I turn wrenches on them sometimes. They are definitely a peculiar motor. I think that one will be an old mechanical engine with a long rack under the valve cover that controls the mechanical unit injectors. When they get worn the engine gets weird operating characteristics. Check and make sure it doesn't have fuel in the coolant. They have brass or maybe stainless cups, depending on vintage, that seal the coolant from the fuel passage. Not to say they are a bad engine, just particular. Always thought about putting one in a pickup. They are pretty long, and not as economical as a cummins. Parts are pretty pricey too. I work on cats 5 days a week, and it is my least favorite engine to work on, mostly because I don't have alot of experience with them. The fuel system has check valves in it to keep head psi in check, and orifices.
I don't think that one is new enough to be a HEUI engine, just an old mechanical, 2 valves per cylinder. They are not a wet liner engine like their larger sisters. Usually we can bore them, but a reman is installed most of them time.
Water pump belts work loose easily.
They can be a spunky engine that makes good torque. Pretty high hp and torque for only 404 cubic inches in a large truck.
Just some info from me,
Michael