Idle problems 2019 s550

[Joseeevelez]

Hey guys, I own a 2019 mustang GT 10-speed automatic. This past Monday, I experienced a rough idle & a change in rpms when I was at the red light. Never done that before. I thought nothing of it but it kept doing it again. I changed the spark plugs, I took it a quick lane by the ford dealership & they didn’t find anything wrong with it. They checked for codes. Only thing that popped up was “unexpected RPM change on crank”. I only have a resonator delete & cat back delete. Nothing else is done to the vehicle. Any ideas of would could be wrong with it? Could it be a vacuum leak? Exhaust leak causing the issue? I’m not sure. Next thing was to replace the ignition coils. Anything helps! Thanks guy!

 

[mwjscn]

Regarding the test, this might be worth reading Part 1 of this article: https://autoditex.com/page/relative-compression-test-67-1.html
tldr: A failure on this test would mean you have a compression issue by monitoring the voltage requirements during cranking. A higher draw means the starter is working harder to overcome a larger amount of compression. A lower draw means the starter isn't working as hard - less compression. The test compares cylinders relative to each other, so while a quick test it may not be perfect. You can read further into the pros and cons in the article.
I would assume Ford is using it's onboard 'smart' battery handling system to detect the voltage requirements during cranking. I'd check if your electrical connections on the battery are all secure, as that could impact both the test and normal driving, and that your battery is generally in good health. You may also want to check the engine compression manually to verify or debunk the 'relative' compression test findings. Additionally, if you find this is only a problem during idle, it may be good peace of mind to do a throttle body reset so it can re-learn the idle trim.