I'm having troubles shifting into third gear. It will go in but it takes some persuasion. I have only had the car for about a month and got no information about the previous owner due to the fact it was purchased from a dealer auction. About a week into driving the car it started to get rough to shift from 1st to 2nd and 3rd to 4th. I read on a few forums saying that the OEM base bushings for the shifter can flex when worn out so I ordered Acuity shifter base bushings and Acuity shift linkage bushings. This fixed the problem for about a week but now it is acting up again. Any ideas as to what would be causing this. Even just a temporary remedy to the problem would help. 2010 Honda Civic si 123 thousand miles last clutch/transmission maintenance unknown
This isn't really a complete answer, but maybe it'll provide a bit of insight. This is a common issue with this car, and while it was apparently fixed in a 2008 recall, I still get some difficulty with my 3rd gear in my 2010 model (usually only after cold starts). So I'm guessing they only slightly improved it. I'm not sure if this problem affects all cars with the K-series 6 speed.
From what I've seen, replacing your transmission fluid seems to work pretty well, though it doesn't fix it forever. The problem may or may not come back when the transmission fluid starts to degrade, but that typically has a fairly long service interval. Popular choices include GM synchromesh (friction modified), Amsoil MTF, and perhaps also Motul Gear 300. You can go to 8thcivic.com and bobistheoilguy.com to research this for yourself. I also find that instead of shoving the gear shifter straight from 2nd to 3rd in one motion, pushing the gear shifter into neutral first, and then straight up to 3rd makes it go in a bit easier when it's notchy. (you can still do this quickly in almost-one-motion)
As for the rough shifting from 1st to 2nd and 3rd to 4th, I'm not sure if I'm understanding exactly what you're experiencing (can you describe it a bit more?), but I'd probably attribute that to tired synchros. I tend to experience some resistance with shifting into 1st and also with quick shifts into 2nd, but it's not the same notchy, blocked feeling that you get with 3rd gear on cold starts. The only proper solution to this (aside from transmission fluid) is a bit more extreme - requiring teardown of the transmission and replacing the synchros.