I'd check all you mentioned... chain stretch is easy to get an idea with the valve cover off. IP timing usually doesn't get too far off (usually goes with chain stretch). At your mileage the chain is probably fine for stretch. You could always stick an A-B light on the pump to see if the timing is still okay.

A compression test would give you an idea, but I seriously doubt the motor is low on one cylinder or more. The 603 seems to be built just as robust on the bottom as the 617



Maybe you have bad motor mounts?

What's the idle RPM?

Leaky delivery valves?

Injectors pop tested and have good spray pattern?

TrapOx in place? Cat plugged?

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I'm not a doctor, but I'll have a look.

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