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post #41 of 53 Old 07-20-2019
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Re: Volvo md7a overheating, temperature fluctuating

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I'm having a new problem with my cooling that has pushed me to try to completely understand all the nuances. It was a bit more complex than I realized.

I'm attaching my work here for comment and use if you find it helpful.

A couple of things I had not thought through.
1 - exactly how the "bypass" circuit works.
2 - that the tiny holes in the thermostat allow air out of the manifold cavity and let hot water up onto the sensor
3 - the need to have enough pressure to overcome the vented loop
4 - effect of back pressure on the system

I hope this helps, I'm open to suggestions for improvement. (!)Also hope the images come across readable!)
Thank you so much for your schematics ! I am struggeling with temp fluctuations on my MD7B and have read this with interest.
Are you sure that the water flows OUT through the thermostathouse/rubber hose when the thermostat is open? In my head, I had the idea that when the thermostat opens, water is running INTO the manifold from the house, to be exausted in the back into the elbow and not through the bypass/rubber hose?
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post #42 of 53 Old 07-21-2019
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Re: Volvo md7a overheating, temperature fluctuating

Based on a brief look, it appears the Volvo system works similarly to the one on the Universal 5411 I had on my previous boat. When cold the water was in recirculation mode. Only a small amount of water went out the exhaust. The thermostat was needed to force significant water out the exhaust and reducing recirculation. If you ran the engine without a thermostat, it would overheat.
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post #43 of 53 Old 07-21-2019
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Re: Volvo md7a overheating, temperature fluctuating

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Based on a brief look, it appears the Volvo system works similarly to the one on the Universal 5411 I had on my previous boat. When cold the water was in recirculation mode. Only a small amount of water went out the exhaust. The thermostat was needed to force significant water out the exhaust and reducing recirculation. If you ran the engine without a thermostat, it would overheat.
Hmm, must be different then. The same amount of water comes out, no matter if the engine is hot or cold, and I know that quite a lot of people run the old MDx Pentas without the thermostat, keeping them continously (too) cold.
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post #44 of 53 Old 07-21-2019
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Re: Volvo md7a overheating, temperature fluctuating

JimsCAL is correct. There is NO doubt that the thermostat opens to allow water to pass through the engine block.

Put your finger on the hose on the elbow then follow it back, away from the elbow. Where does it lead? That’s where the water is coming from.

The small amount of water in recirculation mode, with thermostat closed, is there to provide cooling to the exhaust elbow.

That you see no difference in water volume may be because of two reasons.

First because you can’t perceive the difference. I just cleaned my passages and gave them an acid wash, I have a hard time discerning the difference in water flow between thermostat on and off.

Secondly, if it is truly the same, because your cooling passages through the block and exhaust manifold are pretty severely blocked. You may be getting some small amount of water through, enough to cool the engine. If the passages are blocked the water will still come out through the bypass.

For all of this I personally have no clue why I experienced the wild fluctuations and changing symptoms. FM!

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post #45 of 53 Old 07-21-2019
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Re: Volvo md7a overheating, temperature fluctuating

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JimsCAL is correct. There is NO doubt that the thermostat opens to allow water to pass through the engine block.

Put your finger on the hose on the elbow then follow it back, away from the elbow. Where does it lead? That’s where the water is coming from.

The small amount of water in recirculation mode, with thermostat closed, is there to provide cooling to the exhaust elbow.

That you see no difference in water volume may be because of two reasons.

First because you can’t perceive the difference. I just cleaned my passages and gave them an acid wash, I have a hard time discerning the difference in water flow between thermostat on and off.

Secondly, if it is truly the same, because your cooling passages through the block and exhaust manifold are pretty severely blocked. You may be getting some small amount of water through, enough to cool the engine. If the passages are blocked the water will still come out through the bypass.

For all of this I personally have no clue why I experienced the wild fluctuations and changing symptoms. FM!
Hmm... I can't seem to get this figured out.
Can we agree that no matter the state of the thermostat, the pump delivers x amount of water dependant on RPM's ? So where does that water go, if the thermostat is closed? I would imagine that the same amount of water must come out of the exhaust, no matter the state of the thermostat?
Or am I just misunderstanding it all? (Would not be the first time &#128522
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post #46 of 53 Old 07-21-2019
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Re: Volvo md7a overheating, temperature fluctuating

There are two basic ways the cooling system in a raw water cooled engine works - recirculation or bypass. In looking closer, it looks like my previous Universal 5411 had a recirculation system while your Volvo has a bypass system. In the 5411 when cold, most of the water being pump just recirculates with only a small amount going out the exhaust. With a bypass system, when cold most of the water goes right to the exhaust, with only a little going through the engine. Either will work. One advantage to a recirculation system is the high water flow through the engine during warm up promotes a more even heating of the engine.
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post #47 of 53 Old 07-21-2019
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Re: Volvo md7a overheating, temperature fluctuating

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Hmm... I can't seem to get this figured out.
Can we agree that no matter the state of the thermostat, the pump delivers x amount of water dependant on RPM's ? So where does that water go, if the thermostat is closed? I would imagine that the same amount of water must come out of the exhaust, no matter the state of the thermostat?
Or am I just misunderstanding it all? (Would not be the first time &#128522
The MD7 is a “bypass” system. With the thermostat closed the water must go through a relatively small “bypass” hole in the thermostat housing.

When the thermostat opens FULLY then it can go through the larger path through the exhaust manifold - block - exhaust manifold - thermostat. But the bypass path is still there.

With the thermostat closed the only path is the bypass. With the thermostat open both paths are available, but the cooling path is bigger.

When the thermostat opens the amount of water through the bypass will decrease and most water will go through the cooling path - PROVIDED the cooling path is free from obstructions.

So while the pump turns D revs in one case it is pushing against a small hole and in the cooling case against a larger hole.

Think of it like a bulge pump, in one case it has to push the discharge up 5’ (greater “head” hard to do, low output) but in another case it only pushes up 2’ (less “head”, easier, greater output). The pump is turning the same revs in both cases.
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post #48 of 53 Old 07-22-2019
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Re: Volvo md7a overheating, temperature fluctuating

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Think of it like a bulge pump, in one case it has to push the discharge up 5’ (greater “head” hard to do, low output) but in another case it only pushes up 2’ (less “head”, easier, greater output). The pump is turning the same revs in both cases.
Thanks a lot, it does make sense.
I do seem to get quite a lot of water right from the start, when the engine is cold.
After some minutes it starts to cycle between low green (almost in the white section) and the read section. The cycles are within 20-30 seconds and it cools down rapidly when it reaches the mid section red.

The thermostat is brand new, and I have even tested it - it opens and closes fine, and is a 60 degrees celscius.
What I do seem to hear is that when it starts the descent from the red section (hot) it sounds like the exhaust is dry for a few seconds, before it returns to spitting out water again.
So I guess the manifold is running dry when the thermostat is closed, and then refills when it opens, that's why I thought the flow was the other way around.
But now I suspect that it might be the extra hole in bottom of the thermostathouse/manifold that is clogged?
I guess that would prevent the water from flowing into the manifold (because air cant get out there) - and then when the theromstat opens, it can flow in from the backend, but that will mean that there will be this short pause, where the exhaust is dry...?

But I guess this still doesn't explain how it would drain the manifold during the cold periods where the thermostat is closed...

I recently had to motor 25 nm, and it went fine, and the engine compartment didn't seem particularly warm afterwards, but this can't be good for anything.
The engine starts very willingly, does not smoke, and the oil is the right color. (Hardly uses any oil).
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post #49 of 53 Old 07-22-2019
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Re: Volvo md7a overheating, temperature fluctuating

Yes, I hear and share your frustration in explaining the symptoms. I can’t.

I will say that if your engine haven’t been cleaned in a while it would likely be if it from a good cleaning. However there are risks.

What follows is what I did. Others may have superior recommendations and warnings. Take it for what it’s worth.

First buy a new exhaust manifold gasket. I have reused them by applying a THIN coat of very high temperature silicone gasket material, but it’s not recommended.

Remove the exhaust manifold being careful to not muck up the copper pipe that comes from the water pump. There is this T connection under the exhaust manifold that looks delicate to me.

Using small picks, preferably with a bend like a dental probe but bigger, rig and scrape out all the crud you can. Also real out the copper pipe and crank the engine in decompression to assure you have water flow. Rinse water through the manifold, I used a big plastic syringe I found at Canadian Tire to inject water into the various holes.

Next I used a diluted mix of Muratic Acid and let it sit in the manifold until it stopped gassing. I carefully sloshed the acid around, I taped over some holes so it would not run out. The idea is to get the acid everywhere. I did this 3 times, about an hour each.

I reassembled everything and then ran an acid bath through the assembled engine. Again 3 times about an hour each. I have a T fitting in my intake through hull so I can winterize the engine. I put the diluted acid in a while plastic bucket and ran the output of the thermostat back into the bucket. Crank the engine in decompression to run the mix through. Wait until it stops gassing. Then crank it some more to get fresh acid in.

After this I seemed have have greatly improved water flow.

My understanding is that if you do this often enough you may open some additional passages in the block, head, or manifold. OTOH if you don’t do it your engine is shot anyway.

Contrary opinions on this method welcome.

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post #50 of 53 Old 07-22-2019
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Re: Volvo md7a overheating, temperature fluctuating

Hello all..
anyone find a non Volvo Penta thermostat that works with the MD7a/b and how did you hook it up?
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