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DBboat 05-08-2008 10:51 AM

Heat Exchanger help
I am troubleshooting an overheating problem and need some advice on the heat exchanger and the exhaust elbow. I took my boat out for the first time and it started running hot within 10 minutes of leaving the marina. I was getting water out with the exhaust, however I think it should be more. I've looked at other posts and still have some questions. I've checked the thermostat and the impellors (both on the closed loop side and raw water side) and they are OK. Here's my plan so far:
-Dive over and scrape propeller blades and the raw water intake to remove "critters".
-Run a weak muriatic acid solution (or off the shelf product) into the raw water side of cooling system and flush.

Here is where I need the help. Is there a flush or cleaning procedure for the closed loop side of the heat exchanger? If it needs to come off, how is the exhaust elbow cleaned?

I have a Volvo-Penta MD11C. At the dock, I've had it running both at idle and in forward for >1hr with no overheating.

beej67 05-08-2008 11:56 AM

Before you go too far down that rabbit hole, take the hose between the mixing elbow and the exchanger off, and check it for carbon build up. Also check in the elbow itself to see if there's build up, particularly whether there's build up where the hose from the exchanger ties in. Boats with diesels who are run infrequently, and at low rpms instead of cruising rpms, tend to get carbon build up in these areas, which can impede raw water flow through the mixing elbow.

I have a yanmar, but I *think* the mixing elbow was off the previous engine (a volvo) and I have to take that hose off and scrape around in the elbow with a screwdriver about once a year.

If you do get to the point where you're disassembling your heat exchanger, you can probably see fairly easily down the raw-water side of it for any buildup, once the cover's off. Do a visual inspection before your acid wash. Try the simple stuff first.

erps 05-08-2008 11:57 AM

On my Volvo MD17C, the raw water runs through the reverse gear box first and that is a fairly common failure point for obstructions. I would also pull the end caps off the heat exchanger and look for obstructions and I would check for obstructions in the very beginning of the circut, your thru-hull inlet. Over the years with differnet boats, I've experienced some heating problems from each one of those areas, especially in the fall when we get a lot of loose eel grass floating around.

beej67 05-08-2008 12:00 PM

Also, I find these two statements a bit funny:


I took my boat out for the first time and it started running hot within 10 minutes of leaving the marina.

At the dock, I've had it running both at idle and in forward for >1hr with no overheating.
Can you elaborate on that? Sounds like there might be some clues in there. You ran the boat for an hour in forward, tied to the dock, and didn't overheat, but you ran the boat for ten minutes, in forward, while underway and it did overheat? I can't see how the load on the engine would be significantly different in those two cases. If anything, there'd be more resistance while you're tied up, as your boat tries to tow the dock.

erps 05-08-2008 12:02 PM

Might of sucked up a bag on the way out of the marina?

beej67 05-08-2008 12:07 PM

Maybe so. In that case, dive, check the thru hull on the outside, then shut it off and check the hose between the thru hull and the strainer.

Might be just a debris problem.

camaraderie 05-08-2008 12:13 PM

Since you've indicated a need to clean critters from the prop and bottom there is a good chance that you have a partially clogged intake thru hull that causes heating issues when more flow is demanded under power.

TrueBlue 05-08-2008 12:17 PM

That would be my assumption as well Cam. While underway, the OP most likely had the engine running at a higher RPM, than when idling in gear while tied at dock, thereby developing a higher engine temp and a need for increased coolant flow.

Freesail99 05-08-2008 12:54 PM

The transmission on the Volvo will cause over heating problems, it happened to me. There is a product that can be used to flush your system and remove any obstructions. I can't find the link but I will post it as soon as I do.

DBboat 05-08-2008 03:17 PM

To elaborate for you, beej67, I don't understand why it didn't heat up at the dock. I can only think of two reasons: 1. obstruction or 2. it wasn't loaded up as much as I thought it was. (No tach to verify.) Both have already been mentioned here.

The thorough cleaning is an attempt to get the boat back up to shape. I bought it used recently and it has sat for 4+ years. It was an accomplishment just breaking it loose and sailing.

I'm interested in that flush that Freesail mentioned. I've heard of Barnacle Buster and Salt Away but haven't heard if either work any better or worse than a weak acid solution.

Also (maybe this needs to be another post) there is no raw water screen between the thru hull and raw water impeller. Shouldn't there be one to catch debris coming in? Thanks for all the help!

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