i learned more about my perkins 4-108 cooling system than i wanted to last summer in the bahamas
if you indeed sucked mud into the cooling system it's good to know it's all on the raw water side. all the pieces of the raw water side are external to the engine and can be disassembled and inspected for clogging - including the heat exchanger. don't forget the oil
cooler lines. but you say that the overboard
discharge is ok which i interpret to mean it's pumping as good as it was before you had the overheating
problem. so if you've done all that then the problem is on the fresh water side.
the fresh water side is mostly internal to the engine and will not have sucked in any mud. you can still do an inspection
of it. there are two hoses external to the engine block that go into the heat exchanger. there is a thermostat under the header tank. if you live in a warm climate like i do you can just discard it if you like, or replace it for a few dollars. if you have a hot water heater
in the system there will be two hoses going to that which you can inspect. it may be difficult to inspect the hot water heater
itself but you can probably force water through it to make sure it's not clogged.
now we come to the last bit. the fresh water pump
. it was my privilege
to make perkins history
in the abacos by having the only fresh water pump
failure that my mechanic
had seen in his thirty years of servicing perkins engines. it was the very LAST thing that i inspected, and i correctly predicted that it was the problem by simply spinning it. i loosened the drive belt (it's driven by an external pulley on the same belt that runs the alternator). then i spun the pulley by hand. it spun freely. it shouldn't spin freely because there is a vane on the inside and the vane sits in coolant and the coolant should slow it down. obviously, the pulley and the vane were no longer connected. it was at that point that i visited the local engine shop - marsh harbor marine
. the guy who runs the place is a super dude and a factory trained perkins engine guy. knew the 4-108 inside and out. said he had never seen a fresh water pump
vane seperate from the shaft. but he pulled it anyway (i had trouble getting out the four bolts as they hadn't been moved in thirty years) and sure enough the vane had come loose from the shaft. one week and $400 later i was back in business.
as for the heat exchanger, it's an easy pull. just make sure you put the four hoses back where they belong. it's held on by two straps. remove the hoses, remove the straps, and pull the heat exchanger away. there is a bolt on one end cap that lets you open it up for inspection
. you'll find several dozen small tubes running the length of the exchanger. these are the raw water side and will be where the clog is if there is a clog. you insert a wire in each tube and make sure you hear it hit bottom on the other end. then you'll know the tube is clear. if it's not, try clearing it out with the rod but be careful you don't pierce the wall of the tube. flush the whole thing out with a hose and put it back together. change the zinc while you're at it. then put it back making sure you put the hoses in the correct place.
now you know everything i know. aint much, but when you're away from civilization and something breaks, you learn quickly....