel trims on one bank sound like you may have a problem on one side with a vacuum leak. A positive trim means air is being added and the computer is trying to add fuel to compensate.

I recommend having an experienced technician, such as one from YourMechanic, come to your location to determine why the engine is making a noise and suggest the proper repair.

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