Okay, here's the thing: it appears to be a corruption or change in the fs_smoke_w.fx file. I will send you a file for blue smoke that came with a freeware addon that I downloaded once. Put the file in your FSX/Effects folder, then open your aircraft.cfg for your extra 300gs (make sure you change the security settings to allow your user to edit it! Right click and go to preferences, then go to Security, and click on "Edit" select "Users" and select "Full Control", then OK). In the CFG file, find [SMOKESYSTEM] and change

smoke.1=-10.00, -0.00, 0.0, fx_smoke_w

to

smoke.1=-10.00, -0.00, 0.0, fx_smoke_blue

Run FSX and tell me if it works, in which case we'll both have to find a new fx_smoke_w file...