When the 31 first showed itself, I was told to get a new gas cap too. I just bought one at AutoZone for about $5 I think. Didn't fix it. I have tried to get them to run the scan tool on my car and they couldn't hook it up. Said they didn't have the right equipment for my "unique" vehicle. Furthermore, I actually blew $192 at Steven Mitsubishi in Wichita, KS trying to get this thing diagnosed. After a day and a half, they still couldn't get their scanner to communicate with my ECU (idiots). They ended up telling me it was the EVAP/Purge soloenoid, which I replaced for $10 from the Salvage yard (better then the $197 they wanted + labor + tax). Still no fix. I will tell you this - CEL31 won't just go off when the car's on. If shaking something loose can reset it, you won't know until you retstart the car. I have had this code running several times when I hook up the PocketLOGGER and it shows the same thing: EVAP system leak (small). That could be due to Mike using the DSM VFAQs when he wrote the software, I dunno.

I say you just live with it. It has little effect, if any, on performance. You should just run a keydance a couple times a month to check for other codes and be sure that, if it ever goes away, that you make sure the bulb isn't dead. Trace your vac lines first, then look to replacing EVAP components up front, then it might be the roll over.

Good luck to you sir. Figure this out for me. I think it's one of the more common codes and yet, one of the most difficult. Here's some more food for thought: I currently get 31-25-31-22. Yet, when one goes away, they all go away. Maybe there's some sort of relation to the intake mani? Idle speed (25) can be affected by vac lines on the mani as can the IAC. Coolant temp signal voltage low (22) can affect performance as the ECU needs to know temperature to adjust the A/F mix too.