Normally, my Mustang runs perfectly not no obvious engine noise, smooth shifts, and no check engine light. However, 3-4 time now the engine has developed a VERY rough idle when coming to a stop. Always when warm. The first instance was about two months ago exiting an off ramp the had a considerably steep decline I had been driving slightly fast with over drive disengaged. While breaking it felt very rough and jerk, then at a complete stop it idle almost as if only half the cylinders were firing. I drove a block and it went away, I checked the oil level thinking the level was low and it possible wasn't able to suck up oil from the sump and that could have been the cause. It ran perfectly from then until the next instance. This time it happened approaching a street lamp with a less steep but still noticeable decline. I looked at the gear shifter and actually had skipped drive and was in the bottom 3 gears. I considered maybe then the transmission had issue disengaging so I replaced the trany filter and the necessary fluid but not a flush. The most recent occasion the car was in its full range of gears when coming to the stop. The problem persisted through the process on parallel parking, and in park itself. Ran fine when getting back in. I'm am genuinely stumped now. I've scanned the car and then only code for the powertrain is a misfire in the first 1000 revolutions after startup and the ECU is able to adjust for this immediately after. Code is p0316-FF.

 

Try giving the inside of the throttle body a thorough cleaning as a dirty TB is a common cause of erratic idling.

 

for the P0316, if that's all you got,

I doubt it's fuel related given the time of year but, the takeaway is don't throw parts at it.

Very odd that you'r getting a startup code for a running issue. Does the code show up after this happens or upon the next start up?

 

Okay, I went to give the throttle body a clean, learned the idle adjustment screw was not adjustable. So the throttle body is as clean as possible now. There was noticeable layer of sticky carbon build up inside the TB. It did increase the idle by maybe 50 rpm, and power delivery seems slightly smoother, but I only drove 2 or 3 miles. I think I may use a bit of fuel treatment to clean the valves maybe? I appreciate the video it doesn't quite seem too relevant, but I will see if the problem changes as we finish the transition in to summer. Thanks for the help guys, excited to be a part of the Mustang community now.

 

That's a good idea as it'll also clean the injectors.