MD7A: Fresh water to raw water?

xawdis

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Oct 14, 2022
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My new to me md7a has overheating problems.
This engine is an aftermarket-converted freshwater engine. The raw-water circuit has issues from barnacles on thr intake. that i can fix, and sludge in the exhaust pipe i ready cleared. But when I still noticed overheating even with a stream of water out the back, I checked ant ther is little coolant in the regime, and it is not pulling coolant from the reservoir. Yup, that means a leak, and I found the problem. It seems the is a 5/8 inch hose connecting from a 1/2 in stub st the heat exchanger to a 3/4 stub at the engine. It leaks at both ends. One solution would be to add an adapter and put in the proper sized hoses, and the replace coolant and burp the engine, but is there any reason I should not just bypass the heat exchanger and just convert yo the original raw water system. It seems it eould be much simpler.
I understand that fresh water systems are supposed to run hotter and cleaner, but this engine is already 40 yrs old.
Of course I do not know when the fresh water conversion was done, but i do not see any svmcale in the thermostat housing.
Any comments? The buildup of sludge in the exhaust does bother me, too.
 

alldodge

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Mar 8, 2009
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In most cases there has always been issues changing over the fresh after motor has been run on raw for a while. Would need to see/understand how hosing is done/routed to provide comment.

With things build up it might be easier to go back, I don't know
 

xawdis

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Yeah, I understand the issues with going to fresh after it has been running on salt water, and the conversion was clearly an aftermarket project... I had not seen a freshwater cooled MD7A before.

I believe that by bypassing the heat exchanger I could get water through the whole engine and send it out into the exhaust just after the thermostat, where it now goes to the heat exchanger. My thought is it is a simpler setup... just 1 impeller, etc, and how much damage could salt water do to the 40-yr old engine which was designed for raw water?
And that would attach 1/2 inch to 1/2 inch and 3/4 to 3/4.

The major difficulty i would have with keeping the current setup with an adapter between the different sized hoses would be concern with being able to burp the fresh water engine after adding coolant.
 

alldodge

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There is a method of drawing a vacuum on the cooling system and then filling. This "should" make sure to get rid of any air

 

xawdis

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I had thought of making a vacuum pump from an old gas cap and a siphon pump, but since the fresh-water setup seems to be an aftermarket kluge, and the MD7A was intended as a raw water engine, I would like to make it back to a raw-water engine. One issue I have with that is to make sure the water gets all the way through the engine. So... question...

I would be bypassing the heat exchanger, and want to make sure that in the end the water will flow through the engine and not bypass it. There are two hoses coming between the heat exchanger and the engine. One connects at the bottom of the engine near the water pump, and one at the thermostat housing. So, the important question is, which of these connections goes to the water intake, and which to the exhaust elbow?
Since the temp sensor is at the thermostat housing that that is the heated water that should go to the exhaust, but it seems to me I have seen the opposite.
 

alldodge

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Would need to see what you have, but water comes from the raw pump and then to the thermostat housing or motor circulating water pump. The Circulating pump circulates water thru the block. The rest of the water goes out the exhaust

When temp is reached the thermostat opens and warmer water is pushed out by the cooler water coming into Cir pump from raw water pump.

Go to this site, select the "click here to access new site", then select Engine and drive tab, then enter your motor and select it. Once the screen fills with items select function group and find cooling system

https://www.volvopentashop.com
 

xawdis

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Oct 14, 2022
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Thanks, the volvopentashop site was useful, and it does show that the water flows into the water pump first and out through the thermostat into the exhaust. Actually, the 2 lines from the heat exchanger run to the rear gear and the thermostat. The volvo site does not show the rear gear, but the intake goes to the water pump, and after the water runs through the engine it comes out from the thermostat housing to the exhaust.
This gives me confidence that it will be simple to go back to raw-water cooling, since om my engine, the fittings from the intake and the rear gear match (3/4 inch), as do the fittings at the thermostat and the exhaust (5/8 inch).
 
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