Exhaust manifold 4.108

If I take the exhaust manifold off to check for obstruction will I loose any antifreeze. Not sure if freshwater side runs through it also.

Normally you can remove the exhaust manifold itself without removing the heat exchanger. Make sure you have a new gasket set.
The heat exchanger contains both the closed circuit water (anti-freeze) and the seawater/fresh water cooling.
If you take off the heat exchanger put it in a container, (cut the side off a 5 gallon plastic jar, use the half with the fill/drain cap) filled it with either muriatic acid, or the poor mans version, 12% vinegar and let it steep overnight.
Next day wash it with tap water and it should be nice and clean. you can remove the tube element if your really keen, but if it ain't broke don't fix it.

Already did the acid trick. Only thing left is exhaust manifold

Whats the scoop? Engine running hot? Overheating? Did you change the salt waterpump impeller? Fresh water pump working? Is water coming out exhaust when running? have you opened and cleaned the intake strainer? Made sure you have good water flow through strainer?

Perkins have a bullet proof reputation. The word is, keep the oil changed and full, the water in the block and circulating and the filters changed and they last for 10000 hours.

Have you shot the water channels with a temp gun to see delta on intake, and output of 1. main heat exchanger, 2. transmission, and (surprisingly) the hot water tank. It all flows through all three. If one is impinged it slows down all heat transfer. For my money (after you ascertain waterpump is circulating) disconnect hot water heater from equation. Then check coolant flow around engine and gear box exchanger. Eliminate the outside issues first.

Although I haven't seen the problem personally, hearing about it (anecdotally) about the rubber impellers disintegrating and clogging the heat exhanger tubes (and thus down flow restriction) is an issue. Disassembling the waterpump and seeing the full (or lack of) impeller makes this a (non) issue as appropriate.

Start with the simple stuff first. Waterflow. Impeller. Circulation. Clogs.

Thanks. I solved the problem. I boiled out heat exchanger in muratic acid. Left Marathon last Sunday. Currently in Naples. Engine never got up to 160 degrees while running 3000 rpm. Did 6 to 6.5 kts. Used 29.5 gallons from Marathon to Maples. Very pleased. Thanks everybody.