before overhauling or rebuilding, check that the engine itself is in good tune, specifically the valve clearances (tappets), further check that the injection timing is correct, its set by inserting a dial gauge into the pump head and measuring the stroke of the plunger when engine is at TDC cyl 1.
Once you are sure that everything is OK and checked to be good, then do a compression test (diesel compression testers are a little expensive compared to petrol ones - as they have to measure upto 500 psi compression). If your compression numbers are below 250 psi then its time for a valve job or even a ring job they shatter - if your mechanic ever tried to start it with petrol spray and did not disable the glow plug relay.
if your engine has good compression and shows oil consumption only then replace the valve seals - it is very easily done on any IDI diesel engine without taking the head off. Speak to your mechanic and see if he can actually understand how to do this. If he agress I can explain it to you - all it requires is a jig that is made with some angle iron metal a drill in about 30 minutes with a welder.
For the pump overhauling, ask around in your city of Denso specific diesel labs or contact diesel power systems Pakistan and ask them about testing and setting your pump and injectors. When fitting the pump to the engine the timing is set by a dial gauge (the preload measured is only 0.8mm) and is nearly impossible to get it correct by andaaza.
