Hello and welcome to Just Answer!
I can't see how bad diesel could break a valve in a diesel engine!
You would have other issues long before a valve would break!
Valve damage can occur, if running performance chips or if an injector is over fueling. This will cause excessive heat and cause damage to both the piston and valves.
I hope this helps, let me know!
Yes, bad fuel will not cause a valve to break. I can't believe they are telling you that!
They will try anything to avoid paying for the repair! It seems like they try and blame everything on fuel these days!
I have been working on these Cummins diesels since 1989 and have never seen a broken valve from a fuel issue!
Bad fuel can damage fuel system components, like injectors, pumps and other fuel system parts, but will not damage the valves.
You are very welcome! Sorry to hear you are having issues with getting warranty.