2016 Ford Fiesta S / 103,747 miles / Manual transmission / No error codes.
For the past couple of weeks my car has been idling strange. Usually, after I start the engine the idle increases to around 1200 rpm and then settles to around 900-1000 rpm after about 15 seconds. Now after I start the engine, the idle increases to 1200 rpm but then immediately drops to around 900 rpm then back up to 1200 rpm continually up and down. It feels "rougher" than usual and sounds like it might stall out if the rpm drops any more. Also, the exhaust smells more pungent/rich than usual but the fumes are clear/ no smoking. Any ideas? Thanks.
If you have access to a scan tool, please provide us with the following information so we can help you more accurately:
- Short term fuel trims
- Long term fuel trims
- O2 sensor voltage (or AFR)
- Throttle position
- Ignition timing
- MAF reading
- Fuel pressure
But based off your description, it sounds like the engine is having trouble maintaining a perfect idle (closed-loop). This happens when the ECU cannot maintain a proper air fuel mixture from bad/leaky injectors, dirty throttle body, unmetered air entering the intake tract past the MAF sensor also known as a vacuum leak, worn out spark plugs, etc.
I sprayed out the intake valve with some throttle body cleaner and the idle seems better. I'll give a few days and see if the rough idling comes back.