My new to me 85 E38 was overheating @2000 rpm. Here is what I found. The boat sat for about a year not being run,so the saltwater in the heat exchanger caked approx. half of it off. The raw water fitting where it dumps into the exhaust riser was closed down to about 1/4 inch hole. There was a bronze back surge valve installed between the engine and wet muffler that had lost the nut holding the rubber seal, and the seal was blown into the fitting, restricting the flow.I also fount rust streaks on the outside of the riser, and after scraping them, found it was rusted from the inside out. The good news is that for a few boat bucks, I should have it repaired by Sunday. I have part # for riser if anyone needs it. By the way, its an aluminum alloy. 
