Quick Diesel engine question

[Secondshift]

An air leak would not be visible until the bleed. The air would have been pulled into the line by the negative pressure of the mechanical or the high pressure. Then typically collecting at a filter until the filter failed to supply fuel.

If it were mine I'd run both pumps and carry a back up electric. I wouldn't want to keep starving the high pressure pump. To check the air leak theory I would run it safely less than your 20 minute failure time and do a bleed looking for air.

Sounds like you've taken great efforts to sort this out. I know your frustration.

I'm not sure if we should begin to suspect the high pressure pump at this point. I'm still learning diesels myself.
If it's running fine with the second lift I would continue to do so. We say "If it ain't broke don't fix it." It does seem to have been an unnoticed previous problem. Especially if the electric pump was added to the ignition circuit without an engine compartment switch. Maybe ran this way for years.

Depending on how far you want to pursue this maybe consider a high pressure pump maintenance at your next haul. Just running out of likely suspects.

Hopefully a64pilot will give a response.

Best Regards

[sailorphil9]

Hi All


Didn't want to start a new thread as most of the discussion is here. My engine - old tech Mercedes OM366 5.2L diesel, tank in keel maybe 4 feet max head - has developed a problem which I've been unable to resolve. After being off for a few hours it needs bleeding then will run fine for days. I've changed fuel lines, filter assemblies etc. No sign of fuel leak and I have a shut off valve in the line before the filter.

Just wondering whether a slightly porous diaphragm in the mech lift pump is the problem, but no sign of lub oil problems - levels or viscosity.

Just don't know where the air gets in. So, thinking of fitting an electric pump for bleeding, avoiding lifting the pilothouse floor at sea etc...!

A few of you folks have done the research, so looking for recommendations on pump brands etc. Not much available here in New Zealand so will probably have to ship from US. All sane suggestions appreciated!

[ahun]

This is what I bought:

https://m.ebay.ie/itm/New-Universal-...anner=1&_rdt=1

So far so good.

[jpendoley]

Originally Posted by ahun

Thanks everyone for your input, lot of useful information came together.

During the week the two fuel pumps arrived, so this morning was fitting time.

First came the 'blow test'. The electric fuel pump indeed let's through the fuel when it's not operational with practically no resistance. It's a generic, low pressure fuel pump $16 from eBay, consumable.

I fitted both fuel pumps, bled the system, then went for a test drive. I turned the electric fuel pump off (I put a switch on its wiring) and the engine kept going. Meaning, the fuel lift pump provides enough fuel as normal.

Not for long, in about twenty minutes the fuel starvation started, then the engine stopped. I had to bleed the system again, then I kept the electric fuel pump on, and never had a problem again for the rest of the two hours.

Yanmar 3hm35f engine, fuel tank under the floor, slightly lower then engine, electric pump was not factory fitted.

I had the exact same scenario with my Perkins/Westerbeke40. Electric pump was specifically bought as a low pressure diesel pump that would allow fuel to pass through if the electric failed and the diaphragm was working. Its a solid state cube pump that is supposed allow pull through but to not allow back flow. The electric pump is plumbed in between the Racor and the diaphragm pump-so it sucks fuel through the Racor. Engine runs perfectly with this scenario. If the electric pump is shut off the engine will run for twenty mins to an hour then shut down. This is with a new clean fuel tank.
When the previous electric pump was installed, it was installed ahead of the Racor and was always off-except for priming. Diaphram pump pulled fuel fine and the Engine ran fine until filthy fuel contaminated the fuel system (old tank). Being in front of the Racor, the old electric pump would push fuel through the Racor negating the turbine filtering benefit of the filter (Racor advises pumps be installed on the suction side of their filters lest dirty fuel is pushed through to secondary filter).

A64 has me wondering if perhaps my diaphragm pump is getting weak. I would love to run on the diaphragm alone to avoid the diaphragm failure and sump flooding scenario.