This article was printed in Practical Boat Owner in August 2012.
The question started with a request for help on the Liveaboard forum of ybw.com. Nicky and Paul of the yacht Carmel were experiencing problems with their Volvo Penta 2040, whose coolant circuit appeared to be suffering persistent airlocks. The symptoms of the problem were that coolant was being spat out of the engine at a considerable rate, litres of blue water being found in the bilge at various times. When the pressure cap was released the heat exchanger was always full to overflowing. Although Paul had done everything possible to ensure that all air had been expelled from the coolant circuit it still appeared that air was blowing the coolant out. Initial exchanges between Paul and forum members established that the filler cap was at the highest point in the system, that there was a calorifier in the system that had been leaking but was now repaired and that the whole circuit had been bled through repeatedly.
A similar problem with Volvo engines has been discussed several times on PBO’s Reader to Reader forum. The rubber caps at the ends of the heat exchanger are difficult to fit properly, with the result that seawater passes the seals and enters the coolant system. In order to achieve a pressure in the seawater system, theoretically only slightly above atmospheric, there needs to be a blockage somewhere downstream of the heat exchanger. This is commonly provided by salt encrustations in the exhaust manifold where the seawater is spiked in for discharge overboard. Several forum responders, including myself, suggested that this might be the cause of this problem. However, Paul soon replied that very recently the whole manifold had been removed, cleaned and ground flat as the fittings were quite pitted. A new thermostat was added at the same time. So no blockage then!
Other forum users suggested a range of other possibilities, such as a cut in a hose in the calorifier feed, absence of a coolant expansion vessel, failed cylinder head gasket, faulty thermostat, blocked anti-siphon valve, etc. However, it seemed clear to me that none of these satisfied the observations. At this point two relevant facts emerged. The first was that water was overflowing from the engine when it was stopped. The second was that Nicky and Paul were anchored about five miles away from me. We arranged for them to visit the marina where I was currently moored so that I could give them a hand.
It seemed to me that the cause had to be a perforation in the heating coil of the calorifier, allowing water to pressurise the engine coolant circuit until it overflowed. The first thing I checked was that this was possible from a relative pressure point of view. The pressure cap was rated 75 kPa, just over 10 psi. Most yacht domestic water pumps are rated at more than 20 psi, with some considerably higher. This showed that a leak of fresh water into the coolant was possible, rather than the reverse.
Paul and I set up a test to prove whether it was indeed happening. We removed the two calorifier hoses from the engine, after turning off the domestic water pump, bridging the open nozzles on the engine with a short length of new hose (red in the photograph). We pushed the two ends of the calorifier hoses into a cut off water bottle, turned the domestic pump back on and retired to the cockpit for a beer. By the time we had drunk a can each the bottle was nearly half full of pale blue water. We had identified the source: definitely a leak in the calorifier coil. Paul explained the repair and testing of the calorifier carried out only a short time previously. The testing consisted of pressurising the coil while inspecting the repairs to the external nozzles using soapy water. Of course, nobody thought to check that air was coming out inside the vessel!
As a final check, Paul ran the engine for a short time and monitored it for the next 24 hours. No more water emerged. To allow the boat’s fresh water to be used we joined the two ends of the calorifier hoses together with a short length of tubing. Once the pressure in here reached that of the domestic water, no more leaks should take place.
Finally, Paul contacted the supplier of the calorifier, only three years old, to try to obtain a replacement under guarantee, while checking local suppliers for a replacement in case this was necessary. Nicky resigned herself to cold showers for a while.