Engine runs for hours, then stops, and after a good turning over with the starter, starts and runs again. What’s up?

You haven't given us any details of the motor or the model number but there are quite a few threads on "Yanmar, stops and runs again"

Yanmar cut out, re-starts, runs, then dies | YBW Forum
https://forums.ybw.com › yanmar-cut-out-re-starts-runs...
9 Aug 2016 — It runs very erratically for 20 seconds then stops. I tried it 2 more times, once coming into the river and again when I was back on the ...
yanmar engine cutting out. 5 Sept 2013

Yanmar YSE8 - Runs fine then stops after an hour or so - SailNet
https://www.sailnet.com › ... › Engines › Diesel
21 Nov 2009 — I cannot restart it for around 20-30 mins, I'm guessing until it cools down again. When running it sounds good and seems in good order.

Yanmar Engine runs 4h and revs drop and then stops - Page 2
https://www.cruisersforum.com › forums › yanmar-eng...
1 Apr 2018 — I had a similar problem with my yanmar . It would run fine for 4-6 hours then start to idle down and die. Start up after 15 or so minutes.
Yanmar 20GM20 starts but wont run - Cruisers & Sailing Forums. 14 Jan 2018
Yanmar diesel shutting off - Cruisers & Sailing Forums. 8 Dec 2012
Yanmar 3gm30f - Occasionally sputters and dies on start up. 5 Jan 2016
Yanmar Engine runs 4h and revs drop and then stops. 31 Mar 2018

Thread: Yanmar 2GM Engine cuts out every 20 minutes
http://www.marineengine.com › ... › Yanmar
28 June 2011 — ... running rough-I replaced both filters-now it stalls every 20 minutes-I bleed lines-it starts and stalls again after 20-30 minutes.
 
It’s a 4JH5E. I can’t say anything about the filter when it stopped because the engine is mounted under the cabin sole, and the engine restarted before we got to the “pull up the floorboards” stage! . The temperature and oil pressure were both normal.

Having had a case of diesel bug the whole fuel system was professionally cleaned* and the engine has just had a service.

*What lurks beneath…
 
I did wonder about dirty fuel, which was one reason to suggest looking at the filter (but that job has been done) but you seemed to imply that maybe you were drawing air into the fuel line and I wondered if this might be visible with a part filled glass portion of the CAV filter. It seemed a simple thing to check - though it depends where your fuel filters are located (obviously). I did speculate if it were a very minor leak the air might accumulate over a period of time - suddenly allow the engine to stall, after a few hours of use. Our engines are ever so slightly smaller than yours - but our CAV filter is located such that I can lift up a hatch, slightly, shine a torch inside and see the state of the glass. Just in case you think having 2 engines offers a rather nifty safety factor - fuel is drawn from the same tank, parts wear at the same rate - and if something goes wrong with one - you can bet that the other engine will follow soon after.

As a slight aside I did install remote oil filters for the engines to make access to the oil filters more easy (and cleaner). Its an adapter that fits on the current location for the filter simply connected by hoses to a freestanding filter housing. The ideal would be to spend a bit more time (and money) and install the primary and secondary fuel filters and the oil filter altogether with a bolt on drip tray to allow all three to be serviced without contortions and mess. Our oil filter is only accessible from lying on top of the engine and working head down, bum up - with inevitable spillage of the old oil.

I over enthusiastically tightened our CAV filter - to minimise leaks - the bulb cracked over night and all 200l of our fuel, its gravity fed, filled the engine bay nicely with fuel and the rest of the yacht with fumes. Fortunately the bilges do not interconnect. Removing 200l of fuel was an intellectual challenge.

Jonathan
 
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A chacteristic of diesel bug is that the inlet in the tank blocks, leading to reduced pressure at the primary filter, which causes air to be drawn into it. After a time fuel gets itself past the blockage and the engine will restart.

Worth checking that there is not a strainer inside the tank.
 
Not a Yanmar, but my engine had a very similar syndrome after I'd had the boat-builder put a new sender in the fuel tank. It would run fine, sometimes without issue for some weeks, but ocassionally just stop. Usually it would restart after leaving it a while. This happened first to the engine and a few months later to the generator.

To cut an annoyingly long story short, the cause was that, while servicing the fuel tank, the mechanic had injected a slug of silicone mastic into the pick-up pipes of both the generator and engine - presumably to stop annoying drips. But a different mechanic reconnected it all, and didn't know about (I assume) and certainly didn't remove those slugs of mastic. He also swapped the generator and engine pick-ups so diagnosis was hampered!

Pic here (scroll down to pics 5, 6 & 7)
 
I know that the OP had the tank cleaned but what it sounds like to me is that there is a lump of crud still in the tank, which gets sucked onto the pickup pipe. That blocks the fuel, causing the engine to stop. A short while later the vacuum holding the crud on the pipe eases, the crud falls off and you can restart the engine. This fault will keep repeating itself until the crud is removed from the tank. Happened to me on a catermeran on passage to Gib: had the tank emptied in Sicily but that didn’t sort it. Had to have the tank recleaned in Gib….
 
Does your engine have a mechanical or electric fuel pump?

I had similar problems and went through the fuel blockage and air leak process and also considered a blocked breather pipe. Eventually found that the engine, vetus m4.17, has an electric fuel pump. This has an internal filter in addition to the primary and secondary filters: changing the filter improved things but changing the fuel pump ever better and now runs like a dream.

These engines are often used on small plant like dumper trucks and a sailing friend with a background in agricultural plant said they often replaced the fuel\pumps.

Just a thought......
 
This is now becoming a first rate Mystery of The Sea.

Seventh Cavalry spent a few hours, found some very iffy pipework and a slack union and replaced that. Started and ran the engine and after an hour it cut out again.

Today we set out to navigate from Woolverstone to Fox’s. Needless to say as we set out the wind rose to 27 knots. And the engine cut out twice below the bridge and finally gave up, and wouldn’t restart above the bridge, where the moorings start. We drifted sedately onto the mud in a ladylike manner and hoisted Z. Lee Gilson from Foxs arrived with their workboat and towed us in.

Mark from Foxs arrived and said “I know all about your boat - I used to work for Volspec and they phoned me!”

There are some Really Weird Symptoms - the rev counter flickers between 1,000 and 2,000rpm with no change in engine sound, the gas alarm goes off (with the gas off at the bottle) and the VHF turns itself off with a message reading “Bootlog”.

Is the boat haunted?
 
Kukri said:
This is now becoming a first rate Mystery of The Sea.

Seventh Cavalry spent a few hours, found some very iffy pipework and a slack union and replaced that. Started and ran the engine and after an hour it cut out again.

Today we set out to navigate from Woolverstone to Fox’s. Needless to say as we set out the wind rose to 27 knots. And the engine cut out twice below the bridge and finally gave up, and wouldn’t restart above the bridge, where the moorings start. We drifted sedately onto the mud in a ladylike manner and hoisted Z. Lee Gilson from Foxs arrived with their workboat and towed us in.

Mark from Foxs arrived and said “I know all about your boat - I used to work for Volspec and they phoned me!”

There are some Really Weird Symptoms - the rev counter flickers between 1,000 and 2,000rpm with no change in engine sound, the gas alarm goes off (with the gas off at the bottle) and the VHF turns itself off with a message reading “Bootlog”.

Is the boat haunted?
Click to expand...
You seem to have an electrical problem because,

1 The VHF senses a power cut and goes to reset.
2 The gas alarm goes off because the power has been lost then reinstated and it beeps to tell you it is on.
3 The taco is sensing a power cut and reconnect.

which suggests to me that the power from the batteries is being interrupted. Check cables and connections to batteries, isolation switch/s making bad contact, connections in and out of fuse box/s, and include all the negative connections as well.
 
Check if stop solenoid is power to run ?

YaNmar ignition switches are wired with various solenoids and are a little confusing from memory
Some solenoids operate relays or vice versa
Sorry having a senior memory moment recalling the exact problem but it involved the switch and took several goes to get it right
 
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Update:

The low pressure side of the fuel supply runs from the tanks through a series of valves and unions where a supply to the no-longer-with-us diesel generator used to run to from and where the tank switch over valves are located. The engine ran fine for two hours with a hose from the filter led straight into the dipstick hole in the tank; when reconnected through Spaghetti Junction it stopped in 15 minutes.

Accordingly the source of the air entrainment is now suspected to be there.
 
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