Fixing boat engine cooling system problems

Stu Davies

Stu Davies solves niggly Volvo Penta 2030 engine cooling problems

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The couple next to us in the marina at Albufeira in Portugal are lovely.

They bought their Dufour 36 Classic two years ago from a Frenchman and have been fettling it since, ready to do a transatlantic circuit.

Their Volvo Penta 2030 engine has only done 600 hours (the Frenchman didn’t do much!) and I’d shown them how to change the filters and oil etc.

However, from day one there has been an issue with the cooling system.

The engine sets off its overheat alarm if it is motored hard and the raw water system leaks from hose clip joints.

The previous owner paid for the local workshop to sort it as part of the deal and the intercooler rubber end caps were replaced to allegedly cure the problem.

They stripped it, acid-cleaned the tube bundle and put it back together but the leaks continued. I was asked to take a look.

Identifying boat engine cooling system problems

I noticed immediately that the new rubber intercooler caps were swelling and became very taught as the revs were increased and that coolant water had started to come out of the pressure cap on the fresh water side.

I’d showed my friend how to do the finger dip taste test on water in the bilges and he confirmed that it was salty.

Hmm, salt water coming out of the fresh water header tank. Oh dear!

So a few minutes’ thought and tracing of the salt water system suggested that the bulging of the rubber end caps was being caused by excess pressure because the salt water couldn’t get out of the system fast enough.

Given that the intercooler tubes had been cleaned, the only other blockage could be in the exhaust elbow where the salt water is dumped into it.

An elbow joint of a boat engine cooling system

This is a new elbow showing the raw water injection point which was blocked to about half its size. The interior, which has a sleeve which the raw water flows around, was similarly blocked. Careful poking with an old hacksaw blade and stiff wire cleared it. Care has to be taken because the casting becomes fragile with age and corrosion

Any blockage here allows pressure build-up; the rubber end caps are inflated and water passes from the salt water side to the fresh water side under where the hose clamps hold the rubber caps on. Either that or the tube bundle is holed.

I stripped the intercooler and took out the tube bundle, which looked like it had taken a bashing on removal by a previous mechanic.

The rubber caps were new but didn’t have any Volvo Penta part numbers on them and the areas where they clamped onto the tube bundle and heat exchanger casting were dirty and had corrosion.

I cleaned them up and reassembled them. But it still leaked under higher revs.

Part of a boat engine cooling system

Fixing a boat engine cooling system: The tube bundle in position, each end has a rubber cap to separate the fresh water cooling system from the raw water system. Credit: Stu Davies

The exhaust elbow raw water inlet from the back end cap was badly blocked with salt deposits and I was fairly certain that it was the issue.

Given that a professional workshop had done work on this and must have seen the same as me, I was less than impressed.

I disassembled it again and said to the owner: “OK, we need the tube bundle pressure testing, just to rule that out, so take it back to the people who have done the work and get them to test it first. Then we can go from there.”

He did and sent me a photo and said the bundle was fine. I then took off the exhaust elbow and could see that the main exhaust part was OK, not blocked, but the raw water sleeve and connection were badly blocked.

Having dealt with these horrible things before, I warned the owner that the elbow may collapse as we cleaned it.

A testing time

In the meantime, the workshop staff had started talking about cracked cylinder heads etc. I needed to get that worm out of the owner’s head so I set up a test rig on the pontoon with the end caps clamped on the tube ends as normal.

The pontoon hose pipe was plumbed into one of the end caps and the other end cap was plumbed into the exhaust elbow. It was a perfect way to illustrate the issue.

As the water hose valve was slowly opened the water could be seen bypassing where the rubber cap was clamped onto the tube bundle, and then when the water flow was increased it blew the end caps off!

Continues below…

I then got out various bits of wire and made up prodders; an old hacksaw blade was particularly useful, flexible enough to get in the passages in the elbow.

Slowly but surely I managed to clean the water passage out. I knew I was making progress because by blowing into the water connection in the elbow, the resistance of the airflow could be felt getting less.

I left the hose running at full chat for about 15 minutes to help clean it and videoed it; no leaks from the tube bundle, no blowing off of the rubber end caps. The problem was fixed.

It had been the exhaust elbow all along, not faulty rubber end caps, not a blocked tube bundle and definitely not a cracked head!

hoses and tools on a pontoon

This is the tube bundle set up on the pontoon with the quite high pressure water supply connected. The rubber end caps were clamped onto the bundle and the hose went to one end and the exhaust elbow on the other end. This is a good way to pressure test tube bundles as well. If they are leaking the leak would be plainly seen from the small tubes. It was a game changer in that the leaks past the hose clips contact ends could be clearly seen by the owner, especially when the rubber end caps were forced off! When we had finished he could see that there were no leaks. Credit: Stu Davies

So if anyone is having issues with salt water getting into the fresh water system of their engine with the tube bundles held in place with rubber caps, look at the rubber end caps.

Do they balloon and get very hard as the revs rise? If they do, it’s a good chance the exhaust elbow is getting blocked on the sea water side.

And the overheat alarm going off at high revs? Cured.

The cooling water flow through the intercooler tube bundle was back to normal as was the cooling process with the blockage cleaned out.

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