Engine miss

[raccoon]

Hi all, this last few days I have noticed that when I am driving along steady at any speed over 40 mph I notice the engine seems to be pulling back a bit. Not so much as missing more like you easing off then putting power on again. It is only for a second or so but it goes on every two or three seconds. When I put the revs on she pulls away ok but can be a bit hesitent sometimes.
The car is a 406 HDI deisel 2ltr and is a V reg. I have replaced the fuel filter, air filter but done nothing else. I have noticed what looks like a valve at the rear back left of engine bay with a small pipe laying next to it and when I plug it in the engine does not rev so just left that alone. Any help will be gratefully recieved.

[craigtd]

hi there mate
the valve you are looking at is a boost control valve i think, so if its faulty it will act like you are loosing power,ive come accross this alot when i worked for pug.

[raccoon]

Thanks craigtd. Have you any thoughts of the engine seeming to pull back a bit. When running along with a steady throttle it feels as if it is missing a bit. I have fitted a mass flow air meter a few months ago because of a lack of power but this fault is different to that. It is a little bit like on a petrol enging when you leave the choke on to long and the engine seems to choke off a bit.
Thanks for you reply.