MD7A Poor Water Flow

[gandy]

Hi,

Our MD7A has never passed much water, but until last trip never showed any signs of overheating. Water was never more than comfortable handwash temperature, and the engine never felt (or smelled) too hot.

Last trip when we were motoring against the tide, slightly higher revs than normal but not full power, the exhaust note suddenly changed and nothing but steam was coming out. Reducing down to tickover brought things back to normal. Trying again later if I ran at more than something like 1500rpm or so the exhaust water started to get really hot, but again returned to normal when easing off a little.

I've checked the obvious things for a sudden change, the cooling water intake and the impeller and nothing seems wrong there. When I changed the thermostat a few years ago I took the exhaust manifold off as well and cleaned that all out, also the checked the connections to the cylinder head.

Unless anyone has other suggestions, I think I need to check for blockage in the gearbox. I've seen that suggested. What's the best way to do so?

Any other suggestions appreciated. I have to say that taking that manifold or the thermostat housing off again is a real last resort. Both were so corroded they basically had to be stuck together with silicone to keep them water tight. If they have to come off, that's probably the end of the engine.

Thanks in advance, Tony S

 

[yellowspinnaker]

If you do decide that the manifold has to come off then it need not be the end.

The core plugs come out, rubbish and clag removed, and new core plugs are available for not very much money. If there's damage to the core plug seats in the manifold there are things you can do to remedy the problem or get around it.

The thermostat housing is, I think, bronze, and shouldn't have corroded, but the end of the manifold can and will deteriorate. It can be faced off by any competent auto engineer.

I'm in the process of attempting to source a replacement manifold for my MD6a, which is very similar to yours. My intention is to do the work on the replacement ready for a swap, then recondition my existing one as a spare. As the engine gets older the heat builds up a bit more, and the rust claggy muck is more likely to accumulate in the manifold water bath.

 

[gandy]

Thanks for the comments. The thermostat housing is cast iron, and was so corroded that would probably have to have had 2-3mm ground off to form a good mating surface with the manifold. The biggest corrosion problem however is the two sockets where the branched copper pipe connects. The pipe is supposed to be a push fit with an o ring seal. The sockets in the back of the thermostat housing, and the underneath of the manifold are both corroded so they are rounded off and enlarged, and won't hold the pipe secure or anywhere near sealed. We "got away with it" last time by sealing all these interfaces with loads of silicone, but it took several attempts before we had it assembled with all joints waterproof at the same time.

I wouldn't contemplate taking off either the thermostat housing or the manifold unless I had access to good replacements for both before starting work.

 

[jakson]

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