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We had an ongoing engine overheating problem with our 1997 Maxi 34 fitted with a Volvo MD2030B engine... The symptoms were that the overheat buzzer and warning light would activate after running the engine at max revs (3450 RPM) for 5 minutes. If the revs were then dropped, to say 2000 RPM, the alarms would stop after about one or two minutes. The problem we had was in trying to identify the cause, as we were also experiencing impeller failures and when checking the sea water inlet side to the heat exchanger, there were bits of impeller and barnacles blocking some of the tubes. Having resolved the impeller problem, fitted a sea water filter and ensured the sea water intake on the saildrive was clear, we took the boat out for a trial. To our disappointment the overheat alarms went off after 5 minutes at max revs. During the following week I took the exhaust outlet elbow off to make sure there were no restrictions in the sea water outlet annulus where it mixes with the exhaust. The sea water outlet was relatively clear, however, there was a large build up of coke at the exhaust inlet, reducing the diameter from 35 mm to approximately 18 mm. Was the 75% reduction in area increasing the exhaust back pressure and causing the engine to overheat? A new elbow was fitted and we took her out for a trial. The alarm went off after 12 minutes at max revs, an improvement but not the solution.
To make flushing the heat exchanger easier I fitted two short (100 mm) lengths of 30 mm o/d plastic tube to the inlet and outlet ports. Flushing through with a garden hose removed some of the sludge but there was still a lot left. I then soaked the unit over night in a strong caustic soda solution and flushed through with the hose, but with little effect. The next step was the local Volvo agent who soaked the unit for 24 hours in his acid bath. This removed a lot of the sludge but the unit was still not fully clean.
Having refitted the unit to the engine we took her out for a trial and ran the engine at max revs for 20 minutes with no signs of overheating. Success at last. Looking at the ingredients of Vanish the, main constituent is sodium carbonate peroxyhydrate, which when dissolved in water, breaks down to produce a large amount of foam, I think that it is this foam that gets underneath the sludge and forces it off the tubes.
18 April 2007 |
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The
next item to check was the heat exchanger. The engine coolant was drained
and the heat exchanger removed. The sea water tubes were clear but there was
evidence of a covering of soft sludge on the tubes on the engine/fresh water
side of the exchanger. Where accessible the sludge could be removed with a
finger tip or soft cotton bud through the two large inlet and outlet ports, but
the problem was getting to the rest of the tubes. NB we have the earlier type
of heat exchanger where the tubes are encased with a brass outer cover.
As a
long shot, I decided to have another attempt at removing the residual sludge
before putting the unit back in the engine. I soaked the unit in Vanish Oxi
Action Multi (comes in powder form) using warm water. This is the Vanish that
is used in washing machines to remove heavy staining. The result were
incredible, it was obvious after a few minutes that it was working from the
amount of muck that was in the water. I left the unit for about 1hour, giving
it a really good shake/agitation every five or ten minutes and at the end of the
hour flushed it out using the garden hose. I then repeated the process.
Inspection of the unit confirmed that the Vanish had removed the remaining
residue. As you can see from the picture below, the Vanish removed a further
one cm of sludge from the unit. 