Ongoing problem with our engine. Update. Possibly diesel bug? Advice please.

[Oscarpop]

Our Yanmar 3jh5e kept stalling after we had been sailing. No other time and I could not find a problem. No leaks no crud in the filters.

So his morning I pushed about 2mm of 8mm clear tubing through one of the feeds and sucked diesel up from what I assume was the bottom of the tank . For some reason out boat was built without an inspection hatch in the tank .

Anyway , after a couple of goes, we could see black drops suspended in the fuel as it was being drawn up. Only a couple though. About the size of lentils.

So questions:

I guess this is diesel bug? (We have always kept the tank full , but may have been a little slack in adding the preventative treatment )

As my filters are still clean I am assuming that at present it is a mild case.

I have added the diesel bug treatment to the fuel. The stuff I used is the stuff the rnli recommend.

Is this sufficient? Or do I need to arrange for fuel polishing as well?

The problem is that if we need the fuel polished, there is only the feed access to he tank, and so we would not be able to steam clean the tank afterwards.

All suggestions gratefully received.

 

[vyv_cox]

You don't say what happens after the engine stalls. Does it restart without changing the filter? If so then bug is probably not your problem. Once the filter is blocked with it no fuel can get through, and if you have a filter with a glass bowl you may well see air bubbles going in as the pump tries to suck the blockage.

I have experienced rust blocking the fuel supply from the tank, that was cleared by blowing back into it. This was an old tank with the outlet at the bottom. You might have some debris in there that is displaced when sailing.

 

[pvb]

If the filters are clean, but the engine stalls, it suggests that the problem is in the pipework from tank to filter. If you have a top-mounted pick-up pipe, these sometimes have a mesh filter on the bottom, which can block with bug residue.

You can do fuel polishing on a DIY basis by pumping fuel through a filter and returning it to the tank. If you're able to get some limited access to the tank (maybe through a level sender hole?) you can use a semi-rigid suction pipe to "hoover" crud from the bottom of the tank. However, most decent-sized tanks are baffled, so there's a limit to how much you can get at, and you certainly wouldn't be able to do any steam-cleaning.

My last boat had its main tank in the keel, under the engine, and highly inaccessible. When I got a serious bout of diesel bug, I rigged up a DIY fuel polishing system with an old water pump and a CAV filter. I circulated the fuel for hours, moving the suction pipe around on the bottom of the tank as much as I could, and changing filters when the flow reduced. Then dosed the tank with industrial quantities of bug killer. The problem basically went away.

 

[Oscarpop]

You don't say what happens after the engine stalls. Does it restart without changing the filter? If so then bug is probably not your problem. Once the filter is blocked with it no fuel can get through, and if you have a filter with a glass bowl you may well see air bubbles going in as the pump tries to suck the blockage.

I have experienced rust blocking the fuel supply from the tank, that was cleared by blowing back into it. This was an old tank with the outlet at the bottom. You might have some debris in there that is displaced when sailing.

Yes it restarts without changing the filters.

I figured that these globules of black crud were getting sucked up somehow and stalling the engine .

As I said , it only happens after we have been sailing, and we drop the revs to anchor.
So logically, something is either getting stirred up when under motion , or its introducing air into the system.

I also changed all he fuel lines this morning as there are tales on the internet that suggest that pipe degradation is a cause of the problem.

So if there are black globules in the bottom of the tank, is this probably diesel bug?

 

[Sandy]

Can you remove the tank and clean it?

 

[RichardS]

So if there are black globules in the bottom of the tank, is this probably diesel bug?

Just to answer your specific question, the black globules are probably diesel bug ..... but only a minor infection if the first filter after the tank is still relatively clear as others have said. A bit of dark shading on the paper element, often a band near the top or bottom, after hours of motoring is not necessarily a concern and is not going to stop the engine.

I would do a major dosage of the engine, using the recommended "killer" dosage rather than the maintenance dosage and using a different biocide to the one you are using at the moment and then keep a close eye on the filter. You should find that you can clear it up without any major work and achieve a clean filter after a couple of changes.

If you tell us what biocide you are using at the moment, we have forumites who are knowledgeable enough to recommend an alternative.

Richard

 

[Plevier]

it only happens after we have been sailing, and we drop the revs to anchor

Is that consistent with bug? Running at high power but not at low power?
Not disputing the black blobs may well be bug - but are they the cause of this problem? I question it.

 

[ianj99]

Does it still stall if you throttle back very slowly?

Some injection pumps have an anti-stall device to cure the stalling which occurs when the throttle is closed quickly (the DPA pump on a 4108 has, no idea about Yanmars).

I also don't think its bug related - any blockage would affect high rpm running more so than idling and restarting would be affected.

 

[viva]

I had similar symptoms on my boat. Fuel starvation, filters were clear. Problem was diesel bug was blocking the fuel take up pipe in the bottom of the tank. With my boat I was able to withdraw the take-up pipe from the tank. I had the fuel polished and problem was solved.

 

[seadog30]

Hi

Have a look at the fuel tank breather. If a vaccum builds up while the engine is running fast the fuel pump may be able to overcome a partial block however slow running may not. We had this problem and spent a lot of time (and cash) trying to debug the fuel.

Good luck

 

[mawm]

I had exactly the same problem. I eventually discovered a blocked filter in the fuel separator (it's a thing that seems to be out of fashion). I did not know there was one in it and therefore had never cleaned it. I can imagine that any partial blockage in the negative pressure side of the fuel lines will create the same symptoms.

 

[Birdseye]

Likely your boat has a diesel heater. If so, run that. Does it work OK? Reason is that heaters are far more sensitive to marginal bug cases than engines themselves so in my case the heater is always the first thing to stop working.

second test - change the main fuel filter and take the old filter element and chop it up with a hacksaw. If you have the big the paper element will be clogged. If the paper is clean its unlikely that you have the bug seriously enough to be causing your problem.

I have had diesel bug in every one of the 4 boats I have owned, always I hasten to add, just after I've bought them. On no occasion have I had a problem that occurrs only after sailing. On all occasions the problem has started from the dock side with variable speed running and finally stalling. Your symptoms of stalling only after sailing dont sound like diesel bug to me unless what is happening is that you have a pick up pipe that is clear of the tank bottom and the sailing is stirring up a lot of sediment in the tank.

Best thing to do is to empty the tank and clean it out