is it firing on all cylinders? open the nuts on top of injectors, engine tone should change instantly, if it does't injector is faulty.
is air filter clean
is exhaut pipe good
check fuel filter
also take fuel inlet pipe off back of injector pump make sure gauze filter is not blocked.
good luck had the same problem myself -- I got replacement 2nd hand injectors off e-bay for about £25.00 set of 4 new they are about £80 each
nigel_ccraske@yahoo.co.uk
July 2008