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Old 13-05-2020, 13:01   #1
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MD7A Exhaust/Water Manifold (I am so sorry)

Hi guys!!!


Working on reviving an old lady, it overheated pretty badly so we took off the manifold to clean the channels, which were absolutely blocked.


We cannot figure out what the mystery hole does, but our theory is this: water fills a reservoir in the manifold, but does not 'flow'; it sits there and conducts heat, which will get carried away by the cold water rushing by through the copper coolant rail towards the thermostat housing. The hole in question is perpendicular to the sink where the copper rail will plug in, and it is the first place water touches the manifold from the water pump.


Can anyone confirm this? We've gotta make sure we aren't missing something, cause we can't find any way for water to flow into the 'mystery hole' and out somewhere else.


We did find a post saying it flows into the manifold at this point, and gets pushed into the engine through one of two holes, then all water exits the block through one hole. But we just can't make it happen. Maybe we need acid. Any advice sincerely appreciated!


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Old 13-05-2020, 13:30   #2
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Re: MD7A Exhaust/Water Manifold (I am so sorry)

I had that engine in my first boat.

As you know by now, that engine is raw water cooled, ie, there is no heat exchanger or fresh water, it uses salt water to cool the engine only.

If my memory serves me correct, the cooling salt water that gets pumped thru' the engine also finds its way into that exhaust manifold at that point and all cooling salt water gets ejected out of the exhaust manifold with the engine exhaust. That hole just allows the salt water to travel from the engine block into the exhaust manifold, hence exists with the exhaust gases, ensuring a constant flow of new salt water to cool the engine.

Being a raw water cooled engine, all the cooling passages around the cylinders quickly get crudded up with salt residue, requiring a yearly or bi-yearly removal of the cylinder head to clean it.

When you shut down the engine, there is usually some residual sea water that remains in the engine and that exhaust manifold, which finds it's way down the exhaust to...hopefully....you have an exhaust muffler container, which holds this water until the next time you fire the engine up.

That manifold does not hold any water, as any water in there could easily find it's way back to the cylinders thru' an open valve.

Hope this helps. Volvo does not make that engine anymore.
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Old 13-05-2020, 13:54   #3
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Re: MD7A Exhaust/Water Manifold (I am so sorry)

MicHughV,


Thank you so much for the response, especially so quickly!!


OK, based on your information and some more poking, here's my updated understanding of this manifold's water flow:




what do you think?


I think our problem is, water should be able to flow from purple to red, and the teal marks where we must have a blockage/severe oxidation, do you agree?
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Old 13-05-2020, 14:29   #4
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Re: MD7A Exhaust/Water Manifold (I am so sorry)

To clarify, the red triangle on the left is the bottom, and it seems cavernous in there. I am thinking that water should be able to flow from the coolant rail (plugs in as purple) into that area, where the only way out is through the two holes linked by the dotted line
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Old 13-05-2020, 14:57   #5
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Re: MD7A Exhaust/Water Manifold (I am so sorry)

it's been a while now, so my memory is somewhat faded. I had that engine from the early 80's. That manifold becomes part of the engine's coolant system, so the salt water that is pumped from the salt water pump, goes thru' the engine and also thru' that manifold. That manifold is constructed so as to have a water jacket around it, just like the engine. That manifold has cooling passages just like the engine, but one of those holes also needs to allow the salt water to exit with the exhaust gases as the salt water is continuously pumped thru' there while the engine is running . That cooling jacket will only have water in it, when the engine is running. When the engine stops, all the water needs to drain out of that manifold.


I had to dismantle my engine several times to ream out those cooling passages, as they would get almost completely plugged up with salt residue. It's a pain in the arse. Why Volvo makes a sailboat diesel engine that is raw water cooled is beyond me.

A normal diesel engine with a heat exchanger will have the salt water enter the exhaust system behind the exhaust manifold. The exhaust manifold will have an special elbow there to accept the exhaust gases and also a hose fitting on top, which allows the salt water to enter the exhaust system there.

The MD7A being raw water cooled does not have that elbow, so raw water has to find it's way into the exhaust hose directly from the manifold.

Hope this makes sense to you.
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Old 13-05-2020, 15:49   #6
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Re: MD7A Exhaust/Water Manifold (I am so sorry)

Quote:
Originally Posted by SkydivingSailor View Post
To clarify, the red triangle on the left is the bottom, and it seems cavernous in there. I am thinking that water should be able to flow from the coolant rail (plugs in as purple) into that area, where the only way out is through the two holes linked by the dotted line
This would be my understanding as well. Disclaimer - I have never had a MD7A but I have had a few other raw water cooled engines (small Yanmars and Buhks) and the raw water systems are similar; as well as the problems.

It is not uncommon for the lower section of the waterways in exhaust manifold to fill and cog with hard calcium carbonate and rust deposits (boiler scale and rust ). These deposits can be very difficult to remove and I have seen them fill more than half of the cavity.

It seems to me that it cogs from the very bottom and slowly fills towards the top, especially where is limited water flow. Calcium carbonate precipitates out of solution rapidly at temperatures higher than 60C and this suggests to me that most of problem occurs after shutting down a hot engine. The salt water is no longer flowing though the manifold and the residual engine heat allows this scale to form. As the scale increases , water flow decreases and then scale increases in a vicious cycle.

I suggest you remove the exhaust flange end that is still attached in the pictures although I realise that might be a challenge - to say the least. This should allow you to see exactly how bad the problem really is.

If it isn't too bad, hydrochloric acid will remove the scale but not always. Use with caution, it is not for the faint hearted or in inexperienced hands...
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Old 13-05-2020, 16:07   #7
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Re: MD7A Exhaust/Water Manifold (I am so sorry)

Quote:
Originally Posted by SkydivingSailor View Post
Hi guys!!!

.........

We cannot figure out what the mystery hole does, but our theory is this: water fills a reservoir in the manifold, but does not 'flow'; it sits there and conducts heat, which will get carried away by the cold water rushing by through the copper coolant rail towards the thermostat housing. The hole in question is perpendicular to the sink where the copper rail will plug in, and it is the first place water touches the manifold from the water pump.........
Again, I don't have a MD7A .

BUT... that looks like a drain plug to me if I am understanding the picture correctly.

Raw water cooled manifolds often have a drain plug on the bottom or sometimes a petcock. Used to drain manifold for winterising (or general cleaning).

Such manifolds should be cleaned regularly to prevent scale build as explained in the previous post.
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Old 13-05-2020, 16:54   #8
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Re: MD7A Exhaust/Water Manifold (I am so sorry)

FWIW, here is a picture of another old raw water cooled exhaust manifold to illustrate what you might be up against in the MD7A manifold. The red section is the exhaust side and the yellow section is the water side. The top yellow area was clear, the bottom section was totally clogged. The debris on the bench was some of the chipped out scale.

I was never able to remove the blockages in the bottom section. Even a drill didn't do much.

Hope you have better success!
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Old 13-05-2020, 17:21   #9
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Re: MD7A Exhaust/Water Manifold (I am so sorry)

yep, I had to use a drill bit the same size as the water passageway I was trying to clear and then flush out all the drilling residue...it was always a pain in the rear end. That crud in there can be quite hard.
But, I would do it and re-assemble the head and manifold back on the engine block, using the same gaskets that came off..and the engine always ran....would smoke a bit in later years, but always started and ran...

In the 10 years or so that I had it, I must have performed that surgery at least 3-4 times, always re-using the original gaskets.

That's an old engine, at least 40 years old. I know it's not made anymore. There was a version MD7A and MD7B, the difference being in the gearbox.

Youtube has quite a few references to these old Volvo engines and may be worth your time to look thru' Youtube.

At one time, I had sea water, somehow or another get into the cylinders, seizing them up fast. I beat on the pistons with a sledgehammer till they got to dead bottom, re-assembled the whole thing and damn...if it didn't start right up. A few flushes with clean oil and I was back in business. A remarkable engine, old fashioned to be sure, but tough !!

But that raw water cooling was always a pain in the butt.
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Old 14-05-2020, 09:29   #10
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Re: MD7A Exhaust/Water Manifold (I am so sorry)

Hi

Not sure if this is any help but on the MD11C / MD17D the water from the pump can leave the tube (that goes under the manifold and enter the bottom of the manifold (yellow on diagram) then circulates round the block and exits through the thermostat housing if the thermostat is open to join the rest of the water (which hasn’t been through the block) ending up at the exhaust elbow. This seems a bit counterintuitive as the thermostat is stopping the water getting out of the engine when shut rather than letting water in when open (hope that makes sense).

Hope this helps

Chris
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Old 14-05-2020, 10:35   #11
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Re: MD7A Exhaust/Water Manifold (I am so sorry)

MicHughV, Wotname, thank you so much! I have a much better understanding now. And that image Chriscane, to me it is pornographic at this point. Super appreciative.



I guess I just have a lot more work to do--it seems solid as a rock inside of this thing! I keep scraping, occasionally getting chunks... I think it's time to move from CLR to Muriatic acid
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Old 14-05-2020, 11:17   #12
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Re: MD7A Exhaust/Water Manifold (I am so sorry)

it's a tough little engine that is for sure...I don't recall that I used to flush out the engine with anything specific. After reaming out all the water passages with a drill, brush, etc, I Just put it all back together again and placed a bucket of fresh water inside the boat and connected the raw water intake into the bucket. I would keep the bucket filled with a fresh water hose and then ran the engine to flush out all the debris from my clean out project. I can remember seeing a lot of debris flowing out with the exhaust after first start up. When I stopped seeing the debris, I disconnected the bucket and it was back to business again. I guess you could add the muriatic acid to the bucket of water to flush out little bits and pieces, but I have no experience with that stuff..I do think it needs to be diluted some.

Re-assembling the engine prior to flushing,would limit the problem of accidentally, pouring muriatic acid or water inside the engine cylinders, crank case, etc.

That little MD7 was the only diesel I've had that came with a hand crank...yep, could start it by hand...did it a few times just for funsies...

Good luck to ya....
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Old 14-05-2020, 11:22   #13
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Re: MD7A Exhaust/Water Manifold (I am so sorry)

Ahhh that's a good idea, I think we'll do some closed loop flushing with diluted CLR once we get it back together. That's going to be very satisfying!!


I love the hand crank, we started it exclusively by hand for a while due to a bad key tumbler and poor planning. So satisfying!
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Old 14-05-2020, 11:35   #14
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Re: MD7A Exhaust/Water Manifold (I am so sorry)

If you have seawater and exhaust mixing in the exhaust manifold, the manifold is corroded thru and should be scrapped.....the two never mix in the exhaust manifold.

Here is my recollection of the flow....

Water comes from the pump into the copper tube beneath the manifold. It flows to the first entry into the manifold, which you have colored purple, goes into the manifold and splits. Part of it goes up the dotted line to the small upper hole and into the head, and at the same time it flows into the head thru the lower hole near the corner of the exhaust passage. At the same time this is happening water continues to flow in the copper tube and goes into the thermostat housing, where it divides into two streams. One stream comes back into the manifold thru the small O ring passage and flows into the head thru the hole near the other exhaust passage. The other stream flows up thru the thermostat housing and out to the exhaust elbow, and we can call this stream the “bypass” water. The water flowing into the head thru these streams will fill the head, then down into the block, then back up into the head and return to the exhaust manifold thru the weird triangular shaped opening, and then bump into the thermostat. This is how the engine fills with water, and there will be no additional flow until the engine warms up.

When the engine gets hot the thermostat begins to open and water starts to flow thru the manifold, head and block. As the engine gets hotter, the opening thermostat begins to restrict the water flowing in the “bypass” circuit, thereby forcing more water thru the engine and giving it better cooling. Effectively the thermostat sits there in the end of the manifold playing traffic cop and directing water thru the engine circuit or the bypass circuit.

Hope this helps....and hope it matches the holes in your manifold.
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Old 14-05-2020, 12:07   #15
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Re: MD7A Exhaust/Water Manifold (I am so sorry)

The cooling system is why I replaced my MD7a besides parts availability and price. Salt water pumping through a hot cast block...what could go wrong? The water ports in my head were more than half clogged as I am sure the other water passages in the block were. Good luck


Here is a copy of the Service manual if you don't have one. Oops the PDF is 20.46MB. PM me if your email will accept that and you want it.
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