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Discussion Starter · #1 · (Edited)
Hi Everyone

The Yanmar 3JH3E 40hp in my boat has been a great work horse. Last season I replaced the primary and secondary filters and I have bled the system.

What I am finding now is that after running the engine in gear for 30 minutes or so, it tends to just drop power and then rev up again. Occasionally it does drop power and cut out completely, but most time it just looses revs then up again. I normally run the engine at 2500 revs, but this also happens when running at high revs of around 3500. This weekend the engine ran smoothly for 3 hours (no wind on the east coast of England) and then stopped... but each time it happens it starts quickly and off we go again.

The engine sounds great, no overheating, no smoke of any colour and has plenty of power

Just wondered whether you had any idea, before I get the professionals in the look at it?
 

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Sounds like a suction side air leak... filter seals/gaskets? suction side bleed screw?
 

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Possibly an air leak in the downstream side of the fuel delivery system, bad seal in the filters or connections. Maybe the tank is not venting properly, try running with the filler cap off or loose. Maybe debris being sucked up against the feed pipe screen, if there is one.

If you pressurize the system from the tank outlet downstream by using a squeeze bulb from an outboard or an electric pump, fuel may leak out at any bad connection of filter mating surface.

You could also try a separate gravity fed tank, bypassing the permanent tank.

Paul T
 

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Possibly an air leak in the downstream side of the fuel delivery system, bad seal in the filters or connections. Maybe the tank is not venting properly, try running with the filler cap off or loose. Maybe debris being sucked up against the feed pipe screen, if there is one.

If you pressurize the system from the tank outlet downstream by using a squeeze bulb from an outboard or an electric pump, fuel may leak out at any bad connection of filter mating surface.

You could also try a separate gravity fed tank, bypassing the permanent tank.

Paul T
Like that bolded bit......
 

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Like that bolded bit......
Never had it happen to me, but there are not too many wasps or hornets on San Francisco Bay, only know what I have read. John doesn't say where he is from? I think there are a lot of bugs "back east"?

If any part of the vent system is rubber hose a crimp may develop over time?
Kind of sounds like "slow starvation" I think a bad filter, either plugged or air leaky, would be more constant?

Paul T
 

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Yeah.. slow vent starves feed... stoppage allows tank to equalize, OK for a while and starts over again...
 

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This can also be a problem of a bacterial infection (pseudomonas aerugoinosa) that colonizes as 'strings'. What happens is that such 'strings' when formed on the sharp ends of fuel system 'fittings' or the inlet to the tank dip tube, begin to block the fuel flow volume ... and the engine loses rpm which in turn releases/relieves the pressure by the action of the mechanical fuel lift pump ... or the compression of the 'strings' relax after the engine is shut down.

Rx.: If you're not already 'dosing' your tank with a 'bacteriostat' attempt a 'shock' treatment of Biobor to kill the Ps.Aeruginosa and then see what happens.

Alternative Rx: if the tank is dirty and shedding a fantastic load of fungal debris to the (Racor, etc.) filters, such can also be caused by the extreme pressure/vacuum developed in the fuel delivery system during a blockage that in turn 'extrudes' (unloads) 'through' the filters, re-leaves the obstruction on an intermittent basis, etc. If your Racor filters are 'black-ish' this may be the cause. If so, a Biobor 'shock' application AND a thorough cleaning-out of the tank walls, etc. is probably warranted; ...... and also, do expect to soon to clean out the now 'coke' plugged water injection elbow, prior to your water lift muffler.
 

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Rich H, question is there a way to see if the bacteria is present, is it visible? The Racor was not black in any way when I recently changed it, there was sediment and some water as well. I have just ordered new filters and this Sunday I plan to investigate everything that was posted.

THANKS GUYS FOR ALL THE RECOMMENDATIONS!!

I will keep you posted and take pictures so I can post and hopefully find the root cause and provide a corrective action that others can use...

Please feel free to comment if you have any other ideas...
 
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