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Old 06-02-2010, 22:35   #1
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Yanmar 3GM35F Intermittent Out-of-Control Revving

First of all I typically butcher our engines model number so I apologize if it is wrong. If you know Yanmars you get the picture. It is the 3 cylinder fresh water cooled model. Secondly, we have been having some weird issues with it. The other day when leaving our anchorage the motor revved up it's rpms with no prompting from me whatsoever. A way stronger than usual exhaust smell followed.

We immediately throttled back and shut the engine down and did the usual engine trauma drill, before firing it up again and running it around 1500rpm (nice and easy, normal is around 1800rpm) for the rest of the day without a problem. We suspected maybe kelp in the prop made the engine rev as it tried to work against it, we also suspected that maybe the engine had slipped out of gear.

We are currently doing the Baja bash so have been babying the engine in the off chance there is a real problem. Yesterday we were running the engine back up around 1800 rpm and sure enough, vroom and a big stinky cloud, same thing. We checked the prop to see if we were trailing kelp and we checked the gear shifter but all was nominal.

What could this be?
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Old 06-02-2010, 22:49   #2
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First check your oil level. If it's too high it causes a run-away. If it is then you may be getting fuel in your oil. The injection pump is usually the first culprit.
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Old 06-02-2010, 23:06   #3
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The Fuel injection pump? What is it doing? Can it damage the engine?
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Old 06-02-2010, 23:12   #4
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If the injection pump is leaking fuel oil into the crankcase is what Del is talking about.

The 3GM30s also have a mechanical lift pump. That is another route for fuel oil to get into the crankcase if the diaphragm tears. Even just over filling the crankcase with oil can cause runaway.

Diesel engine runaway - Wikipedia, the free encyclopedia

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Old 06-02-2010, 23:16   #5
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Thanks guys I will check the oil level in the morning. Could a high oil pressure be an indication of this problem. Is there anything else that could cause these symptoms? In what way might a bad fuel pump cause a runaway?
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Old 06-02-2010, 23:56   #6
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Thanks guys I will check the oil level in the morning. Could a high oil pressure be an indication of this problem. Is there anything else that could cause these symptoms? In what way might a bad fuel pump cause a runaway?
If the crankcase starts filling up with fuel it gets hot and vaporizes, and then gets sucked up into the crankcase vent, and into the intake, in turn giving it more fuel then the IP is set for, causing the motor to speed up.

High oil pressure- you could have something else going on there. The IP could be forcing fuel back up the "Lube Oil Line". The fuel pump pressure is 1800+ psi and the oil pressure ranges from 10 to 80 psi depending on the RPM.

Here are some pictures of an IP. The fuel passes by the seals and then into the crankcase or diaphragm as Cal40john said.
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Old 07-02-2010, 00:18   #7
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From my quarter birth, where I write this, all night long I have been smelling the slightly charred smell of oil spilled on an engine block. The quarter birth is on top of the diesel tank. I bet you some oil got sent back into the tank on the fuel return line and that is what I am smelling. I can't wait to investigate in the morning when everyone else is awake. Thanks guys. I will let you know if that was the problem. Would the oil have to be grossly over-filled or just slightly?
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Old 07-02-2010, 06:45   #8
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From my quarter birth, where I write this, all night long I have been smelling the slightly charred smell of oil spilled on an engine block. The quarter birth is on top of the diesel tank. I bet you some oil got sent back into the tank on the fuel return line and that is what I am smelling. I can't wait to investigate in the morning when everyone else is awake. Thanks guys. I will let you know if that was the problem. Would the oil have to be grossly over-filled or just slightly?
that is physically impossible.

check the exhaust elbow for blockage.

how high will it rev in and out of gear?
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Old 07-02-2010, 06:59   #9
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Maybe the transmission is slipping Check the oil level in the trans
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Old 07-02-2010, 09:47   #10
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that is physically impossible.

check the exhaust elbow for blockage.

how high will it rev in and out of gear?
I am not really sure we are too busy trying to shut things down to check the rpms. I would say, just by ear, somewhere around 2200 to 2500rpm.

Why would a blocked exhaust elbow make the engine rev?
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Old 07-02-2010, 10:33   #11
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I am not really sure we are too busy trying to shut things down to check the rpms. I would say, just by ear, somewhere around 2200 to 2500rpm.

Why would a blocked exhaust elbow make the engine rev?
first you need to stop freaking out about this. Read the owners manual, normal operating RPM on a 3GM is 2800-3200 RPM.
The engine should free rev in neutral to 3700 ish.

Please start the engine and let it warm for 90 seconds, engage forward gear and apply full throttle. Record the maximum RPM achieved.
Now, decrease throttle to idle and engage neutral. WITHOUT engaging gear. Apply full throttle. Record the RPM.

Report back both RPM reading here.
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Old 07-02-2010, 10:48   #12
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We are not freaking out just being cautious with an engine very far from anything resembling someone who can work on it. 2800 to 3200 although within the operating range of the engine sucks down a lot more fuel than 1800 rpm . Saving fuel has been an objective in a place that is also far away from anyone who sells diesel.

Before we go out and apply full throttle to an engine we are trying to go easy on please tell us what will the exercise listed above accomplish?

In response to the post that suggested we check the transmission oil level: we just did and it seems fine, although the dipstick on the transmission is very tough to read.
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Old 07-02-2010, 10:52   #13
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3GM30F

I own (2) 3GM30 and have experienced this same symptom. Mine had very low torque at low rpm, wouldn't run a 90amp alternator at idle until it was completely warmed up. Cruising a 2700rpm, the engine would suddenly decide to run up over 3000rpm. Black smoke, black water. I was not making oil.

It turned out to be the injector pump and injectors. Nothing mechanically wrong with it, I had an injector shop tear it down, clean it, and re-calibrate it. It runs like a new engine once again. They did the same to the injectors, I needed one nozzle.

According to the shop, the problem comes from not running the engine enough. Diesel will varnish similar to gasoline if it sits too long. Their suggestion was to run the engine once a week for 20min. to keep this from happening again.

BTW, the diagram in the above message is NOT a 3GM30 IP.
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Old 07-02-2010, 11:05   #14
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We are not freaking out just being cautious with an engine very far from anything resembling someone who can work on it. 2800 to 3200 although within the operating range of the engine sucks down a lot more fuel than 1800 rpm . Saving fuel has been an objective in a place that is also far away from anyone who sells diesel.

Before we go out and apply full throttle to an engine we are trying to go easy on please tell us what will the exercise listed above accomplish?

In response to the post that suggested we check the transmission oil level: we just did and it seems fine, although the dipstick on the transmission is very tough to read.

until you establish the engine is running out of normal parameters, there is no failure your reporting.
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Old 07-02-2010, 11:08   #15
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I own (2) 3GM30 and have experienced this same symptom. Mine had very low torque at low rpm, wouldn't run a 90amp alternator at idle until it was completely warmed up. Cruising a 2700rpm, the engine would suddenly decide to run up over 3000rpm. Black smoke, black water. I was not making oil.

It turned out to be the injector pump and injectors. Nothing mechanically wrong with it, I had an injector shop tear it down, clean it, and re-calibrate it. It runs like a new engine once again. They did the same to the injectors, I needed one nozzle.

According to the shop, the problem comes from not running the engine enough. Diesel will varnish similar to gasoline if it sits too long. Their suggestion was to run the engine once a week for 20min. to keep this from happening again.

BTW, the diagram in the above message is NOT a 3GM30 IP.
About how much did that cost? And I am sorry the model in question is a 3HM35F not a 3GM, sorry about the discrepancy.
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