Yanmar 4JH3-TE Overheating at Full Load

[Tidetracker]

Hi, all...

Well, this one is a mystery. I've always done my own diesel maintenence and repair, but this is the first Yanmar I've owned, and I'm stumped... so is a diesel mechanic friend of mine.

It started in the spring, on our first outing. Normally, if I want to 'get somewhere', my cruising speed is usually 2800 or 2900 RPM, giving me 8.25 knots. This is the fifth season with this boat, and the engine has 475 hours on it. I've taken this boat all the way to mid-coastal Maine three times, and have spent many hours cruising the waters outside of Narragansett Bay, RI.

The trouble started the first time out. After about ten minutes at our usual cruising speed, the hi temp alarm started to sound (our panel only has the idiot light and sounder, no gauge). We shut it down, checked out the usual suspects, and restarted it ten minutes later. No problems, as long as we kept the RPM down to 1500-1800 RPM.... but running it back up to 'normal' RPM's caused the alarm to go off again.

Normal water flow out the exhaust... normal coolant level in the expansion tank. I opened up the raw water pump, but the impeller looked pristine, no broken, torn, or missing vanes.

So, I gave a new temp sensor a try. No luck... same problem.

This time, I figured that the thermostat might be stuck closed... so I replaced it (surprisingly expensive!). Still no luck, same symptoms.

A friend ran across something unusual... a report that Yanmar raw water impellers sometimes have had poor bonding between the rubber and the metal hub, and could slip under high load.... so I replaced the impeller with a brand new one (turned out to be VERY hard, even with a proper puller, because of limited clearances). The old impeller looked like it had come straight out of the factory, with no evidence of slipping on the hub (although it would have been hard to see, regardless).

We thought this had cured it, and the following weekend, sailed down to Jamestown for an overnight. The next morning, we had to rush back, so we throttled up to 'normal' cruising speed... and the alarm went off again.

At this point, after consulting several different articles and forums, I've got it down to two possibilities:

1) The heat exchanger is clogged with marine growth... not enough to impede water flow, but enough to reduce the thermal transfer characteristics, by coating interior surfaces with slime. There's a lot of marine growth in our area.

2) Possible problems due to electrolysis (more on that in a bit).

The first problem is a bit of a dilemma. Unlike the previous engines I've owned, which had a separate heat exchanger sitting on a clamp at the rear of the engine, and easy to remove, the Yanmar is much different. It appears that the raw water circulates through a heat exchanger which is actually part of the exhaust manifold, and then to an engine oil cooler (much like a regular heat exchanger, except it's virtually buried underneath the turbo) before exiting via the exhaust elbow. The engine oil heat exchanger is similar to heat exchangers I've known, with the usual tube structure, that could be rodded out, if growth was clogging it. I have no clue, however, how to clean the other heat exchanger, which looks like it's buried in the exhaust manifold.

Has anyone ever had to clean theirs out? How about the oil cooler... anyone ever remove it for cleaning?

Now, about electrolysis. I usually haul my boat out every other year.... in the intervening years, I keep it in the water. The bottom paint is good for two seasons, and it makes commissioning a lot easier, in the spring, when it's in the water. Usually, my zincs are pristine, even after a full year.... but I hired a diver a few months ago, to make sure that my engine raw water intake wasn't clogged, and he told me my zincs were completely gone... maybe the 'winter slip' I was in had a lot of leakage from adjacent boats... in any event, I'm told that the engine oil cooler is very sensitive to electrolysis... and I'm, also told it's extremely expensive to replace (which I'm not surprised about.. I couldn't believe that a simple thermostat, from a Yanmar dealer, was $69!) I haven't hauled the boat yet, figuring that another month wouldn't hurt anything (with regard to the lack of zincs).

So, that's the situation. Anyone familiar with any of this? I'd appreciate comments and suggestions.