C15 6nz Coolant in fuel separator

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      Guys have a Freightliner century with a C15 6nz, today my brother and I were doing a service and found the fuel filter and separator full of coolant! What can it be and where should I start?

I think she may have a fuel cooler behind the left side tank will chase the lines tomorrow. Had a mate say could be injector cups/orings?

Anyone have any idea? Spewing we’ve had this truck for years and has never missed a beat.

Thanks