Hi. I've looked over the forum and cannot see if this has come up before so I decided to see if anyone could help. 

I have a 2011 1.6 cmax TDci which has a problem 😞

it's very difficult to start when cold and struggles to idle or rev. Once warmed up it's revs ok but the diesel knock is terrible between ,1000 and 2000 rpm and I also get a lot of smoke. I done a leak of test and should you know it ran perfect again but if you reconnect the leak of pipe it goes back to being bad again. I've tested with another pipe and the pipe from the filter to the hp and it's the same. Just runs normal without it connected. Sorry to be a pain but any ideas or advice will be amazing. Thank you 

I was wondering if there is a one way valve further down the line back to the tank or something.