MD7A Exhaust Manifold Port identification - SailNet Community

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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Diesel
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Diesel This is a new forum dedicated to diesel engines and their applicable accessories.


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  • 1 Post By Rockter
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  #1  
Old 03-30-2012
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MD7A Exhaust Manifold Port identification

I have a MD7A on my Cape Dory 28. I just got it 3 months ago and replaced the impeller, cleaned the thermostat housing and removed the exhaust manifold due to the large amount of debris. There was quite a bit of clogging so it was worthwhile. I believe I've figured out the flow pattern from the manual diagram at capedory.org but I don't know what this port is for, does any one know? It seems to have an insert maybe lead as an overheat relief valve? I first thought it might be a water port that had completely clogged but it looks like it goes to the freeze plug on top. I don't want to dig around to hard as I'm unsure as to where it goes. Its the smallest port just above the exhaust port. I put a probe in it in the picture. I know this a long shot but I couldn't find an answer for this. I did see some great pics from harbin2 Volvo MD7A Overheating Solved? Thanks
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MD7A Exhaust Manifold Port identification-img_20120330_042827.jpg   MD7A Exhaust Manifold Port identification-img_20120330_043225.jpg  
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Old 04-04-2012
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Re: MD7A Exhaust Manifold Port identification

Sweeney
I just got your private message. Not sure why you were having trouble. Maybe you have it fixed by now.
I spent a lot of time understaning the MD7A cooling system. There are bypasses around and though the thermostat. Some of it is still somewhat of a mystery. For example, I believe the thermostat seldom opens unless you are running higher rpms. The engine runs very cool (maybe too cool). Below is a list of what I believe are the ports on the head/manifold. Hope this helps.
Ron
Function of holes from front to rear.
1. First, there are 4 bolt holes from the upper forward corner to the lower aft corner. The bolts pass through these holes and thread into to the head.
2. Second, there are two large rectangular holes Ė these are exhaust ports where the exhaust passes through the manifold to the muffler.
3. Large polygon shaped hole Ė this comes from the thermostat housing in the head. It passes water that has been inside the engine block out to the thermostat housing.
4. Small round hole below the large polygon. This passes raw water from the thermostat housing cover into the engine.
5. Small round hole below the forward exhaust port (smaller than the other two). Iím not sure what this does or where it goes.
6. Large circular hole in center of manifold. This hole is only about 1/8Ē deep. It is dish shaped and Iím not sure what purpose it serves. It does mate to a hole in the head that goes somewhere in the engine.
7. Small circular hole between the two aft large holes. This passes raw water coming up into the manifold from the copper tube, into the engine.
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Old 04-05-2012
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Re: MD7A Exhaust Manifold Port identification

On my olde Volvo MD17C, I just scrape away the scaling with a screwdriver and coat-hanger wire. The two vital ports are the coolant fluid entry ports. They are the scaled ports on the second tier up from the bottom. One of mine almost bridged off with scaling and starved the forward cylinder of coolant. The result was overheating and scored bores, now re-bored in 1998.

The motor is a good one, but the astronomic price of the spare parts will mean I will never buy another Volvo.
Never.

I had to take the exhaust manifold off last season. The cost of the exhaust gaskets? Sorry Sir, you have to buy the whole top end gasket kit. That will be £120. Really, only $180? Gosh, for 3 wee gaskets like that? What a bargain!

When the time comes for engine replacement, next motor will be a Kubota, in Beta Marine guise, or perhaps even a Lister-Petter. We shall see. It will not be a Volvo.
.
erps likes this.

Last edited by Rockter; 04-05-2012 at 03:06 AM.
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Old 04-07-2012
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Re: MD7A Exhaust Manifold Port identification

Yea, the parts are expensive. I didn't need to replace the gasket so the cost of my overheating fix was just my time. Anyone know that the hole is that Sweeny points to with his shiny pick?
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Old 04-07-2012
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Re: MD7A Exhaust Manifold Port identification

Thanks for the responses I cleaned and bolted up the exhaust manifold. I'm still curious as to what the small hole is over the exhaust port is. The flow diagram shows three ports as harbin2 points out, so maybe the small port is for overheating (total guess). I bought the exhaust gasket for $41 and the smaller gasket tor rear pipe for $10 at Volvo Penta Parts & Accessories Marine Diesel & Gas Engine | Power Products Systems, LLC part #'s 859194 840902 respectively. The small water supply gaskets 3 needed are 829714. Gasket around thermostat 416033 and the o-ring 955989 totaled $15 if anyone needs these numbers. Hope to test it this week. Thanks for the help.
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Old 06-03-2012
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Re: MD7A Exhaust Manifold Port identification

I just removed my exhaust manifold on my md7b this weekend after it became clogged. The clog was in the hole feeding from the raw water under the thermostat. Reattached and started the engine. Water flowed out exhaust but about 1 minute later engine rpms slowed and white steam flowed out the exhaust. Killed engine immediately. I think the issue is that the the gasket needs replacing and a small amount of water dripped into one of the exhaust holes in the manifold. There was some small leaking between the manifold and block. Would you agree with this determination? I ordered a new gasket which should be in mid-week. Any other suggestions from those who have done this before, before I affix the new gasket? What would you suggest is a good method to cleanthe surfaces? I'm pretty far over my head at this point so any suggestions would be greatly appreciated. Thanks in advance
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Old 06-04-2012
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Re: MD7A Exhaust Manifold Port identification

Thanks for the parts link, didn't know about these guys.
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Old 06-07-2012
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Re: MD7A Exhaust Manifold Port identification

Quote:
Originally Posted by usa1136 View Post
I just removed my exhaust manifold on my md7b this weekend after it became clogged. The clog was in the hole feeding from the raw water under the thermostat. Reattached and started the engine. Water flowed out exhaust but about 1 minute later engine rpms slowed and white steam flowed out the exhaust. Killed engine immediately. I think the issue is that the the gasket needs replacing and a small amount of water dripped into one of the exhaust holes in the manifold. There was some small leaking between the manifold and block. Would you agree with this determination? I ordered a new gasket which should be in mid-week. Any other suggestions from those who have done this before, before I affix the new gasket? What would you suggest is a good method to cleanthe surfaces? I'm pretty far over my head at this point so any suggestions would be greatly appreciated. Thanks in advance
I had this exact same symptom on my MD7A and it was a faulty thermostat.
The small hole you cleared is a water bypass that allows a minimum water flow when the thermostat is closed. There is not enough water flowing through that orifice to cool the engine so if the thermostat doesn't open, the water starts to boil off, hence the steam. You definitely want to test that thermostat. just drop it in a pot of hot water and see if it opens. If it hasn't opened before the water approaches boil, replace it. (I believe it should be open at around 160F)

A matt knife (aka box cutter), or a razor blade is my favorite tool for cleaning off old gaskets. Just remember to scrape with the length of the blade, don't gouge with the point.

Last edited by SchockT; 06-07-2012 at 09:17 PM.
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