Why does AFR drop and fuel trim go negative when coming to idle?
When returning to idle after the engine is fully warmed up, the air-fuel ratio drops and short-term fuel trims dip temporarily for about 10 seconds, accompanied by a slightly different exhaust note. Cleaning the MAF sensor, air filter, and throttle body does not fix the issue, leaving drivers dealing with fluctuating RPMs in traffic.
What people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
- 1Checking the EVAP purge valve and monitoring O2 readings
- 2Cleaning the MAF sensor
- 3Replacing or checking the air filter
- 4Cleaning the throttle body
- 5Checking for a vacuum leak
In their words
Unedited, grouped by where they were said, most upvoted first within each place, each linked to the thread it came from.
“Vehicle drives fine with normal afr and fuel trims while driving”source ↗
“However sometimes returning to idle when fully warmed up.. such as in traffic.. my afr will drop and my short term fuel trim with go to around minus 7 for about 10 seconds”source ↗
“It happens most days and will sometimes hear a slightly different exhaust note while this happens before it returns to normal”source ↗
Where this came up
People with this problem also raised
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