Intermittent overheating problem

[Mike H.]

I get an overheating alarm on my 1994 Yanmar 3jh2e when I push it over 3000 rpm or when I reduce rpm to low after running the engine for a period of time.

I think the elbow is original. I boiled it out in acid about 150 hours ago. The engine has 750 hours now. Water flows out of the exhaust, but I can't say if it is enough.

Question, would you start the troubleshooting by replacing the elbow?

Thanks,

Mike

 

[JoeWhite]

I do not believe that the exhaust elbow will cause overheating. Overheating usually is caused by either insufficient fresh water or more likely there is air trapped in the fresh water system. It needs to be burbed. I am not familiar with this engine and do not know how to burb - Lets see if others know who.

 

[Ralph Johnstone]

Once the raw water passage in the elbow becomes plugged with corrosion products, the next step is for the engine to overheat ............... simply a case of nowhere for the waste heat to go. However, ater the acid bath, I'd think yours is still pretty clear.

All the theory behind it is nice, but doesn't do you a bit of good in solving the problem. Start simple and see if the mixing elbow is cool and/or if the water from the exhaust is cool. Should be comfortable to the touch. Careful here as although you may have lots of raw water flow, the heat exchanger may be scaled and insulating the heat transfer from the sea water to the fresh water.

Backing down on the RPM and causing an alarm makes it a little more interesting. If you don't have a temp. guage, invest in a cheap IR gun. Make sure you always shoot the same spot on the engine water pass . You're not interested so much in the exact temperature but in the amount of CHAGE of temperature as the engine changes RPM.

You MAY be looking at a faulty temp. switch, a scaled heat exchanger, a plugged heat exchanger, a pluged water pass in the elbow, a damaged raw water pump, or a fouled etcetera . Take your pick. Just work from the simplest (cheapest) to the more expensive.

 

[splax]

I think the most probable cause to be air trapped in the system or a damaged pump and/or impeller. The symptoms you describe suggest to me a vapor lock caused by a bad impeller, but there are many things that could combine to cause the indication of overheating.

 

[Steve Dion]

Mike: Have you checked the impeller? Check it out before the elbow.

I would agree that the elbow can cause overheating. Take a good look at your raw water intake along with the hoses. The elbow is a little pricey so I would leave it for last.

if you r/r the elbow be sure you use high temp never sieze on the threads.

 

[Bill Roosa]

the overheating at low RPM after running normaly is the key. The engine has residual heat that needs to be gotten rid of after running at operating temp. the reduced flow of raw water at low RPM is not removing that residual heat fast enough. since it runs OK at normal RPMs that would indicate a heat transfer problem aka the head exchanger need cleaning OR the thermostat is not opening fully and restricting the flow of anti to the heat exchanger.

 

[Jeff]

I had a high RPM over heating problem with my 30GMF a few years ago. Turned out to be a worn out cork gasket at the strainer bowl. Evidently air was being sucked in at high RPM's. I replaced the gasket and no more problems.

 

[Mike H.]

Thanks everyone.

Could it be old coolant? I heard that it needs to be replaced periodically.

Also, my transom has soot on it. Not much, but enough to notice. It was after a week with about 30 hours of motor sailing. Is that a clue?

 

[Mike H.]

Based on my past experience with this exact same problem and reading other blogs about overheating problems with yanmar 3jh2e engines and reading a horror story about ruining an engine caused by a clogged elbow, I'm going to pull my elbow and replace it first. Yanmar recommends replacing this elbow every 200 hours if in salt water.

Steve, your boat is in fresh water that would explain the long life of your elbow.

Wish me luck sailors!

 

[Ralph Johnstone]

and reading a horror story about ruining an engine caused by a clogged elbow, I'm going to pull my elbow and replace it first. Yanmar recommends replacing this elbow every 200 hours if in salt water.

You can find pretty much find any science fiction you want on the web and destroying an engine from a plugged elbow is one of them. This assumes that your temperature alarm is working properly.

You'll get all sorts of other indications of a plugged mixing elbow long before any damage occurs on the gas side. In fact, having a plugged mixing elbow may actually be good for your exhaust valves .............. it slightly reduces the pressure differential hence less wear across the seats and valves. Mind you, this gain is probably offset by a slight increase in exhaust gas temperature .

If you've ever seen the inside of a mixing elbow, it makes you wonder how ANYTHING gets through there.The below elbow brings to mind the process of giving birth (let's see if this gets censored). This mixing elbow was removed as a matter of routine maintenance when the engine was running perfectly. Shows how much these things have to plug up before they DO cause problems.

 

[Mike H.]

After talking with the parts guy at Svendsen's, I will check the water flow, impeller, heat exchanger and change the coolant first before removing the elbow.

I'll probably boil out the heat exchanger in muriatic acid if I find gunk in there.

Also will probably change to propylene glycol coolant.

That's what I'll be doing this weekend. Better pump out first.

 

[centerline]

I dont know the yanmar engine specifically, but in my opinion its not likely the sea water pump or it would continue to heat up and remain hot all the time, so its probably 1 of two things, either your engine water pump is bad, or the thermostat is not opening and closing fully... its not circulating well enough to carry the heat away as quickly as it needs to to keep the temperature spikes from occuring.

when running at high rpm, you build heat too quickly and the flow is so low that it cannot be carried away, and at an idle you dont have enough pressure being developed by the water pump to circulate any water at all... but at the right rpm there is a balance between the heat building and the flow of water making its way to the heat exchanger so it can return to cool the engine.

a restricted sea water system could cause the spikes, but after heating up it would take an unusually long time to cool down.... from your description it leads me to believe the engine water pump is the culprit.

 

[Mike H.]

Update:

I drained the coolant, it was brownish green with a lot of crap in it. Pulled the heat exchanger and elbow. The elbow was corroded, but not has bad as three years ago when I boiled it out last time. The heat exchanger had growth in 5 of the tubes that had completely closed the tubes off. There was growth on the inside of the front cap. Inside the exchanger around the tubes, there was a lot of sludge coating the tubes. I also found some red confetti inside the exchanger, probably from 4th of july or something.

I boiled the exchanger, caps and elbow in two gallons of muriatic acid. Looked like an evil scientist with fizzing and smoking right there on the dock. Rinsed everything off, reassembled with new 0 rings on the exchanger and flushed out the system with preston radiator flush for about an hour at 2000 rpm. NOTE, (Used my dinghy foot pump to push the coolant and flush out of the hoses and water heater plumbing. Worked great.)

Drained the flush, including water heater, and refilled with almost 2 gallons of the red Yanmar coolant.

Ran engine in gear about 2700 RPM for about an hour and no alarms!

True test will be when we go out again