Sounds to me like a compression test is in order.You could swap #5 & 6 injectors with two that are working like #1 & 2 and see if the problem moves with the injectors,i wouldnt bet on that, but its easy to do so give it a try.I am not familiar enough with that particular engine to know if it could be a head gasket blown between #5 & 6 and not affect the cooling system,there are engines where this can / does happen but i dont know if the 603 is one of them. My $.02 Don

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