MELTED PISTON/INJECTOR FAULT?

This sprinter was smoking & running very rough, had no compression on number 4 cylinder, done around 280,000 miles, customer decided to have a remanufactured engine.
We sourced and fitted a full engine from a reputable engine builder, we sent injectors and pump to be tested by diesel service centre (they were only 2 months old) they tested ok and were refitted.
After reassembling and running up to temperature, all seemed good sweat as a nut, extended road test carried out (twice) then handed back to customer with a daily check notice for the driver.
After 200 miles (first day out) I had to recover it for loss of power & running rough.
Tested compressions, number 4 cylinder very low, removed cylinder head to find piston melted and cylinder bore scored.
So Guys
what do you think?, thought I had the injectors covered, I have checked for broken wires in the engine wiring loom they all look good.
Could the ECM cause this? Could it be an injector even though they were checked?
NO FAULT CODES STORED.
I am going to have to put this right, but my biggest fear is that it may happen again with the next engine.
Yours in hope!!!
A very desperate
Lindsay Flaxmer