Volvo MD7A Overheating Solved?

[harbin2]

Since I purchased the boat in the fall of 2008, I have had a minor overheating problem. It exists when the sea water is warmest (July to September) and when the engine is being run over 2,000 rpm (max design is 2,600). I have dealt with the problem by motoring between 1,200 and 1,800. After reading several Sailnet posts suggesting blocking in the exhaust manifold, I decided to remove the manifold. This only took 20 minutes and I was happy to see a lot of fairly easy to remove crud in all of the cooling water passages. Most of it was located at the interface between the manifold and the head so it was easy to get to and fairly easy to remove. The gasket seemed undamaged so I didn’t replace it. Between the engine manual (showing a diagram of the cooling flowpaths) and probing around in the manifold and thermostat cover, I believe I have figured out exactly how the cooling system in the engine works and I’m confident I have solved the overheating problem. I’ll know for sure next spring when the boat goes back in the water. Attached are photos. If anyone has any comments or questions, I might be able to help with, please let me know.

Photos:
Head before
Head after
Manifold before
Manifold after
Thermostat Housing

[jetboat7]

Hello,

Thanks for sharing your photo's, I'm fairly certain thats whats going on with my Cal 30, did you run it yet to be certain it is fixed?

Good Luck,
Todd Basinger

[harbin2]

Just ran it yesterday and today and it runs great. Raw water is cool this time of year so that is a factor but I ran it at 2,400rpm for over and hour and it was cool the entire time. I'm very happy. Thanks to several posters here on Sailnet for the advice.

[Faster]

Congrats! You'll have a lot less anxiety while motoring this season!

[jetboat7]

That's good news, I'll remove my manifold as soon as I have the gasket set.
Thanks again

[harbin2]

Having a gasket set isn't a bad idea - for two reasons.
1. If the existing gaskets get damaged in removal. You don't want to waste a lot of time now that it's boating season.
2. Just to have new ones and ensure good compression, no leak by etc.
I removed mine at the end of the 2010 boating season so I figured I'd take a chance not getting one - I'd have all winter to do so if I needed it. As it turned out, the existing gasket came off very clean. It is metal faced and stuck to the manifold. So I put it back together using the old one. No leaks and it works fine. I had priced just the manifold gasket at about $45. The only other gaskets are the ones for the cooling water tubes. I reused those too but do have extras of them - they are a weird o-ring type rubber gasket with a square cross section.
The whole job was pretty easy - a lot less time that what I spent agonizing over doing it. My biggest concern was getting deposits down into the cylinders as I was cleaning out the head exhaust ports. I used a vacuum as I was chipping away at the carbon.

[jetboat7]

Hello again,

This boat is in the Santa cruz California Harbor at all times, after previously removing the thermostat housing (which was much more clogged up than yours in the photos you posted) I went through the whole system other than the manifold, I'm fairly certain that is my problem. I'll let you know after I take it out.

Thanks,
jetboat7