RPM plunge at warm up idle

[MocaST]

Hello all. I recently purchased a 2017 Fiesta ST used with 42,000 miles on it. The car has been well maintained and is bone stock. I'm very much loving this car but am a bit worried about a sudden plunge in Rpm when warming up the car. Car starts first crank every time and has nice smooth idle. After about 5 minutes give or take, the car has a slight glitch/plunge. It's not significant enough to kill it but it drops down to about 400-500 rpm and jumps right up to about 900 like its good and warm. Any advice would be much appreciated. Thanks! -Jeremy

 

[SteveS]

Are you letting the car sit and idle for 5 minutes after starting it? That's not good for cars. Here's one article of millions that explain why: https://www.popularmechanics.com/ca...ng-up-your-car-in-the-cold-just-harms-engine/

 

[green_henry]

I had similar symptoms a few months ago and attributed them to a bad tank of gas because it got better on my next fill-up and disappeared completely after the following fill-up. I put some Red Line Fuel System Cleaner in the next tank. No issues since then.

 

[MocaST]

Are you letting the car sit and idle for 5 minutes after starting it? That's not good for cars. Here's one article of millions that explain why: https://www.popularmechanics.com/ca...ng-up-your-car-in-the-cold-just-harms-engine/

Thankyou so much. I feel like such a fool. Growing up I always helped my dad work on his old muscle cars and I eventually started building Hondas and selling them for profit. It's ashame I've just been doing what my dad always did for his old cars with carburetors. All the unnecessary wear I've done to so many innocent engines 😒

 

[MocaST]

I had similar symptoms a few months ago and attributed them to a bad tank of gas because it got better on my next fill-up and disappeared completely after the following fill-up. I put some Red Line Fuel System Cleaner in the next tank. No issues since then.

Sounds great! Thanks for such a quick reply. You and Steve helped me out a lot. I will be filling up tomorrow at BP with some good ole 93 octane. Have you had good results in the past as well with the Red Line Fuel System Cleaner?

 

[green_henry]

I have been using Red Line Fuel System Cleaner for a long time, but it can hard to find sometimes. I use Lucas or Seafoam when Red Line isn't available.

 

[MocaST]

I have been using Red Line Fuel System Cleaner for a long time, but it can hard to find sometimes. I use Lucas or Seafoam when Red Line isn't available.

Okay great! Thanks man👍🏻

 

[TyphoonFiST]

I could be wrong but after you start it the vehicle goes into open loop for emissions based purposes until certain conditions are met then the idle will drop down into closed loop and then go back up and normalize. If you have a Tuner they can set the idle for you if you'd like to see if it helps your issue. Good luck!

 

[MocaST]

I could be wrong but after you start it the vehicle goes into open loop for emissions based purposes until certain conditions are met then the idle will drop down into closed loop and then go back up and normalize. If you have a Tuner they can set the idle for you if you'd like to see if it helps your issue. Good luck!

Thanks TyphoonFist! I'll look into this. Very interesting. I just recently moved to the western end of North Carolina and I no longer have a tuner. Haven't looked into tuners in my area as of yet. I know there are some in Atlanta area and other parts of NC.