Changing Coolant on a Yanmar 4JH3E Diesel Engine

We've had our boat for two years now, and we have put 500 hours on the engine. It's actually overdue, but the time came to replace the coolant. The coolant on the Yanmar 4JH3E should be replaced every year, or 250 hours. Our engine had the Yanmar brand coolant (YG30R), but we couldn't get a hold of any without paying shipping charges, so we researched an alternative. Initially, we found a suitable replacement with Havoline Extended Life Coolant, and this is what we put in the engine. After further research though, we think next time we will use either Texaco or Shell "ELC" coolant because both brands have added nitrite and molybdenum for protection against pitting and erosion. The engine has three drain cocks: 1. On the cylinder block, on the port side aft. 2. Underneath the fresh water pump. 3. The exhaust manifold. I saved a plastic water bottle to be able to drain the old coolant in tight spaces. After draining coolant from all the drain cocks, I filled the engine with distilled water and ran it until it reached operating temperature. You do this so the thermostat opens up and the cooling pump moves coolant through the heat exchanger, therefore cooling the coolant with sea water (exchanging heat). This gets the coolant inside the heat exchanger moving so you can drain it with the distilled water. If you were going from green coolant (Ethylene Glycol ) to the pink/red/orange stuff (Dexcool), you may want to flush with distilled water a few times, since the two types of coolant are incompatible. When mixed together they form a gel-like substance that stops coolant flow and consequently the engine overheats. Since I switched from Yanmar Dexcool to Havoline Dexcool, I only flushed with distilled water once. Many boat engines have a portion of the coolant circuit diverted to a water heater. When draining, you can ensure the coolant in the water heater circuit is drained by disconnecting from the pump and blowing the coolant into the engine. You need to be careful because coolant is poisonous. After draining the distilled water with the leftover coolant, fill the engine with coolant. Make sure the coolant is either pre-mixed (ready to go in) or concentrated (mix yourself). The Havoline was concentrated, so I mixed with 50% distilled water and 50% coolant. After filling with coolant through the pressure cap, run the engine with the cap off until the thermostat opens. When this happens, the coolant level will drop. Fill to the top again and close the cap. Never open the cap on a hot engine.

During the project I replaced a portion of the coolant hose because the rubber was cracking a little.