Overheating MD7A

[John Tier]

I have a 1982 CD 28 with MD7A that is overheating (raw water cooled). Last season I replaced the water pump, and every year I replace the impeller. In the spring I flush out the engine. I am now going to flush out the engine with muriatic acid to see if this will unclog something. There appears to still be a blockage somewhere, as I am pumping water out the exhaust, but it looks like the flow should be heavier. What are the chances that the exhaust manifold is shot/clogged ? Any help would be appreciated before I start to take the manifold, muffler, etc. apart.

Thanks,
John



tierj@novell.com

[Steve I.]

I have a 1982 CD 28 with MD7A that is overheating (raw water cooled). Last season I replaced the water pump, and every year I replace the impeller. In the spring I flush out the engine. I am now going to flush out the engine with muriatic acid to see if this will unclog something. There appears to still be a blockage somewhere, as I am pumping water out the exhaust, but it looks like the flow should be heavier. What are the chances that the exhaust manifold is shot/clogged ? Any help would be appreciated before I start to take the manifold, muffler, etc. apart.

Thanks,
John

I just had the same problem, it turned out to be the mixing elbow on the exhaust.It was almost blocked,at least enough to not allow enough exhaust to exit which will cause overheating. ( like putting a potato in your car exhaust pipe ) You will also loose power if blockage exists. Good luck !!!!



chiping@tcol.net

[Bill]

I have a 1GM and it too had a mysterous overheating problem for almost 2 years. i checked the mixing elbow, thermostat, impeller and strainer all to no avail. I finally did two things at once. I replaced the water pump, and in doing so, I found a "TINY" leak in the intake hose to the water pump. I dont know if it was the water pump or the pin holes, but it has not overheated since then...I cant imagine the leak causing a "vapor lock", but I dont know. Just check for leaks to!

Bill
S/V <a href="http://www.rhapsodysails.com/sailing.html">Rhapsody</A>

I have a 1982 CD 28 with MD7A that is overheating (raw water cooled). Last season I replaced the water pump, and every year I replace the impeller. In the spring I flush out the engine. I am now going to flush out the engine with muriatic acid to see if this will unclog something. There appears to still be a blockage somewhere, as I am pumping water out the exhaust, but it looks like the flow should be heavier. What are the chances that the exhaust manifold is shot/clogged ? Any help would be appreciated before I start to take the manifold, muffler, etc. apart.

Thanks,
John

cd25d@rhapsodysails.com

[Ken Cave]

If anything, check out and replace the thermostat! I have talked to lots of folks that have problems with their engine, and none of them have ever checked the thermostat!

The former owner of my CD 28 (with the Volvo MD7A) even rebuilt the top end of the motor-but never checked the thermostat!

When I purchased it, I found that the thermostat was stuck open-and thus-always running a cold engine. The thermostat can also get stuck closed!

My local NAPA auto store found me a thermostat for about $17.00 that was made for the motor-and works like a charm!!

Ken Cave
Dragon Tale



bcavd@whidbey.net

[Mark Yashinsky]

If you are going to change the thermostat, you might want to reverse flush teh engine (w/ the thermostat out). There have been others w/ seawater cooled engines that have had cooling problmes because of organic stuff (seaweeds, grass, fish parts) that have been picked up (you would be surprised) and have plugged passages. See what comes out. Freshwater cooled have had similiar problmes of this happening to the heat exchanger (seawater passages). You could also have a corrsion buildup on the passages that is preventing good heat tranfer to the raw water. But be careful w/ the acid rinse, as this could start the engine self-destructing, by loosing up the corrosion and starting internal leaks.

[Steve Alarcon]

John,

Consideration of the conditions under which the motor overheats will help in troubleshooting. For example, if it runs normal at cruising RPM (say, 1800 or so) but over heats at higher RPM, then you likely have some blockage that restricts cooling flow. In that situation I would look to the item already mentioned, but also take a look at the strainer mounted at the intake on the hull. Those little holes are notoriously easy to plug up with bottom paint!

Steve Alarcon
CD36 Tenacity
Seattle



alarcon3@prodigy.net

[Larry DeMers]

All of the suggestions written here are good ones. I would only add that impellor blades that have broken off in the past are still rotating through the engine. I had a blockage for years, overheating when we were at 2100 rpm or higher..on Lk. Superior yet. Well, we tore the engine out and rebuilt her for other reasons, and found 5 impellor blades in the cooling passages..by reverse flushing with the thermostat out.

Good Luck,

Larry DeMers
s/v DeLaMer
Cape Dory 30 Sailing Lake Superior

I have a 1982 CD 28 with MD7A that is overheating (raw water cooled). Last season I replaced the water pump, and every year I replace the impeller. In the spring I flush out the engine. I am now going to flush out the engine with muriatic acid to see if this will unclog something. There appears to still be a blockage somewhere, as I am pumping water out the exhaust, but it looks like the flow should be heavier. What are the chances that the exhaust manifold is shot/clogged ? Any help would be appreciated before I start to take the manifold, muffler, etc. apart.

Thanks,
John

demers@sgi.com

[denny]

I know this is a late post but maybe someone may check the history log. The MD7A is a very good engine. But, saltwater will take its toll over the years.I had a similar problem my MD7A when I bought my boat (CD28) back in 95. I ended up pulling the head, exhaust manifold, etc. (no big deal) gave it to a local engine rebuild shop where I had it acid washed and cleaned and I repainted both. The block I rodded out the best I could with wires and a vacuum with small tubes hoses. Yes everything was a mess from the years of neglect. I also most imporantly cleaned the cooling passage throught the transmission (very important)using stiff wires and brushes and flushes. I removed and cleaned all tubes and rebuilt the water pump and replaced the thermostat. There is a lot gunk, but everything needs to be clean.

A lot of work but no big deal. After everything was back together I had no more over heating problem.

But, TO ensure the system stayed clean, I hooked up the port fresh water tank to the cooling inlet at the seawater strainer through a tee and bronze ball valve, 1/2" I think. This tank is 30 gallons. Now every time I come in from a day sail I open the valve to the fresh water tank and close the seawater seacock and flush the engine with fresh water for about 15 minutes. I usually get two flushes before refilling the tank. It makes a big difference. Everything stays clean. I keep a close watch on the tank when flushing so as not to run it dry and fill it at my convenience.

I check my system every year and replace thermostat and water pump impeller as needed, but there is no doubt that these items last twice as long or more. Anyway this works for me.





dfike@awod.com