Yanmar 3gm stops at random underway!

[rpeebles]

Engine runs fine for a bit then coughs and stops. It can be started and if you let it idle for a while it will resume good behavior. During the idle period if you up there throttle it will appear to “misfire” and engine will buck. Then it will run fine for about one half hour then stop. Wait. Start. Runs at idle coughs and misfires and shudders. Then after several misfires etc will run ok for a bit. Then stop. Repeat. Not good on lee shore big bumps and stiff breeze. Thanks in advance!

[bstreep]

We have a similar Yanny on our generator. This sounds like fuel delivery. Any chance you are sucking air somewhere down the line? Could be as simple as an old fuel line. Or a bad gasket/seal at a filter or the pump. Could also be a clogged filter. These things only require fuel and air - so it's one of them.

[Compass790]

Tank vent or pickup tube blockage to add to the list that bstreep gave you.
Definitely sounds like fuel delivery or air ingress.
I assume filters are not clogged but thats usually slowdown type behaviour.

[chris mac]

We have 3gm30's. Same situation if there is a fuel blockage prior to the racor. It takes 10-20 minutes to use the fuel from the racor, then it dies.
If it's a gravity feed from tank, it can slowly refill. Allowing the cycle to continue.
When it sputters and dies, check your racor(or equivelant) to see if it's full or not.
Check your fuel hose also, the first fitting( likely the shut off valve) may have some debris in it.

[rpeebles]

Thanks! The engine bangs and shudders a couple times on restart. The bang is like a pre-ignition. Two or three bang bang. What causes this! Could it be a clue to my problem? Also many posts say if it’s an air leak the engine will not restart without help from primer. My engine restarts just fine after stall if you wait a minute or two. But back to my original question what causes that apparent pre-ignition?

[tkeithlu]

Diesel engines deliver fuel as a mist at just the right moment in the compression cycle. In your case, there is residual fuel in the cylinder(s), and it ignites whenever the combination of compression and temperature crosses the threshold. Ergo, BANG, and not at the right moment in the cycle.

[rpeebles]

Originally Posted by tkeithlu

Diesel engines deliver fuel as a mist at just the right moment in the compression cycle. In your case, there is residual fuel in the cylinder(s), and it ignites whenever the combination of compression and temperature crosses the threshold. Ergo, BANG, and not at the right moment in the cycle.

Thanks for the prompt reply! And what would cause residual? Bad injector timing?

[MikeHoncho]

Leaking injectors potentially

[Excalibur5]

Had a very similar issue with my 3gm - took a while to figure it out.

It turned out to the the racor filter housing had nylon stoppers to plug the unused ports on it. The nylon material had cracked over time, allowing little bubbles of air into the fuel stream. Over a long run, the bubbles would build up until it stalled the engine.

In my case, the engine would start back up after bleeding the air out of the filters, but run rough until it bled the tiny bit of air out of the injector lines. After a bit, it would actually bleed itsself of air out of the injector lines, and run fine.

I suspect you have something similar happening, enough air to get in to stall the engine, but not enough air to fully lock out the injectors from firing. Check all your fuel lines from the tank to the engine and all the fittings. I'm pretty sure that's where your issue lies.

[sinnerman]

Could be slime/gunk in the bottom of fuel tank being sucked up and plugging the fuel pickup tube. Once fuel in the lines/filter runs out the engine dies.

After the engine stops the suction holding the gunk in the pickup tube falls away and unclogs the tube. (Till the next cycle.)

Have a diesel polishing guy come and clean the tank, via the tank access covers if there are any. Or remove the tank for cleaning.

Check for gunk buildup at the fuel tank brass shutoff valve, Racor filter inlet, check/ball valve at Racor filter inlet if installed, and fuel pump inlet.

Some electrical fuel pumps (cylindrical) have an internal screen filter as well

[Cpt Mark]

Same engines, no bang bang but had same problem when I bought the boat and brought it up north in the wrong time of year. Air leak into the lines. I replaced lines, tightened, checked connections etc.. eventually something I did made it work right ever since. A few weeks ago the other engine died. Fuel issue. I tried a lot of things as I'd teed the fuel line off to connect my diesel heater to assuming it was that. Last thing I'd touched so 1st thing to check for. Eventually I carefully checked my fuel line and where it went into the engine the top of fuel line was swollen. It has looked like that since I've owned the boat but apparently it had gotten worse. Cut back 1 inch and all is fine.

[Cheechako]

Tank vent issue? Little bees build a nest in your hull outlet and clog it up sometimes.
Could be the pickup tube clogged from debris in the tank also.

[Russ]

Air leak guaranteed. Yes check your vent first, it's easy. After that, it gets a bit harder. I have been there with my Yanmar.
First, wrap a small piece of paper towel around EVERY fitting from tank to engine. Running the engine may do it and show damp diesel on a section where the fitting is loose. Gotta remember the diesel engine SUCKS diesel so the air gets sucked INTO the fitting and for that reason you might see nothing even thig there's a leak. IF no luck this way, don't give up. You need to get a small pump and PUMP diesel through the system preferably staring at the tank end of the hose. This WILL show a leak if it exists in the system between the tank and the engine. Yes, I did ALL this to find my leaks (2) even though I have all copper flare pipe fittings!
PM me if you need more help. It isn't likely to be anything else (except the vent, Yes I had a spider web clog one of my vents once, too, same scenario engine running stopping, etc.
Russ

[dreadpiratk]

Originally Posted by Russ

Air leak guaranteed. Yes check your vent first, it's easy. After that, it gets a bit harder. I have been there with my Yanmar.
First, wrap a small piece of paper towel around EVERY fitting from tank to engine. Running the engine may do it and show damp diesel on a section where the fitting is loose. Gotta remember the diesel engine SUCKS diesel so the air gets sucked INTO the fitting and for that reason you might see nothing even thig there's a leak. IF no luck this way, don't give up. You need to get a small pump and PUMP diesel through the system preferably staring at the tank end of the hose. This WILL show a leak if it exists in the system between the tank and the engine. Yes, I did ALL this to find my leaks (2) even though I have all copper flare pipe fittings!
PM me if you need more help. It isn't likely to be anything else (except the vent, Yes I had a spider web clog one of my vents once, too, same scenario engine running stopping, etc.
Russ

What Russ said. We’ve had this exact issue with our YSM12. Ultimately I took apart and re-made every fitting in the fuel system. I still don’t know which one was leaking but the problem stopped. Make sure you check your primary fuel filter and the bleeder valve for it(if you have one). Ours was stripped and leaking a small amount of air I had to re- tap it a size larger.

[Compass790]

Originally Posted by dreadpiratk

What Russ said. We’ve had this exact issue with our YSM12. Ultimately I took apart and re-made every fitting in the fuel system. I still don’t know which one was leaking but the problem stopped. Make sure you check your primary fuel filter and the bleeder valve for it(if you have one). Ours was stripped and leaking a small amount of air I had to re- tap it a size larger.

Lotta good suggestions here. We had to do the same on our YSM8 except replaced all the lines with non-yammer A-1 fuel hose. If your 3GM has any original fuel lines they need to be replaced. Our braided original fuel lines were actually loose on the crimped fitting the rubber had shrunk & hardened so much. A fuel line crumbled in my hands on removal but it was 37 years old.
Yanmar reckons 4 year replacement on fuel lines. lol, Thats OTT but pretty sure 3GM originals will be past their use by date