The Renault turbodiesel in the 440 is pretty simple as it's still almost completely mechanical (fuel pump and such).
Missfire in a diesel would be caused by either low compression (eg. leaking piston rings), a clogged/dirty/bad fuel injector or fuel pump going bad. It could possibly also be water in the fuel, but that's usually caught (and can be drained off) in the fuel filter.
Cam timing should not be a problem (it's not that sensitive) unless it jumped a tooth. But then I'd get a little nervous as the belt may not be in very good condition anymore when that happens and it *is* an interference engine. So if the belt goes it destroys the valves and pistons. Has the timing belt been changed according to the normal maintenance intervals?
So first of all I would try to have a compression and leak-down test done on the engine. If that's OK then have the injectors checked and cleaned.
After that it may be a problem in the fuel pump. It could be related to the fuel enrichment under boost not working properly or (more seriously) the fuel pump itself is not putting out a good pressure and flow.
Diagnosing that really requires a shop which specializes in (turbo)diesel engines and can do all the relevant tests and has the equipment to do it.
Hope this helps a bit..
Bye, Arno.
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