Strange Theory about Engine Overheating

[Moody46CC]

I have a Mastervolt 6 Genset. Sits in a plastic, insulated case above the boat engine. It has a Mastervolt exhaust gas separator, a device which separates the exhaust gases from the exhaust water. The separator is downstream from the exhaust mixing elbow. The gases are vented out the back of the boat through an opening in the transom, while the water is discharged via a thru-hull under the boat. The purpose of this is quiet running, i.e., no above waterline water discharge to make noise.

The engine for the Genset is a two cylinder Mitsubishi diesel. The engine overheats intermittently. The Genset is 2000 vintage with 258 hours on it. Sometimes it runs under load w/o any problems; sometimes it shuts down after running with a load for 45 minutes; sometimes it shuts down right after start-up. The discharge of the water under the boat makes it impossible to determine the cooling water flow. The usual suspect is the impeller, but it is fine. All vanes and ribs intact; no missing pieces. I changed it last summer. Looked at it a week ago. The engine has two heat sensors, one for the coolant and another for the exhaust discharge. There is also an oil pressure sensor, and any of these three can shut down the engine. I think they all work—as well as the thermostat. I say that because I have run the engine w/o the thermostat, and it has over heated. I have had a heat gun on the engine, and when it does shut down, it has really overheated. Not a false alarm. The coolant temperature sensor is set to shut it down at 220-234F, and it does. Heat gun readings confirm it. One can also see air bubbles running from the coolant tank in a clear tube to the coolant expansion tank. Caused by boiling, I assume. Plenty of oil. Plenty of water coming in via the intake seacock and strainer. The strainer is a clear Vetus model, and I can see the water flowing.

One theory is broken impeller pieces floating around in the cooling jacket. I am contacting the boat’s former owner—I bought the boat last year and have not had much occasion to use the genset. Haven’t figured out how to get to them if it is broken pieces of impeller, and I’m not sure I can check for scale as the heat exchanger is under the engine and generator. I’d have to remove the Genset from its plastic container, which means the whole thing has to come out. And one would think that were the problem scale, it would always overheat--nothing intermittent.


A mechanic suggested another theory. Both the intake and discharge seacocks are located next to each other on the bottom (hull). The mechanic thought that if the gas-water separator were malfunctioning, it might be discharging bubbles (of air and gas) along with water under the boat and next to the intake opening. The bubbles get sucked in (intermittently owing to current at anchor or in a marina) and air gets into the cooling system. I have to add, the one time I ran the generator while underway (and with a full load on it), no issues. Ran it for two hours and was doing 6.5 knots.

Does this explanation (about the seacocks being too close) make any sense to anyone? Can always crank it up and get a diver to look and see if there are any bubbles.

[Auspicious]

I have the same generator (2006 Mastervolt 6) with the same gas/water separator. I have more severe overheating problems than you describe.

Of course there is no guarantee we have the same underlying problem.

I have pulled replaced all the sensors to no effect.

I pulled the water outlet from the thru-hull and run it into a big bucket -- I'm getting expected water through the system.

My next experiment is to run Barnacle Buster through to clean the heat exchanger.

I'm in Galesville MD, not too far North of you. Thursday next week is my next big attempt working on the generator.

[Moody46CC]

I'm all ears to your experiment with barnacle buster, first as to how you will do it, and second, if it works.


Contact me via private e-mail and let me know if it works.

michaelbarrett12@att.net

[socaldmax]

Is there any way to monitor your intake water temp. and the outlet water temp?

This is way out in left field, but I've seen car engines overheat when their thermostats were removed, causing coolant to flow too quickly and not absorb enough heat from the engine block.

I'm not familiar with these, is this a raw water cooled engine?

[ericoh88]

FYI

1. I have an entec genset and the mechanic did a top head overhaul.

Found bits of old impeller fin inside the fresh water hose stuck at the pipe for the heat exchanger. Happened to find this bit this round because I wanted to replace all the old hardened hoses.

2. A diver was cleaning the bottom of boat, said the intake was partially covered with barnacles.

3. Saw a video a few holes in the engine block for salt water cooling was completely clogged with salt.

4. Old hardened impeller may loose some ability to suck and move water.

5. Not sure if you mentioned the heat gun confirmed the shut down due to over heat. My salt water and Gresham water use 2 Johnson pump with the same shaft and separated by a seal. That seal leaked and let salt water into the fresh water side and corroded the thermostat.

6. Is it possible to route the exhaust gas/water mix straight to existing outlet above the water line by pass the water /gas separator ? You will hear the noise of water coming out though .

Hope you able to tackle the issue, I know how frustrating to have the genset working and not reliable.

[GILow]

Have you checked oil for traces of water? This has a slight headgasket failure feel to it to me. Particularly the bubbles heading to the cooling tank.

As for the separation of the inlet and outlet, a simple elimination of this variable might be to bung a bit of hose about 6 inches long in either the inlet or outlet, and see if the temporary separation makes a difference. (Depending on the water temperature where you are at the moment that will be a fun or miserable task.)


Matt

[Cheechako]

The temp or flow sensors on gen sets are often problematic right from the get go. Does it perform fine if you bypass the sensors?

[Moody46CC]

I had a nice long answer, but in proofing it, sent it to electronic infinity. Wonderful!

Let me try to answer.

Cheechako: planning on replacing the sensors, but I can try to by-pass them. I suppose the way to do it is to remove their leads.

Gilow: have not noticed any oil in water. Oil looks normal. I am going to have an oil analysis done in fall for both the genset engine and the main engine. Was going to do it last fall and just forgot. The water temp in Va is fine for diving under the boat and trying to insert a length of hose in one of the seacocks. But the jellyfish are not fine. Got to play that one by ear this coming weekend.

Ericho: I did use a heat gun and the engine really did overheat--over 230F at thermostat and 180+ at exhaust elbow. I do suspect bits of impeller, and I can easily pull the impeller to see if it has hardened. I like the idea of testing the discharge flow by routing the discharge to an above the waterline outlet. I might be able to do that one--the nearest is 10" away, but it might be possible

Socaldmax: my mechanic mentioned your theory about the thermostat, but it has overheated with it in and with it out. I am going to replace it. I can take the temperature at the inlet side (thermostat housing or just before it) and at the exhaust elbow just below the valve where the water is injected into the gases.

Some food for thought to share with my mechanic, and I appreciate it.

[sailorchic34]

OK basic engine trouble shooting 101. Air bubbles in the overflow coolant tank is caused by a blown heat gasket. Classic sign of a blown head gasket. As is engine overheating which can be caused quite easily by the same combustion gases heating the coolant. Both are classic signs of a blown head gasket.

Oil may or may not be in the water, same goes for water, milk colored in the oil, it may or may not happen with a blown head gasket.

You can also have a blown head gasket without bubble in the overflow tank. But bubbles in the overflow is 100% the sign of a blown head gasket. Coupled with overheating and and its a classic textbook case. Surprised the mechanic did not know that.

Bubbles could also be caused by a cracked block, but lets not go there. Check the head gasket.

[Ex-Calif]

Originally Posted by sailorchic34

OK basic engine trouble shooting 101. Air bubbles in the overflow coolant tank is caused by a blown heat gasket. Classic sign of a blown head gasket. As is engine overheating which can be caused quite easily by the same combustion gases heating the coolant. Both are classic signs of a blown head gasket.

Oil may or may not be in the water, same goes for water, milk colored in the oil, it may or may not happen with a blown head gasket.

You can also have a blown head gasket without bubble in the overflow tank. But bubbles in the overflow is 100% the sign of a blown head gasket. Coupled with overheating and and its a classic textbook case. Surprised the mechanic did not know that.

Bubbles could also be caused by a cracked block, but lets not go there. Check the head gasket.

+1 -

Unless...

The water is being discharged and when engine cools rather than drawing the coolant back into he engine block air is entering via a bad water cap.

Inspect / change the water cap first.

What happens to coolant level during the overheating events? Are you losing coolant?

If not SCs scenario prevails. Coolant should not have bubbles in it...

[sailorchic34]

Originally Posted by Ex-Calif

+1 -

Unless...

The water is being discharged and when engine cools rather than drawing the coolant back into he engine block air is entering via a bad water cap.

Well that is true.

But a leaking cap, would not cause the engine to overheat, not if everything else is working ok. Plus it would stop blowing bubbles after a short time, after the engine was up to temp.

Where as a blown head gasket can cause overheating and continuous bubbles.

I've a dollar that says its a head gasket.

[socaldmax]

Originally Posted by sailorchic34

Well that is true.

But a leaking cap, would not cause the engine to overheat, not if everything else is working ok. Plus it would stop blowing bubbles after a short time, after the engine was up to temp.

Where as a blown head gasket can cause overheating and continuous bubbles.

I've a dollar that says its a head gasket.

I'll throw in a dollar next to hers. I think she's right. Somehow I completely missed the bubbles in the first post.

[Moody46CC]

I did put a new radiator cap on the coolant tank--not the expansion tank, but the coolant tank on the top of the engine block.

When it overheats, one can hear the boiling and see bubbles of air going from the coolant tank to the expansion tank. The latter is connected with clear tubing. And no, the actual coolant amount has not dropped. I put a tick mark on the plastic expansion tank, and when things settle down after overheating the coolant fluid level returns to the tick mark.

[Moody46CC]

On the blown head gasket theory. Makes sense, but what about the fact that the behavior (overheating) is intermittent?

Thanks for all this information.

[HG02]

Originally Posted by sailorchic34

Well that is true.

But a leaking cap, would not cause the engine to overheat, not if everything else is working ok. Plus it would stop blowing bubbles after a short time, after the engine was up to temp.

Where as a blown head gasket can cause overheating and continuous bubbles.

I've a dollar that says its a head gasket.

From an old diesel fitter she would be spot on