Signature Series 600 Issue

I'm going to make myself look silly here, but this is how my mind works (simple)! And I am not familiar with Cummins but yours ought to be similar to a pre-acert cat for the basics maybe.

Ok, so it takes fuel, air, and timing to make her go.

1. You have put a clear line or sight glass on the return fuel line and found no air, replaced injectors, replaced hoses from the tanks to the pump (does it have a transfer pump?), replaced/cleaned/opened caps to rule out a vent problem, so you probably have ruled out anything in regard to fuel supply problems. This riddle is based upon the fact that it works well for awhile when ambient is cool, but not long at all when ambient is warm. You have checked oil samples to rule out losing fuel into the oil. You are not gaining pressure in the coolant system, so injector interface with both oil and coolant should be ruled out.
2. Air - should be no way that this is related to air restriction, bad turbo, or leaking CAC because it runs well when cold.
3. Timing - again it runs well when cool, so it should not be related to timing or valve problems, unless it is not advancing at whatever rpm you are at @ 55 mph.

Again, I know nothing about your engine, but it would seem that perhaps there is a sensor that is sending a signal to the ECM regarding fuel pressure (and maybe fuel temperature) that is incorrect and derating the engine. Is there a fuel temp sender on yours? Are you actually losing fuel pressure as in you have checked it with a manual gauge? As many shops as you have had it into that have plugged into your engine a sender problem should have shown itself.

I'm just talking out my butt here because I don't know crap, but maybe this will trigger a thought by some of the smarter guys.

 

If I remember, there is a pretty fine mesh screen where the fuel line connects to the engine. We were finding aluminum shavings clogging it on brand new trucks where the fuel tanks were tapped for fittings. Of course, this was 23 years ago, so memory is a little fuzzy.

 

I ran into the exact same problem back then on new ones.

 

The isx runs extremely hot fuel. It also uses fuel to cool the ecm. In Australia and South Africa they even run fuel coolers. I wonder if you have an issue with the fuel getting hot. I installed an automatic transmission cooler on mine. It did lower the fuel temp by about 20-25 degrees.

My issue wasn’t fuel pressure loss though. So it’s sort of a wag. I assume you have replaced the supply pump.