Hey guys, new to the forum and have a question about my brother's 740. It has what sounds like spark knock when it hits max boost. It knocks maybe 3 times and puffs a little bit of greyish smoke out of the tailpipe. It only does this under full throttle acceleration...once it's fully spooled up it will hold 15psi and doesn't knock. I seafoamed it about a week ago and have begun using premium fuel but the problem persists. Ideas/advice on how to alleviate this? Thanks.

 

I'm not a turbo authority....but with black smoke and a potential misfire I'd say you have an excess fuel condition.

check the fuel pressure, leaking fuel pressure regulator, vacuum lines, map sensor, coolant temp sensor temp to voltage.