I swapped a jdm b18c into my 97 gsr. Converted everything to USDM spec so running a us ecu. I'm getting a weird CEL when I first start the car in the morning. After it starts and runs for about 30 seconds. I reset the ECU and start it back up and I get no more cel. I jumped the wire under the passengers side and the code read 91. I couldn't even find that in my helms but found it posted on a forum and it read "fuel tank pressure circuit". What in the hell is this. I can't even find any documentation on it anywhere. But like I said, I get it after starting the car in the morning after it's been sitting over night. Once it starts, I reset ecu and it's gone for the rest of the day. Tomorrow I'm going to open the fuel cap and release the pressure before starting to see if it has something to do with that. Any thoughts?