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  1. #1

    Default Volvo MD7A Rebuild and Cooling System Questions

    In November 2018 I purchased a 1984 Dufour 2800 that was fitted with the original Volvo MD7A. This has been running absolutely fine, albeit with a little white smoke under moderate to hard throttle. A couple of weeks ago on a trip out, I noticed significant more smoke and a lot less water flow than I’m used to seeing. I have very little mechanical experience so turned to the internet and forums for suggestions. The impellor was checked and all looked fine. I read on lots of sites how the water channels through the exhaust manifold get clocked up so I set about removing this to inspect further. To my amazement, 2 of the 3 water channels were completely blocked with the third more than 50% restricted. Having read a very detailed blog about someone flushing one of these engines, I purchased some Rydlyme Marine descaler, various hoses and a submersible pump. A bucket of the Rydlyme solution was then pumped through the tube from the inlet seacock; I bypassed the water pump with some flexi house and plugged the bypass hole within the thermostat housing to make sure the solution went around the engine. I had to modify the existing thermostat to do this and replaced it in the housing to get a watertight seal. The return to the bucket came from the top of the exhaust block. I ran the solution both ways around the engine for 2 hours each way. The results were amazing.

    Ironically, once refitting a new thermostat and reconnecting the relevant pipes. Very little water was still coming out of the exhaust. It turned out to be a blocked water inlet seacock. This was an easy fix and considering 2.5 of the 3 channels were blocked in the exhaust block, I consider the flush a good job to have still done.

    One of my questions is does anyone know what and where the 3 holes in the exhaust block go / do and what the function of the 2 inlet pipes into the block do? One is via a T piece underneath the block and the other through the various routes in the thermostat housing cover.

    My second question is what advice would people give if rebuilding one of these engines. I’ve just managed to pick up another MD7A that I’ve seen running. My plan was to strip it completely and rebuild it. Partly for fun and to learn what’s actually in there and secondly to have a ‘good’ engine ready should anything ever happen to the one currently fitted. What tests and tolerances would people recommend I check both before and during the strip / rebuild and how might these results affect my actions. I’ve read things such as compression checks, reseating valves, skimming heads, reboring and cylinders rehoned but don’t know if these are things you should do anyway or only if some tolerance is out etc. Any general advice with this would be greatly appreciated.

    Many thanks,

    Damon

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    Last edited by damonlm; 05-30-2019 at 04:49 AM.

  2. #2
    Join Date
    May 2008
    Location
    Kent, the garden of England, UK
    Posts
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    Default Re: Volvo MD7A Rebuild and Cooling System Questions

    Quote Originally Posted by damonlm View Post
    In November 2018 I purchased a 1984 Dufour 2800 that was fitted with the original Volvo MD7A.

    One of my questions is does anyone know what and where the 3 holes in the exhaust block go / do and what the function of the 2 inlet pipes into the block do? One is via a T piece underneath the block and the other through the various routes in the thermostat housing cover.

    My second question is what advice would people give if rebuilding one of these engines.

    Damon
    My reply to these questions on the YBW.com Reader to Reader forum is as follows:

    When the engine is cold, and the thermostat is closed, water flows along the pipe under the exhaust manifold to the thermostat housing. It then flows through the bypass passage in the thermostat housing and on to the exhaust outlet elbow, via the anti-siphon device if fitted.

    When the engine is hot , and the thermostat is open, water also flows into the manifold water inlet passages from the tee piece and the thermostat housing then via the ports you have ringed in red into the cylinder head. Hot water leaves the head via the roughly triangular port to the back side of the thermostat.
    The hot water flow, controlled by the thermostat, then mixes with the bypass flow and flows then to the exhaust elbow.

    ( in some engines, but not it appears in this case, the open thermostat closes off the bypass)

    Dont spend shed loads of money on an old engine like this. Parts generally are expensive and many are no longer available. Keep it going with second had parts etc while you can but put any serious money towards a modern replacement such as a Beta

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