Volvo MD11D overheating

[bliss]

hi - I have a 25 yo ( only 800 hours ) Volvo MD11D which has an overheating problem - I guess i am thinking about the impeller and the thermostat and water inlet valves etc

1 - the boat is pumping water out the back but it does seem to pump less as it warms up ...... ? why

2 - i have had the impeller out twice - and changed them but the same problem persists

3 - it is about 4 years since i put a new thermostat in - if the thermostat was stuck it would explain why the boat steadily heats until overheating with running .... but wouldn't explain why i think it is pumping less as it gets hot

4 - i also had a hot water service in the boat - a stainless tank with a central heat exchanger and the hot exhaust water was diverted through it - i removed this a few months ago and the water now should be diverted back out through the exhaust .... i guess after all of these years this could be corroded up .... but engine seems to pump water ok when cold ... pumps less when hot ... ??

5 - can you end up with some sort of air lock in the system ??? as it heats up

6 - i have been for a swim and there is nothing around the saildrive skeg

When i have put questions to this forum in the past i have always been helped by the informed replies .....

Any ideas ??

[DougR]

Water flow on the MD11D (raw water cooled) is as follows:
From the saildrive valve to the sea water pump, then to a "T" fitting on the bottom of the exhaust manifold. At the T fitting the flow can split, depending on the status of the thermostat. One way is into the exhaust manifold, the other is to continue to the thermostat housing, then to the exhaust elbow and overboard.

When the engine is cold and the thermostat is closed,the water flows from the sea water pump, past the "T" fitting and into the thermostat housing. From there it flows out the top of the thermostat housing and to the exhaust elbow. Very little water flow restriction in this path.

As the engine warms up, the thermostat starts to open. The movement of the thermostat partially blocks water flow thru the thermostat housing, and this causes some of the water to flow from the T fitting into the exhaust manifold water jacket. From the exhaust manifold the water flows into both cylinder heads, down into the cylinders, around and back into the heads and back into the exhaust manifold. Then it flows past the partially open thermostat and out to the exhaust elbow.

As the engine gets hotter the thermostat will completely close off the bypass opening in the thermostat housing forcing all the water flow thru the manifold, heads and cylinders.

Over the years the water passages in the exhaust manifold and heads will get plugged with corrosion, rust, etc., and this restricts the amount of water that can flow and will cause the engine to run hot. The passages need to be cleaned, either by using a cleaning solution or by disassembly and mechanical cleaning.

As an aside, the MD11D engine went out of production in 1982/3, so it's a little older than you think.

DougR

[bliss]

Doug

thanks for your time - sounds ominous - the engine sat in the shed at my father's house until about 1990 before he finished fitting out the boat .... i feel like i should try and change the thermostat or take a look at it before going down the other track but your logic seems sound - i can't remember if there is a gasket when you replace the thermostat ??

and how / where do you put the cleaning solution in ?? if the build up of corrosion is the problem then i suspect it needs the alternative treatment

[gah964]

Originally Posted by bliss

Doug

thanks for your time - sounds ominous - the engine sat in the shed at my father's house until about 1990 before he finished fitting out the boat .... i feel like i should try and change the thermostat or take a look at it before going down the other track but your logic seems sound - i can't remember if there is a gasket when you replace the thermostat ??

and how / where do you put the cleaning solution in ?? if the build up of corrosion is the problem then i suspect it needs the alternative treatment

oh boy

[Shawn67]

On the MD7A (also raw water) a common problem is the cooling ports between the head and the exhaust manifold get clogged up and the engine progressively overheats. Pulling the exhaust manifold and physically cleaning these up is the solution. I haven't done this on mine yet but it needed it at the end of last season. I will be physically cleaning this up, then running multiple gallons of citric acid through the engine and then converting to fresh water cooling.

Thread on another forum with good pictures of what gets clogged up on the MD7A.

Volvo MD7A Overheating Solved? - SailNet Community

Shawn

[gah964]

Originally Posted by Shawn67

On the MD7A (also raw water) a common problem is the cooling ports between the head and the exhaust manifold get clogged up and the engine progressively overheats. Pulling the exhaust manifold and physically cleaning these up is the solution. I haven't done this on mine yet but it needed it at the end of last season. I will be physically cleaning this up, then running multiple gallons of citric acid through the engine and then converting to fresh water cooling.

Thread on another forum with good pictures of what gets clogged up on the MD7A.

Volvo MD7A Overheating Solved? - SailNet Community

Shawn

and that's why I said "oh boy" tear it down and rebuilt it that's where your at now.

[Sandero]

I had an overheating problem with an MD17... I tried everything... and I mean everything. The loss of water and cooling was due to crude at the intake inside the strainer below the seacock. I cleared it by removing the hose and poking around down there while water gushed in for few moments. CLEARED and then the engine ran cool.

[mark maulden]

I have an MD17C FWC I rebuilt several yrs ago. If you're at all handy with a wrench, you can have the manifold and two heads off in under two hrs assuming decent access. Take them to a machine shop and have them boiled out. You can look at valves at the same time. Also take the elbow in. Then everything is cleaned out. Make sure all copper tubing is clear. You'll have to buy some gaskets...Easy Peasy


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