Volvo MD7A Rebuild and Cooling System Questions

[damonlm]

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

[TassieBloke]

Hi Daminlm,

I don't much about Volvo marine engines and I'm any number here will chime in.

But what I do know is that you are to be commended for trying it yourself AFTER having visited forums etc. I personally am all for getting in and doing it yourself, because when you are out there, and things go pearshaped, you will know your own motor better than most.

As for rebuilding one as a 'spare', the first thing I would do is get hold of a workshop manual specific to that motor, as most of the info required to do that will be there.

The economics of it are another debate...

Good luck with it[emoji106][emoji106]

[DougR]

Hello Damon,
The water flow on the MD7 goes like this:

Water comes out of the seawater pump and goes up to the exhaust manifold, where it has two options, 1. Go into the exhaust manifold via the "T" fitting or 2. Continue past the "T" fitting over to the thermostat housing and enter the thermostat housing.

When the engine is cold the thermostat is closed and will not allow any water to flow within the engine, so all the water flows past the "T" fitting, into the thermostat housing and then immediately out of the top of the thermostat housing and into the exhaust mixing elbow.

As the engine begins to warm up, the thermostat begins to open, and this allows some water to begin to flow into the "T" fitting and into the exhaust manifold. At this time water could be flowing in both paths.

When the water flows into the exhaust manifold via the "T" fitting, it circulates inside of one section of the manifold and then exits the manifold via two of the holes and into the cylinder head. From the head the water flows thru holes down into the cylinder block, circulates around the two cylinders and then comes back up into a different section of the head and then exits the head back into the exhaust manifold thru the third hole. Now the water is in the section of the manifold where it touches the thermostat and it warms the thermostat, which will begin to open. So the thermostat senses the temp. of the water in the manifold and head and controls the flow in the engine. As the engine water gets hotter and the thermostat opens more and more, the thermostat also begins to block off the water flow passage in the thermostat housing so that water can no longer take the "short cut" around the engine, and this forces all the water to flow thru the engine.

So the thermostat sits there acting like a traffic cop directing water either around the engine, or thru the engine depending on the engine load and temp. This way water is always flowing out of the thermostat housing and into the exhaust elbow, and the exhaust hose won't burn up.

Hope this makes sense....

DougR

[MicHughV]

The Volvo MD7A is...I believe....a raw water cooled engine......ie, it uses seawater to cool the engine. I had the Volvo MD7B engine in my first boat, the MD7B is the same as the MD7A, but for the transmission. The MD7B had a downward sloping transmission.
Regardless, being seawater cooled, eventually leads to the water passages around the cylinders getting clogged up with salt residue preventing proper cooling of the engine, which can likely lead to other problems.
I made a habit of removing the cylinder head and cleaning out the water passages around the 2 cylinders and the cylinder head. Once cleaned, I simply replaced the cylinder head gasket and cylinder head and the engine ran fine. I did this about once every two years over a span of 12 years or so.
It's a robust engine that is for sure.
Hope this provides some guidance.

[Compass790]

Don't know yr Volvo but will just give a little rebuilding advice from my experience. First off start with what tassie bloke said. You NEED a workshop manual, even if you decide not to rebuild still good to have.
Then I would test compression & oil pressure . If compression is low you may be in for pistons, rings & liners + valve grind and maybe exhaust valves. If oil pressure is low you may be in for new crank main bearings & big end bearings.
Price these parts up & gaskets as you may be shocked at what Volvo will charge. You may be able to cannabalize some parts from your spare engine.
I don't THINK it would make economic sense to rebuild it if you cannot do most of the work yourself. When we totally rebuilt our Yanmar I reckon I could have bought a new engine if I'd worked overtime doing the same number of hours I spent on the engine or close to it anyway.
But the advantage with rebuilding is you totally understand your engine & it makes troubleshooting so much easier. If you can do the rebuild at your leisure on a spare engine rather than having an immobile boat it is so much better but price it up first.
There is also the alternative of buying a good used engine that has to be considered

[Rex Delay]

Some years ago I rebuilt an MD2B Volvo. Parts alone were more than buying a new anything. I would not do that again!

[barnakiel]

MD7A can be fresh water if someone added a conversion kit. Many owners did. These engines will go on forever ... until a part breaks and then ...


Here (Europe, Canary Islands) proper overhaul is not possible (because nobody can do this). Elsewhere (continental Europe, Sweden, Germany, Denmark, etc.) it costs about as much as buying a new unit. (give take 8 k)



You must make the $$ calc for yourself. Some will rebuild, some will scrap and buy something new.


Cheers,
b.
MD7A (converted with a Martec kit)

[Shawn67]

Easy to convert to fresh water without a kit too. Just need the add on Johnson double pocket pump (assuming you have the later Johnson raw water pump) a heat exchanger and hoses. The add on double pocket pump becomes your raw water pump. Plumb from the strainer to the pump to the raw water inlet on the heat exchanger and then into the water injection point on the exhaust pipe. Fresh water circuit is from the outlet on top of the thermostat housing to the fresh water inlet on the heat exchanger then to the inlet on the transmission and leave everything else the same.

Shawn

[Shawn67]

Originally Posted by MicHughV

I had the Volvo MD7B engine in my first boat, the MD7B is the same as the MD7A, but for the transmission. The MD7B had a downward sloping transmission.

Different transmission and the MD7B also had a different high pressure fuel pump that raised the RPM limiter on the engine so that it put out more HP. Went from the MD7A 13hp at 2600rpm to MD7B 17hp at 3000rpm.

Shawn

[MicHughV]

having owned 2 volvo diesels in my various sailboats...all older and earlier versions....the MD7B and the 2003 series...going back to to early 80's for the MD7B and early 90's for the 2003 series, I can say that I have 12-15 yrs or more years of experience with these volvo engines.
If it is already in your boat, that s one thing..but if you are contemplating a new engine, I would seriously consider alternatives !! In subsequent boats I had Westerbeke, Perkins and Yanmar..so all told, 35 yrs or so messing around with diesels.
Every diesel manufacturer has it's plusses and minuses and it's hard to quantify one make over another. If I had my druthers, I would chose Yanmar as #1, for a number of reasons...followed by Perkins and Westerbeke in that order....a distant 4th would be the volvo. I think every volvo owner I ever met had a love/hate relationship with their volvo's. When the run, they run great, but lordy, when they don't...bring your check book !!!
Where you cruise, or plan to cruise is big factor in engine decision.
To make a case in point. I hail from the NE Florida area.This is Yamaha territory. It would be fair to say that 99.9% of outboards here are Yamaha. If you have Mercury, Suzuki, etc....finding parts or mechanics or dealers would be a challenge.
For this reason, I would choose Yanmar, as in all likelihood, it is more of an " international" diesel engine, which means parts and service are likely more readily available.
Bottom line....know where you will be travelling...because when the **** hits the fan, as it invariably will...it's nice to know you can find parts or a dealer.

[hpeer]

Originally Posted by DougR

Hello Damon,
The water flow on the MD7 goes like this:

Water comes out of the seawater pump and goes up to the exhaust manifold, where it has two options, 1. Go into the exhaust manifold via the "T" fitting or 2. Continue past the "T" fitting over to the thermostat housing and enter the thermostat housing.

When the engine is cold the thermostat is closed and will not allow any water to flow within the engine, so all the water flows past the "T" fitting, into the thermostat housing and then immediately out of the top of the thermostat housing and into the exhaust mixing elbow.

As the engine begins to warm up, the thermostat begins to open, and this allows some water to begin to flow into the "T" fitting and into the exhaust manifold. At this time water could be flowing in both paths.

When the water flows into the exhaust manifold via the "T" fitting, it circulates inside of one section of the manifold and then exits the manifold via two of the holes and into the cylinder head. From the head the water flows thru holes down into the cylinder block, circulates around the two cylinders and then comes back up into a different section of the head and then exits the head back into the exhaust manifold thru the third hole. Now the water is in the section of the manifold where it touches the thermostat and it warms the thermostat, which will begin to open. So the thermostat senses the temp. of the water in the manifold and head and controls the flow in the engine. As the engine water gets hotter and the thermostat opens more and more, the thermostat also begins to block off the water flow passage in the thermostat housing so that water can no longer take the "short cut" around the engine, and this forces all the water to flow thru the engine.

So the thermostat sits there acting like a traffic cop directing water either around the engine, or thru the engine depending on the engine load and temp. This way water is always flowing out of the thermostat housing and into the exhaust elbow, and the exhaust hose won't burn up.

Hope this makes sense....

DougR

DougR,

Just want to say THANKS for this great explanation.

I’m having a very weird and erratic cooling problem with my MD7 and your explanation will surely help me sort this out.

I’ve got to think about this some more in light of my experience and perhaps ask a follow on question.

For now I wanted you to know your explanation was very helpful.

[Mongo_at_sea]

Originally Posted by hpeer

DougR,

Just want to say THANKS for this great explanation.

I’m having a very weird and erratic cooling problem with my MD7 and your explanation will surely help me sort this out.

I’ve got to think about this some more in light of my experience and perhaps ask a follow on question.

For now I wanted you to know your explanation was very helpful.

Hi, Another MD7B owner here (1984 model) , adding another wrinkle. Yes its an old thread, but the MD7 goes on forever.

I wanted to review the cooling flow description.. based on above, if the thermostat opens fully and the engine block cooling lines are blocked, will water still come out the exhaust?

Three years ago I took off the manifold, cleaned it up with rydlyme. I cut the cooling tube into three pieces and added high temp silicone tubing connectors so that I could install the cooling tube flexibly into the manifold, thermostat and water pump without leaks. It seemed to work fine.
This fall I noticed water was slowing, and I took it apart to find that where I had cut the cooling tube, blockages had formed. I took it apart and cleaned it again, but I am wondering if the engine is getting too hot and causing salt precipitation. If so I assume the cooling channels in the block are blocked as well. How difficult is it to clear these? I was thinking of converting to freshwater cooling, but first I need to verify if the cooling channels are blocked or there is no point.
1.Anyone try running rydlyme through the system without making it worse?
2. What is max temp the engine should run at when fully warmed and thermostat is operational?
3. anyone experience blockage forming in the cooling tube?
4. If engine full heats up and engine channels are blocked, will water still come out?
thanks

[barnakiel]

Originally Posted by Mongo_at_sea

I was thinking of converting to freshwater cooling, but first I need to verify if the cooling channels are blocked or there is no point.

Disconnect the cooling water intake and connect a hose.
Disconnect the cooling water exit, place a bucket under.


Run water thru. See what happens. You can circulate water with de-salting compound, cleaning the passages.


Less water going thru in normal opeartion can be many things :
e.g.
1 - water intake outside the hull can be blocked with growth or debris,
2 - water intake seacock, valve or filter may be clogged,
3 - water may be obstructed at the transmission (if you have msb),
4 - water pump impeller may be damaged or badly worn,


etc etc etc


If it were engine running too hot, the engine hot alarm would have sounded.



If you are converting to fresh water cooling then you are cleaning the passages anyways prior to conversion, so ...



b.

[Mongo_at_sea]

I may have not mentioned that after I cleaned it again the water flow is fine. the blockage was in the cooling feed tubes. There are two water paths through the manifold/thermostat. one to adjacent to the exhaust area and next to the thermostat (to open it when warm enough) and the second through the open thermostat and then through the engine. I am curious if there is a way to test if the second path is working or if the engine cooling passages are blocked. Based on the description by DougR referenced in my first post, it appears that the first path is blocked when the thermostat is open. Is that true? if that is true, then if water continues when the thermostat is open then the cooling passages are open.
My 1894 engine lights do not work, on my list to trouble shoot.