Overheating Yanmar 3QM30

"...Recently my raw-water cooled Yanmar 3QM30 diesel engine had been overheating during trips and fairly suddenly after years of reliable performance. The engine idled without problems but increasingly overheated when run for extended periods at continuous revs (only water temp sensor). Engine maintenance has always been done on schedule. Fuel and air filters, thermostats and water pump all checked as ok. No signs of water intake blockages, insufficient air, compression issues at start-up, contaminated fuel or engine water leaks. Just noticeably greater engine temperatures and more steam than normal from the wet exhaust. With over 8000 hours on the clock and no obvious cause for the overheating, I was beginning to think a replacement engine was on the cards.

No not yet! The engine idles ok and there is no starting or exhaust problem, so more investigation was needed. The air intake and exhaust manifolds were removed and cleaned – not a backpressure problem. The cylinder head end anode plates were removed. Some calcification was coating the head walls, but wait a minute shouldn’t there be water circulation pores between the cylinder head and engine block?? They should have been visible but were solidly blocked by calcification!! This was clearly contributing to the overheating. I was both surprised and thankful to have found this before major engine damage had occurred and not in a situation when the boat was solely dependent on motoring. The regular use of vinegar soaking/flushing clearly hadn’t kept the smaller cooling passages clear. Even though raw water was being pumped through the engine block, cooling water from one of the dual flows was not circulating around the cylinder head.

So how to unblock the pores ? End pores could be reached but there was no access to physically unblock all pores. Removing the cylinder head would only be considered a last resort. Dissolving the buildups would be less abrasive and more complete. Increase the frequency of vinegar flushes? No, solution proven to be too weak. I considered stepping up the strength to 10%+ hydrochloric acid but ruled that out due to the risk of acid damaging the engine’s rubber impeller, seals and hoses. After canvassing the situation with other boat owners, one owner suggested using Salt-X. I hadn’t heard of Salt-X, but following some investigation and discussion of my situation with the supplier, I decided it would be worth trying.

First use was filling the engine’s cooling system with a 30% Salt-X solution and leaving it to soak for a week. At the end of that week the brown colouration of the initial flushed discharge clearly showed something had changed. During a 30min run at the mooring at varying revs the engine operated as expected, with no unusual signs or sounds and the wet exhaust discharge quickly returned to its expected colour. The operating temperature seemed cooler for the time but the run was too short to gauge any real change.

The sequence of draining, refilling the cooling system with the Salt-X mixture and leaving it for the week was repeated for a further 3 weeks. A similar coloured discharge occurred after each end-of-week flushing. After the 3rd week (4th flush) I noticed small white fragments were flushed out – yes! good to see! A check of the head cooling space showed it to be clean and the visible pores were open. The head bolt tensions were also checked and found to be ok. I have since taken the yacht for an extended run and found the operating temperature, particularly around the cylinder head, to be thankfully much cooler. I’m confident Salt-X has removed the cause of the engine’s overheating.

Out of this whole experience, I’ll now be using Salt-X as a regular maintenance agent. Salt-X has provided a very effective solution to resolve what could have turned into a major engine problem to fix. Roll on the warmer sailing weather!..."

Bruce Jenkins, Gosford, Central Coast, NSW.