Bouncing RPM?

[skiski329]

So this has been happening for a while, but i haven't been able to catch it on camera because it doesn't always happen.
It only happens on start up and usually doesn't happen more than 3 times.
When it does happen, I fire it up and right away the rpm goes up, then dips down to normal rpm, jumps up 200 to 300 rpm, then falls back down. that happens 3 times within about a second or two, then it goes back to normal.
I've been looking for a technical name for the problem, cause, and solution. Any ideas?
so far i'm leaning toward vacuum issue and maybe that's what's throwing my evap code that i keep on clearing (cuz i haven't found the cause yet)
Let me know if any pictures or video (i can keep trying to get it, but either my camera is too late, or i forget to pull it out).
Thanks!

[Extreme Dimensions]

vacuum leak is what i'd put my money on as well. I believe the technical name for what you are describing is idle surge. a common method of finding vacuum leaks is to spray some carb cleaner or starter fluid around any hose you think may be leaking, if the idle changes after spraying a specific hose, then you've found your culprit. Do a visual inspection first so that you aren't just spraying extremely flamable liquid all over your engine bay.

[skiski329]

^ Awesome! i never knew that. it's been -13F to -15 F so i've been filling it up every day and notice it happens on start up after i fill up. so i'm thinking when i open the gas cap it breaks the vacuum then it struggles for a bit because of the leak. I'll have to wait till it warms up a but, but glad i know how to approach it now. it thought i was going to have to spend a bunch of money on vacuum tests.