strange happenings...overheating volvo

[mrplastic]

I have a very strange problem...I have twin Volvo AQD40's one of which started overheating suddenly some months ago. It was originally diagnosed by a main dealer as an impeller breaking up...heat exchangers, oil coolers and all associated parts were removed, chemically cleaned etc etc. The whole caboodle re-assembled and viola....still overheating!! Now after further investigation it is definitely a raw sea-water. What happens is that when the engine is started the pump delivers a good flow at the discharge to the exhaust elbow....then for some strange reason and it seems at random....the pump just stops pumping. No water flow at the discharge into the elbow...nothing...bugger all...the pump shaft is turning (this can easily be seen through the aperture in the pump body). The impeller is brand new and in perfect condition, the splines on the pump shaft are perfectly ok...no blockages in the system.....has anyone else come across this problem? any solutions offered? all advice appreciated.

Thanks in advance
CLP

 

[pvb]

A possible cause...

A possible cause, and it's very rare, but maybe worth checking, is the suction side of the hose to the pump. It's sometimes possible for reinforced rubber hose to degenerate allowing the "inner" surface of the hose to become detached from the rest of it. When subjected to suction, the inner portion can close up and prevent flow. As I said, it's rare, but hose is relatively cheap to replace so it might be worth a go.

 

[byron]

Re: A possible cause...

PVBs answer is a very likely cause. This sympton often happens with the Perkins HT6.354 and when all else fails it proves to be the pipe. These reinforced pipes do deteriorate and when they do they have a tendency to flatten when they get warm.

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[hlb]

Has the boat got outdrives. I had trouble once with the little pipe behind the leg, breaking. Fine whilst at displacement speed. But the minute we started to plane, the pipe came out of the water and sucked fresh air. Took ages to find the trouble.

<font color=blue> Advice is what we ask for when we already know the answer but wish we didn't.
Haydn

 

[ccscott49]

Are you sure the impellor is perfect, sometimes the brass centre comes away from the rubber, you cant see it, until you actuall try and move it, but I think the pipe delamination, is a very likely suspect aswell.

 

[mrplastic]

Re: A possible cause...

Thanks for the advice. I have got outdrives but this occurs with the boat stationary and moving. Also, the inlet pipe to the pump is metal tube, probably copper or brass. I have tried 2 brand new impellers...no difference. Sometimes I get flow....sometimes nothing. Engine revs make no difference. I beginning to suspect the pump itself or maybe the pump drive which I believe is connected to the injection pump. If that's the case I'll post here again! Thanks

CLP

 

[longjohnsilver]

Re: A possible cause...

I have seen both in the past, a fuel pipe on a previous boat collapsed but looked fine from the outside, also saw an impellor as described by Colin above, either way the solution should hopefully be simple and cheap.

 

[byron]

Re: A possible cause...

The pipe we are all referring too is far too big to be metal it is probably around 2" or more dia. Look under the engine.

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[mrplastic]

Re: A possible cause...

Thanks for that Byron, I will do. I'm fairly new to boats and learnin' loads and lovin' it! My own understanding was that this metal pipe ran along the the lower level of the engine and eventually connected directly to the outdrive leg. If the pipes as big as you say, I assume that it must reduce quite a lot in size somewhere then as the inlet to the pump is no more than 25/30mm dia. I certainly hope it's a hose as it'll be much cheaper to fix!

clp

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