Overheating Engine

Hermit Scott · May 2, 2010

You should take the exchanger off and clean it. Take it to a shop that does it.

higgs · May 2, 2010

Get a new sending unit.

BobM · May 2, 2010

Get a cheap IR thermometer to figure out what the temps really are. Is the engine running up to normal operating RPM no problem? If it is, then the exhaust elbow is probably okay. You could flush the raw water side with some acid to clean it out.

Stu Jackson · May 2, 2010

Keep it simple. Most HXs I've seen don't have clogged tubes unless they have been horribly neglected. The main cause of insufficient cooling is the inlet port. You're in salt or brackish water, right? Take the HX off. Look in the ports. Often sediment (salt) drops out there and blocks the port. My old 2 inch HX was notorious for this, as is my newer 3 inch HX. The tubes have always been just fine because the velocity of the water through the tubes keeps them pretty well cleaned out from use (assumes you use your boat more than once a season!).

You alos don't have to take it to a radiator shop. Put some LymeAway in a bucket and run the engine for a bit.

RichH · May 2, 2010

The logical and time tested approach for analyzing heat exchangers is:

1. Get the engine specs for WATER OUTPUT AT EXHAUST OUTLET ... this will be specified as gallons/liter per minute ... PER SPECIFIED ENGINE RPM.

2. Run the engine at the spec. rpm and MEASURE (stopwatch and bucket) the output flow.

3. If the output flow is measurably LESS then spec. then check the whole raw water circuit for blockage, slipping/bypassing impeller vanes ..... restriction. This includes slab rust formation INSIDE the exhaust manifold.
3a. Start with the raw pump discharge DISCONNECTED, quickly run up to spec rpm and MEASURE the flow, quickly stop the engine. If OK, then reassemble and start breaking into 'downstream' connections until the blockage, etc. is found; IF NOT OK then start checking UPSTREAM of the pump (inlet strainer, etc.).

4. If the flow from EXHAUST outlet is OK. Then check the Hx for fouling and/or use a commercial 'boiler descaler' (RydLyme, etc.) to remove the FOULING from the raw water side. Pump the Rydlyme, etc. through the entire raw water circuit - from the raw pump to the injection nozzle on the exhaust system). If the exhaust manifold is included with the raw water circuit, then also descale the exhaust manifold. (Note: cast iron exhaust manifolds are notorious for forming 'slab rust' in their cooling passages --- thus partly blocking the 'flow').

For a heat exchanger to operate correctly it must have
1. **Correct Volumetric flow of (raw) water**. - see your engine specs.
2. The ht. transfer surfaces should be 'relatively' clean.

The Hx tubes when originally calculated should have included partly fouled tubes ( Coef.U - fouled ) so they dont have to anally cleaned ... just free of 'most' scale.

Be logical in your pursuit of the problem ..... .

Steve Dion · May 2, 2010

I do not know much about this engine, but overheating is common on the Yanmars when the mixing elbow starts to plug up.

Bill Roosa · May 3, 2010

thermostat

If the heat exchanger and flow check out you might want to check out the fresh water side. If the thermostat is sticking or inop it will cause the engine to either overheat or run too cold. Also check the fresh water pump and pulley. The fresh water pump almost never give you a problem however. The leak more than they fail as a pump. As long as the fresh water pump belt is reasonably tight it should pump enough water. The alternator usually shows signs of slippage before the water pump if they are the same belt.

Groundhog246 · May 3, 2010

You mention a reduction in flow out the exhaust and flow from the raw water pump as ok. Pull and check/clean the injector at the mixing elbow of the exhaust. Calcium buildup there is common in salt water due to the heat.

jake9920 · May 3, 2010

heat exchanger

Have you taken the side covers off the ends of the heat exchanger and look inside you may be supprised.

Fred T · May 3, 2010

Just one addition to the previous advice. Have you checked the thermostat itself? It can be tested by placing it in a pot of water with a thermometer; then, gradually heat the water and record the temperature at which the thermostat opens. If out of spec, get a new thermostat.

Also, some engines require a special thermostat that creates some back pressure to assure that coolant follows the proper channels within the engine. Using an automotive-type thermostat, while cheaper, may cause problems in those engines.

Fred T

Ron Wiggins · May 3, 2010

Check the anti-freeze

Is the anti-freeze mixture correct for your engine? The specific heat of pure anti-freeze is less than pure water. If you have pure anti-freeze (or higher than 50-50 mix) in the fresh water side it might not be capable of removing enough heat from the engine. To check this, remove the anti-freeze and put in pure water. If it still overheats, this wasn't the problem. If it does not overheat with pure water, properly dispose of the old anti-freeze and install the correct anti-freeze mix.

Brian D · May 3, 2010

A friend of mine has a Yamaha 33 and he uses Salt Away after every run of the engine. The reason? He had to have major work done on his cooling system because he neglected to flush out his system. My Yamaha outboard had the same heat problem. So I back flush after every outing if I can. Try running some Salt Away and some CLR diluted in water through the cooling system to flush. That might help. Good luck.

Seajay · May 3, 2010

Check the intake through hull pipe. It sometimes gets clogged before the filter. Need to mtake the hose off the thru hull (closed) , take a 3/8 or 1/4 rathet extension ( or coat hangar), put throiught the thru hull (open) and withrdraw it after rubbbing it around the valve to clear it. Sometimes weeds will get in there there, but due to the right angle turn to the filter, they clog up the pipe. Will take on just a little bit of water, not enough to worry about.
Jim

Plumper II · May 3, 2010

I had a similar problem but it was an air lock on the freshwater side.

John · May 3, 2010

I had a similar problem with my engine when we first got our boat. Knowing even less than I do now, I assumed it was the heat exchanger. I got a gallon of Rydlyme and filled the hx with it and let it sit for a day and then flushed it out. I also changed the thermostat at that time. That fixed the problem. I figure if I'd done all the tests that are advised above, I'd have put in twice the amount of time. (Incidentally, Rydlyme is a very good product; I even used some to clean our toilet bowl at home. And it's not supposed to be as toxic as many other products. I have no interest in the company, by the way.)

McQueen Smith · May 4, 2010

Check for air in the closed system. Air rises to the top, creates a bubble, and one the temp hits 212 degrees, becomes steam and super heats. Just a thought, since this was the recent problem on a neighbors boat, from relocating his overflow system. He simply bled the highest point in the system of air.

Deadline · May 4, 2010

You would never think of this...

Given a choice between the paper gasket and the O ring to reseal the waterpump cap after replacing the impeller, I opted for both.

The result was an air leak so imperceptible that water did not leak out of the pump. But the air leak lead to an air lock and blew apart the exhaust hose. It was spectacular.

Took the Yanmar mechanic four hours to deduce.

Stu Jackson · May 4, 2010

How to BURP (M25 series engines)

McQueen Smith said:
Check for air in the closed system. Air rises to the top, creates a bubble, and one the temp hits 212 degrees, becomes steam and super heats.

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