Overheating

Neil_P

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I have a similar problem to Julian. (gonefishing) The starboard engine (Volvo AQ151) overheats when reving above 3500rpm under load. All appears ok at slow revs, load or not and at higher revs with no load.

I think I may have picked up a plastic bag on the leg as I noticed one just as it went under the starbord side of the bow a couple of minutes before the problem. Could this have been drawn over the intake and starved the raw water system?

The problem was first noted when travelling at 20 knots plus but remained after slowing. After the engine had been shut down for a few minutes it cooled immediately on restarting but overheated after speeding up.

I have checked filters etc. Thoughts are that the raw water impellor may be damaged by a lack of water or the high temperature.

Any other ideas?

Neil



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h4nym

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Unlikely that the impeller is damaged by LACK of water... but easy enough to change anyway! Do visual checks indicate any damage / wear?

I had a problem not that long ago with an overheating port engine, and it transpired it was the water pump beginning to seize up. That would be somewhere to investigate...

Good luck

H

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studgies

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I used to have an AQ151 and it had a similar problem, I also know of at least one other person with the same fault. It turned out to be a restriction at the water outlet end of the manifold where the manifold joins the rubber hose going to the transom fittings. There was a build up of rust and general debris here blocking all bar one of the outlet holes.

Worth a try.

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bernard_foster

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How old is the engine?, this sound identical to the problem i have recently had with my 40 Year old AQ80, turned out to be the water ducting in the main block, gummed up with rust, works great now

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BarryH

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The 151 uses a rubber impellar running in a brass pump housing. The water cools it and lubricates it. Lack of water will kill the impellar fairly quickly!

Check the mainfold as someone else said. They are known to corrode between the exhaust outlet and raw water outlet in the back end. This restricts the flow of water and lets water enter the exhaust ports on the two rear cylinders. Cant see that this will be the prob though, it don't happen just like that.

Theres no strainer in these lumps, well there is but its on the inboard side of the raw water pump!. Most likely to be a partial blockage in the water inlet side. Whip off the pump front cover and also the rubber hose from the leg to the copper inlet pipe.

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byron

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As another overheating source I am always inclined to look at any rubber piping taking water to or away from the engine. After a while they can lose their rigidity when hot collapsing under pressure and restricting flow. I only say this because it has happened to me and I also saw it happen on a Broom Crown.

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scottie

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if you only had the problem when planing the hose connection on the a frame could be the problem as they corrode and leak air in when the boat rises out of the water
ps check that the water intake at the foot of the drive a vertical 1" hole is clear as well as the grids in the sides


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martynwhiteley

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I have also experienced a similar problem with one of my old (now replaced) BMW petrol engines, and the problem was (as prev. suggestions) a blockage in the manifold outlet/exhaust riser.

Rust and crud (some from broken melted impellor bits) was almost, but not quite, blocking the outlet water ducts in the riser.

Visual checks before stripdown showed water to be flowing at idle, and I spent a lot of time and money fitting new hoses, impellor, and finally a pump, all the no avail.

Eventually I tried blowing into the water inlet on the manifold, and realised that there was a severe restriction. After removing the manifold and the riser, the problem was clear. Following a manifold clean and a new riser, the cooling was perfect.

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Dave_Knowles

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Hi,

I had the same problem when I first bought my Cleopatra with a 151A. After blowing the rubber connection on the maifold and being rescued by Seastart (the best investment I have made on the boat) I phoned around Volvo agents and all said it would be the manifold so after picking myself up from the floor after hearing the price I bought one. On taking the old one off I found there was nothing really left inside. It had all rotted out so put on the new one (a fiddly job) and the teperature is now normal. Thge only other place is the header tank which can become blocked and can be cleaned out and refurbished by you local car radiator repair company.

<hr width=100% size=1>Take care.

Dave

Dave Knowles
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