YSM8 Revs up Dangerously

[Harborless]

Just finished getting the engine system connected all up. Started it up and cranked on the first try. Ran it ten minutes putting t in and out of gear at the dock.
All of sudden after I revved engine slightly and was throttling back down the engine started revving up uncontrollably. It would not shut off from pulling the choke and was speeding up so fast it would have blown up had I not went into the engine box and pushed the compression valve shut.

Waited a few minutes and piddled around. Started back up on first crank. Ran for two minutes. Same issue.

Engine is 1979 YSM8 Yanmar. Any Ideas?

[Rockter]

Certainly sounds like the governor has failed.
To stop a motor on the compression valve lifter is a risky business.
It is better to have a wee device that shuts off the air supply.
I wish I knew more about governers to help but that looks like where the problem is.
.

[-OvO-]

I'm not familiar with the YSM8, but I'd be surprised if it has a choke -- most diesels don't. The T-handle that is commonly used on sailboats to kill the engine does look just like the choke control on old-fashioned carburated gas engines, but it actually kills the fuel supply.

If you've cut off the fuel supply and the engine keeps running, it's getting fuel via an unorthodox method (it's possible but highly unlikely that the fuel kill mechanism is broken). The fact that the engine is not responding to throttle control also indicates the same thing. If the governor has failed, removing the fuel should still kill the engine.

One good possibility is that the fuel being burned is not diesel, it's motor oil. This can happen if you overfill the crankcase. Also, if the rings are shot, they can allow oil past them which will burn.

Another possibility is that diesel fuel has leaked into your crankcase somehow, same effect.

Another thing that causes runaway diesel engines is accidentally putting gasoline in them, but that usually happens immediately, not after a minute or two.

So, two things:
1. what color is the exhaust (white, grey, black, blue?)
2. Check the dipstick and see if the level is high, or if the oil has been contaminated with diesel.

[Faster]

Nice post,OvO..

I agree.. sounds like governor, although why it would run even for a few minutes is a bit odd. Severe blowby is another good candidate. Disconnect the PCV line (usually goes to the aircleaner housing or intake manifold somewhere and look for puffing fumes.

Next time it happens get a rag or towel and cover the air intake (don't use your bare hand) to kill the engine..

[Harborless]

Originally Posted by -OvO-

I'm not familiar with the YSM8, but I'd be surprised if it has a choke -- most diesels don't. The T-handle that is commonly used on sailboats to kill the engine does look just like the choke control on old-fashioned carburated gas engines, but it actually kills the fuel supply.

If you've cut off the fuel supply and the engine keeps running, it's getting fuel via an unorthodox method (it's possible but highly unlikely that the fuel kill mechanism is broken). The fact that the engine is not responding to throttle control also indicates the same thing. If the governor has failed, removing the fuel should still kill the engine.

One good possibility is that the fuel being burned is not diesel, it's motor oil. This can happen if you overfill the crankcase. Also, if the rings are shot, they can allow oil past them which will burn.

Another possibility is that diesel fuel has leaked into your crankcase somehow, same effect.

Another thing that causes runaway diesel engines is accidentally putting gasoline in them, but that usually happens immediately, not after a minute or two.

So, two things:
1. what color is the exhaust (white, grey, black, blue?)
2. Check the dipstick and see if the level is high, or if the oil has been contaminated with diesel.

Thanks for the well thought out answers. I too thought it was the governor but I wanted a second opinion. I know that about a year ago (before I ought the boat) it had engine work done-- replacing old parts and such.
I myself replaced the transmission and gear box.

You are right. the Choke is a fuel cut off switch and it did not do anything when I pulled it to turn off the motor. I figured perhaps the fuel injector is stuck open to wide or something.. Exhaust smoke is grey and water comi g out is clear. Engine cranks first try every time.

I will check the diesel-- I know it is not marine diesel but I hardly think it would make that type of difference.

The problem seems to only happen after I have throttled up and then bring it back down to neutral. If I just left the engine i neutral it would run all day with no difference. Only when I engage gears (forward and reverse) and idle up and down that the problem happens. I will take a look at the fuel injector and governor. Any other ideas feel free to share! thanks for the help!

Here are a few pics of the engine:

[Harborless]

[Faster]

Sorry, HL... I edited your link (to show pic in post) before I realized you'd linked to several. Please edit your post to put the original link back in.. (I've lost it...)

[Lake Superior Sailor]

Is this in the boat hooked up? AS fuel shut off isn't! also missing line by charger....Dale

[Harborless]

No that is just photo of the engine. It is currently all hooked up. Push to start so to speak

[Lake Superior Sailor]

Really need to bring in the real patient, hard to tell what's wrong with you by looking at your friends.. Dale