If the vacuum readings, and compression check out, and there are no vacuum leaks. I would look at your Manifold air pressure sensor. Especially when the engine is running very rich, and has a shaky idle. This was the fix for my 2011 5.7 GSI . I had replaced virtually everything else before this. My engine did not throw a code for this, but on the Rinda Diacom the manifold atmospheric pressure, and vacuum pressure were not reading in parameters. My engine was also running very rich, and I could also smell gas. It had been at a repair shop for 3 months and the tech was unable to identify the problem. When I brought the boat home, I thought I would replace the M.A.P. sensor, as that was the only other electronic sensor that was not replaced. Bingo she purrs like a kitty. The Rinda I bought had a very good troubleshooting section that really helped me. The tech even thought I had mechanical issues with the engine, as I just had it rebuilt. I hope you find out the cause and post it. Good luck Dave-R