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MD21B Cooling problems Is there a sea water filter

L

laurelturbo

New member
"Hi,

My MD21B has trouble w

"Hi,

My MD21B has trouble with cooling at above 3800RPMs. I started some research on it, and found out that my first suspect would be the filter on the sea water going to the heat exchanger.. The problem is, I cant find it.

Heres how it works: Sea water goes through the valve in the bottom of the boat and in on the port side of the transmission. Then on the other side of the transmission it comes back out and goes directly into the oil cooler, and from that to the pump and heat exchanger.

Now, correct me if Im wrong, but somewhere along this route there MUST be a filter? The MD21A has them just before the oil cooler, but on MD21B theres just a pipe to the transmission..

Any ideas?"
 
"The 21B has a tube cooler opp

"The 21B has a tube cooler opposite the 21A that has a plate cooler. 21B does not have a factory installed strainer. To 'clean' the raw water part of the cooler the end covers have to be removed. If it is a sudden incident, it may be a partly blocked cooler, defective impeller or perhaps a blocked water inlet.
Any reduction in water going out with the exhaust?"
 
"Hi,

I bought the boat rece

"Hi,

I bought the boat recently, and the previous owner claims never to have crossed 3000. Theres no problems before ~3800+.

Are you saying theres no filter at all, that the oil cooler is the closest thing to a filter Ive got? Or is it the heat exchanger I need to clean out?

Thanks!"
 
"You probably have no strainer

"You probably have no strainer, unless the boat builder has put in one.
Whatever manage to pass through the water-intake, gearbox and oil-cooler, will even manage to pass through the heat exchanger.
Considering the age of the engine, start by checking the impeller, then the exhaust outlet on the engine. The water outlet might be blocked.
Then you have the possibility of dirty boat bottom/water intake, partly blocked oil-cooler and heat exchanger."
 
"Ok,

Thanks a lot, Ill have

"Ok,

Thanks a lot, Ill have a look at that! By the way, the water passes through the exhaust manifold, then into a hose on the exhaust elbow that goes to the outlet seperate from the exhaust. Im not shure where water enters the exhaust itselfe for cooling... Any clues?"
 
"Not all the water goes out th

"Not all the water goes out through the hose from the exhaust outlet. A lot goes goes down and mixes with the exhaust. If that outlet is blocked, not enough water will pass out from the engine cooling system."
 
"So the water from the heat ex

"So the water from the heat exchanger goes partially through the hose and out on the side, and partially into the exhaust?

Edit: Would it not be smart to mount a filter?"
 
"Robert

I was having a sim

"Robert

I was having a similar problem with my AD41 deasel but at 2000 rpm.
Changed impeller, belts, thermostats, clean fresh water filter and topped off anti freeze to no avail.

Took it to a mechanic who found a couple of small leaks. He also removed and cleaned the heat exchanger but found it was clear and clean. At the suggestion of a fellow poster he used a IR gun to test tempertures at various points along the cooling chain and engine. Turns out the exhaust elbow (heat riser) is the culprit with scaling. Temperture at the exchanger was 185 which would build quickly at high r's. Previously I could run at 1500 r's all day long, but even that started to change as the blockage got worse. I'd say try the IR gun for starters just to save the time of taking all the other items apart or replacing them like I did it's cheaper. By the way cost of new riser in my case is about 350.00 which looks like it might be worth if to avoid a repeat of the problem next season.

Hope this helps either now or in the future.

Chip"
 
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