You have probably had an occasional experience of the overheating warning light coming on while motoring during a cruise. Most of the time the problem can be identified and fixed quite quickly. Common reasons are: The cooling water inlet is clogged with weed, the impeller has failed, the thermostat has stopped working. But what if none of these turns out to be the problem?
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| The very efficient Beta Ten
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I have a very efficient inboard diesel engine on a small day boat I own. During a return trip from Hampton court to Limehouse Basin on the Thames, a round trip of around 60 miles. The engine light started to glow on the way down river. Stopping and checking for possible causes did not reveal the problem, The stop was long enough for the temperature to go down and initially I thought the engine had cured itself. Sure enough though, the glow returned. Not a full blown red light but enough to make me feel uncomfortable.
Knowing that the engine manufacturer had told me that the unit was designed to run at higher temperatures than the safety margin set on the thermostat, I decided to carry on. There are few convenient stopping places on the Thames through the centre of London.
We made it to Limehouse with the warm red glow. Thinking it would be a good time to make a proper examination I was lucky to get the resident boatman to take a look. He could find nothing amiss. So we repaired to the bar at CA House, the headquarters of the Cruising Association.
Starting back up river all was well for a short time, then back it came, the very annoying soft red glow. What could it be? I can easily blame myself for not spotting a problem but this one had been checked by someone who I respected as an experienced engineer who had often helped out in the past. We got back home but the light stayed on.
Over the next few weeks I tested everything the manufacturers suggested. The tube stack was cleaned out, the thermostat was changed, The impeller was eased out of its housing, while I was at it I changed the oil filters and of course the weed filter came out again, clean as a whistle. Still no improvement. The electrical connections were all dismantled and cleaned. I even made a visit to the engine suppliers who were very helpful but mystified. They gave me a demonstration of all the possible problems on the same engine. I left with some replacement parts..
The next week end I fitted the new wiring and switchboard. Another round of checks and now for the test. After twenty minutes or so on it came again. The dreaded red glow. OK once more back to the manual and the troubleshooting pages. This time I followed every item in turn including the belt tension. It seemed within the spec. But just to be safe I put a spanner on all three adjustable bolts. The third one is out of sight and difficult to reach . As soon as I put some pressure on it there was movement. To be honest it was loose. At last the solution was obvious. As the engine warmed up the belt had become slack and a small amount of slippage was occurring, enough to decrease the efficiency of the water pump and in turn the cooling process.
Sounds simple when you know, but as often happens, a solution turns out to come from an unexpected source. The only consolation was that I was not the only one to miss this one, obvious when explained. It will be the first place I look next time.
