Left bright and early on saturday morning and had a trouble free trip up to Dielette on the French coast at 30+ knots all the way. Spent the day shopping and sampling the local food and beverages as you do, and whiled away the time before we could return, as the tide wouldn't be high enough til about 9pm.
Set off at 7pm and about 5 mins out from port, the overheat alarm went off
and the engine cut down to 3000 revs to its 'self-protect' speed. Now this has happened on three ocassions before, and each time I have stopped, checked for bags around the intakes and restarted, and all has been fine. But not this time. Each time I restarted or tried to accelerate it would overheat and the tell-tale would peter out. I found that I could just about keep on the right side of the alarm by going no more than 2000rpm, about 7 knots. Whilst this meant it'd take 3.5 hours to get back I wasn't unduly concerned beacuse it was flat calm and the tide was in our favour. Sod's law applied just as we got to the Ecrehoes reef (the only rocky bit of water before home) and the bloody thing cut out so I decided it was time for a pan-pan and the inshore lifeboat came out to resuce us.
So after all that rambling, the question is..... what can I check.
After the first time it happened I checked and replaced the impellor, and I removed and checked every water pipe on the engine, but there was no sign of any blockage. I've even pumped water water through the block and it comes out just fine. A couple of weeks ago I put a new thermostat on, the only thing I haven't changed is the pressure relief valve.
The symptons are
1 - On first three occasions it happened after about half an hour of motoring, but was fine after a restart for the rest of the days boating.
2 - This time no amount of restart would help. The strange thing is that the journey up was trouble free, with the only difference from normal being that the engine stayed tilted down during the day on the pontoon.
Short of removing the head or block, how can I check the water galleries within? Why should it be so seemingly temperamental. The one thing I did notice, was that the pressure gauge would read about 4psi at limp home speed of 7 knots, but would occasionally (say every 20 mins) drop to 1psi, the engine would get really rough then after 2 or 3 mins the pressure would build up to 4 psi again. If i tried to accelerate the pressure would instantly drop down. Bizarre!
Hope someone can help, because it's 2 weeks before the local agent can look at it
Edit: And to cap it off, it worked absolutely fine yesterday went I went to move it to it's normal mooring.