AQAD40B Volvo blowing cooling hose

Have noticed in recent outings that my port engine was running slightly warmer than my starboard engine. Last trip running the engine around 1500rpm everything appeared ok but on increasing rpm up to 3000 and after a couple of minutes the rubber end plate attached by two hose clamps to the rear of the heat exchange blew off resulting in the bildge filling with fresh and salt water. Had an engineer look at and he couldnt see any reason for this happening so put it back together after checking the heat exchanger. The same problem occurred again when doing a sea trial. Appears that the raw water system is being pressurised to dislodge the rubber end plate. Temperature of the engine is not excessive when this happens. Anyone have any ideas as the engineer is a bit mystified.

Did you check for raw water blockage immediately after the end cap? Make sure aftercooler water passage is not clogged and that exhaust elbow is not rusted shut. Raw water has to exit the system, if it can't it will build pressure and blow the weakest point in the system (that funky end cap because of its large surface area and soft material).

Hey, at least you have stainless oil pan

The engineer removed the exhaust elbow and although it was obstructed somewhat it was not rusted shut. I was under the impression that the aftercooler was before the heat exchanger so a blockage in the aftercooler would not cause an increase in pressure at the end cap or am I wrong here? As yet we have not had a chance to sea trial it again following removal of the exhaust elbow (waiting on parts). Thing is I want to cover all possiblities as to the cause of this problem before doing another sea trial.

Normally - Pump - After Cooler - Heat Exchanger - Manifold - Elbow. Could also be an engine and/or transmission oil cooler in the line somewhere as well. Elbow would be the most likely cause of your problem.

Thanks for the input. We have replaced the exhaust elbow as well as the rubber end cap on the heat exchanger. One can run the engine at 3000 RPM (no load) without the end cap being dislodged although there is an obvious build up of pressure in the end cap as can be heard with the engine stopped and the clamp on the end cap is loosened slightly. On a sea trial at 3000rpm the end cap is under severe pressure and it would be only a matter of time before it was dislodged. The temp of the engine is about 80-85 C and there is no loss of fresh water coolant.


As the pipe from the end cap goes directly to the exhaust elbow it would appear that we still have some restriction further down the exhaust track or stern leg. I have noted on the sea trial that this engine with WOT does not attain full RPM (about 400-500rpm short) Maybe this is an indication that there is some restriction in the exhaust system??? Guess the next step is to lift the vessel and check for marine growth in the leg or some other problem.

If your engineering shop is any good they can measure exhaust back pressure. They need to do this as part of the commissioning process for a new engine installation.

You might want to check for water in oil, and oil in water....its possible that your head gasket is blown and engine compression is leaking into the cooling passages. Good luck

SB - seems the problem is in the raw water side. No 'normal' way for a blown head gasket to pressurize the raw water side is there?

if everything else checks out, pressure has to come from somewhere

Prior to hauling the vessel to check for blockages in the leg the mechanics decided to check the raw water pressure at the H/E and compare this with the starboard engine (probably should have done this earlier) and guess what, both were the same so a blockage was not the problem. They think the rubber end plate is not being secured sufficiently to the H/E to withstand the raw water pressure. Apparantly there is some sort of rim on the H/E that should assist the end plate being secured. This rim was damaged in places. Anyway they have built this back up and sea trialed it and all now appears ok. Hope this is the end of the problem with summer coming on. Thanks for all your input, it helped.