Join Date: May 2006
Location: Left Coast USA
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Re: Volvo Problems
Search, this is my post from several years ago.
MD7a - know it well. You can use a non contact thermometer for troubleshooting. It works great for showing the hot spots. I had cooling problems with mine and got a good understanding of the cooling system, problems and solutions. The water passages are most likely rusted shut, but while time consuming it is not difficult to fix. If you can get a hold of a manual, it will help but I will try to describe what to do. This was 10 years ago so it is the best I can remember.
The MD7a was not designed for salt water cooling. The internal passages are i parallel and are quite small so it doesn't take much for them to plug. The good news is you can clean them out with a stiff piece of wire. Your bilge pump is going to get a workout. I would start by removing the exhaust manifold. You can then clean the manifold. You will find lots of rust in it but you can scrape it out if you poke around with the wire and flush it out with water. There are some small passages that were so plugged on mine that I needed to drill them out. You can also poke around in the head and loosen as much rust as possible too. Remove the engine drain valve on the block below the injector pump. Poke around in there to loosen rust. Run a hose into the water passages in the head to flush the rust out. (plug the exhaust ports so you don't get water in there) Replace the exhaust manifold and gasket. Leave the thermostat housing off.(I think) You can hook a hose to the copper water inlet that goes to the bottom of the thermostat housing to flush the engine.
Now this is really the key part. Pull the soft plug (freeze plug) on the front and rear of the head. This gives you good access to the head water passages and allows you to clean the most important ones which are directly below the plug openings and are undoubtedly plugged. There are 2 water passages that feed water from the head into the block. If you poke around while running water you should get a lot of rust flushing out once you get them open enough to flow.
The plugs can be replaced with the rubber double washer through bolt type sold in auto parts stores. You could try to replace them with the Volvo ones but it is not needed.
If you have a manual this will make more sense as you can see the holes in the head and block in the pictures.
The basic problem with this engine is if the water inlet is not shut off when the engine is not used for some time the water level will drop to sea level back through the cooling pump. This leaves a water air combination in the engine which allows rust to form.
After you clean the engine and get it working properly you can start shutting the water off when leaving the boat for a few days. Hang the engine key on the handle so you don't forget to turn it on.
I know this might sound daunting but if you have some mechanical abilities you will figure it out. Be aggressive as possible in the cleaning process as you won't be able to get at all of the passages anyway.
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