Hard to start when hot?

I have a transit 2.4 duratorque 2007 diesel, when its hot its hard to start, there was a rattle from a sensor on the right of the engine on top of some sort of breather, with a pipe going to the fron grille at the bottom, i replaced the sensor, the one that came off was caked in gunge like in breather pipes, when the new sensor was fitted it got worse and the rattle is still there when you stop the engine, it has poor mpg and only just past its test for smoke, any ideas anybody, this sensor is just infront of the fuel filter, thanks.