Hello I'm Neal and I am here to help resolve your issue. If I cannot help I will opt out and refer your question to another expert.
What vehicle or equipment is this engine in?
Fuel in coolant is almost 100% bad sleeves or seals.
Pull the injectors and replace the injector sleeves, it's a quick and easy job with the CAT tooling avaliable, should be able to do it in 3-4 hours easily.