Erratic RPMs at idle/start-up | Solved with fuel system cleaner

[MrAmerica1776]

I have a 2015 Mitsubishi Mirage ES with a CVT, 128,412 miles. I’ve noticed recently that when I start up my vehicle that the RPMs will throb/surge from 800 to 1200 or so until the vehicle is fully warmed up, driven, etc. It doesn’t happen all the time, it’s more of a hit or miss. My thoughts are that it’s either the battery, which is aging, something with the fuel system, or perhaps the car converter. Anyone have similar issues?

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Forum member grevster had a similar problem that was solved with some fuel system cleaner. It's important to note that this is not a fuel treatment, but a cleaner such as Techron or as grevster used, Gumout:

Here is an update, I used a bottle of Gumout all in one cleaner in a full tank of 87 octane gasoline (sorry, i could not find any techron locally) and drove 2 hours continously on the highway around 110 - 120km/h. The next morning, when i started from cold, i still had the erratic idle but this time it was not bouncing between 1100 rpm - 750 rpm but it was bouncing around 900 - 750 rpm. So the amplitude of idling bounce reduced.

I gave another bottle of Gumout this time in half tank Shell 91 octane gasoline. It is still in the tank not finished yet but when i checked this morning, i didn't see any bouncing at all. Of course i need to check couple more morning cold starts to make sure but i guess it is getting much better and cleaner really helped. Thank you all!

If you are having a similar problem and have tried a fuel system cleaner, please post your results.