misfiring 90

my baby is sick it seams to misfire when under load but as soon as teh revs come up it all stops.......also the fan belt slips.......but that is new so i guess that may have stretched a little and needs tightening

Any ideas on the misfire no idea where to start? already got a new fuel filter on.

correction i should say once the revs come up she stops misfiring and is perfect. Not losing any oil or water either

Sticking/badly seated valve causing the compression to drop so at low revs all the diesel air mix is escaping before it gets the chance to combust and at higher revs its catching it.

Which engine is it?

its a 200Tdi.....this sounds serious?