There are quite a few threads on this as it is the same on all the 16 valve jtd's.
You will be better to get a new manifold rather than try to clean it out as the swirl valves will probably be worn and leaking, the new manifolds are modded to stop this and are about £170, mostly listed for vauxhall's.
It is a big job involving taking off the fuel injection pump as the manifold wont clear the lower pump mounting stud which also has to be removed, this can be done without removing the timing belt as the pump pulley can be bolted to the back plate to keep the tension on the belt but it is quite fiddly not to drop the woodruff key and then to get it back in again. So if your belt and water pump is any way close to needing changing you'd be better off doing it all at the same time, its an easy belt to change.
Set aside a good day or better still a weekend to do it, dont expect to start it one evening and drive to work in the morning..
Oh, and you have to drain the coolant as there is a feed between the manifold and head, if you dont it will fill a cylinder up with coolant.
Last edited by JON156VELOCE; 03-04-12 at 16:13.