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post #1 of 5 Old 07-25-2013 Thread Starter
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Calling all Engine Experts....

Ok, looking for an engine differential diagnosis.

1985 Yanmar 2GM20F

Motoring out into channel, kick it up to 2400 RPMs at my usual spot, throttle goes all way forward, RPMs decreasing instead, oops, swing boat around with lots of traffic, yell at wife and sister-in-law to break out dock lines (sister-in-law can't tie a cleat hitch), barely make dead stick landing at end of closest finger pier before engine dies. Make visual inspection, no water or diesel leakage, try and start engine again, starts, then dies about 5 seconds, no smoke, but notice what looks like a carbon or exhaust sheen slowly coming from raw water exhaust. Call BoatUS, use unlimited towing back to slip.

Tow boat guy, old salt, says it's definitely a fuel problem.

Wait 2 weeks, then go and change both fuel filters, notice beautiful diesel and no sludge in either primary or secondary. Use bleed screw on secondary with hand pump, get good flow and pump until no bubbles. Do not bleed anywhere else, for example, at the fuel injectors or high pressure line. But I have only bled at secondary filter with hand pump and have never had any issues with air in lines. Try and crank engine, it cranks, then immediately dies, again, with a little of that carbon sheen coming out the exhaust at the transom. I put my finger in it, definitely not oil, like exhaust carbon in water.

Other things, maybe related or not.

Mixing elbow extremely rusted on outside. When I bought boat 5 years ago, mechanic who did work on engine said that elbow worried him, he would change it given how rusted it was on outside. Also, I have very poor access to the back of my engine, but leaned in and noticed that hoses leading to mixing elbow are also deteriorating.

Fuel: I usually keep tank at 3/4 to full and use biocide. This year I let tank get to 1/2 and have had some bumpy rides, but again, no sludge in filters.

Black spray: I have noticed back in my engine housing, under and around the mixing elbow, a small amount of black spray on the bilge paint.

Fresh water hose: while doing an examination, I noticed that the fresh water line was resting on top of the engine block and gradually making a hole in the rubber. I don't have any fresh water or coolant leakage yet and not overheating or loss of coolant, but I will have to re-route or secure regardless.

So what do you all think?

s/v Romaniţa
Deale, MD
Chesapeake Bay

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post #2 of 5 Old 07-25-2013
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Calling all Engine Experts....

I wouldn't call myself an expert but I work on my own diesel and like the challenge of troubleshooting. I consulted my copy of "Marine Diesel Engines" by Calder. Fuel flow or quality issues are definitely high on the list. With fuel out of the way, excessive back pressure in the exhaust seems likely given your description of your exhaust elbow. The elbow could be clogged or failing. I suppose excessive carbon buildup in the elbow or exhaust hose could also be blocking and causing excessive back pressure. The sheen maybe unburned fuel in the exhaust or carbon residue from the build up.
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post #3 of 5 Old 07-25-2013
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Re: Calling all Engine Experts....

I'm guessing that you've got several hours of troubleshooting ahead of you.

Things that I'd check first:

1. Mixing elbow. Like Eng said, a coked up elbow will choke out your engine. The black spray could very well be a strong indicator that you've got a serious blockage and/or an impending failure. Given your surveyor's thoughts, I'd lean heavily towards replacing it. While you've got the exhaust torn down, also check your waterlift muffler if there's one installed.

2. Tanks and fuel pick-up tube. Sometimes these suck a piece of sediment up tight against the tube opening, enough to restrict the flow. Once the engine dies (and the fuel pump stops sucking), the clog drops back to the bottom of the tank and allows fuel to flow freely until the next time the suction develops.

3. All fuel hoses and connections. As Principal Mackey would say "Any air getting into the system is bad, OK?"

If it ain't one of these three, then deeper digging is needed.

As for your plumbing runs -- it sounds like you are looking for encouragement to drop some green to replace; that wouldn't be a bad thing to do.

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post #4 of 5 Old 07-26-2013
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Re: Calling all Engine Experts....

Our 2 boats had this problem.....We pulled the pick up which was clogged. We also replaced fuel filters. Would recommend pulling the fuel tank and cleaning it then replace pick up, fuel lines and filters. Both Diesels run really well now. The reason I say replace fuel lines is: we had cleaned the tank and replaced pick up and filters and it died on us again. So we replaced the filters and was running great again. However in the filter there were little pieces of rubber which could only be from the fuel we replaced the lines. Haven't had any problems this year. I am a big believer in clean fuel.
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post #5 of 5 Old 07-26-2013
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Re: Calling all Engine Experts....

BTDT. Blocked injection nozzle in the mixing elbow. The 2GM mixing elbows seem to fail at roughly 500 hour intervals. You can pull it off, clean it out (somewhat) with a rat tail file and further clean it by soaking it in a mild acid solution to get a bit more use out of it but you'd be better off replacing it.


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