A4 B7 P0455

[user18]

Hello everyone. I recently purchased a 2008 Audi B7 A4 2.0T Quattro Tiptronic with roughly 120k miles. After a couple of days, I experienced some issues. I got a message on the dashboard that said “Close the fuel cap” and after a while of driving it went off and the check engine light went on. When I scanned the codes I got a P0455 Evap system (Gross leak). After reading some information I started with the simple fixes, erased the codes, and got a new gas cap. That didn’t solve it. After I replaced the N80 valve, erased the codes, and still had no luck. The same thing happens but now is more sporadic, sometimes I get the message but no check engine light afterward, and vice-versa. The car has also some other symptoms that I noticed, when it’s cold the rpms fluctuate especially between 1800 and 2100, it is very noticeable when you use cruise control, afterwards, the message and the light come on. When it warms up it doesn’t do it anymore or it’s not as noticeable (I did some research online and found that it could be a bad torque converter on lack of maintenance on the transmission). Another symptom is that after fill-ups it takes longer to start. The PCV has been replaced by the previous owner. My mechanic did a smoke test on the intake manifold and there were no apparent leaks. He also looked underneath (the car was not on the lift) and he said there was no apparent leaks of fuel. I don’t want to spend a lot of money diagnosing the car and wondered if this was a recurring issue that could have a simple fix. Thank you in advance!
PS: Obd is showing other codes that may be relevant P2261 Turbo boost not detected mechanical (from what I read it can be the Turbo diverter valve) I got the part and I have to get it installed.