I have a 2008 Honda Accord EX 2.4L. The long term fuel trim was at -3%, then I decided to clean the MAF sensor using CRC MAF sensor cleaner. My long term fuel trim went down to -10+% and short term fuel trim went down to -13%. The car runs like a champ after cleaning the MAF sensor! Lots of power. The engine does NOT burn any oil. Why is my engine running so richly?? Does it just take some time for fuel trims to adjust after cleaning the MAF sensor?
Your engine is not running rich, a negative fuel trim means that the ECU has reduced the fuel in the fuel-air mix to keep it from being rich.
If your MAF was dirty and under-reporting air flow before then the ECU would adjust the fuel trims based on the downstream lambda sensor data, now that its clean the ECU thinks that there's more air flowing in, so the trims change. This is a case where you want to reset the fuel trims.