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Old 05-01-2008, 17:00   #1
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Lift Pump Diaphragm Failure

I have searched through 300 posts on this forum looking for an answer to this question but the closest discussion, which didn't answer the question, was in a really old thread: Rather than resurrect that dead discussion, I decided to open this new thread.

We recently brought our boat out of layup and motored for about 4 hours to a cruising anchorage in San Carlos, Mexico. Several days later I pulled the dip stick to check the oil level on one of our Yanmar 3GM30FC engines and was greeted with a gusher of diesel fuel/lube oil mixture. On the advice of several locals and a Yanmar tech in Mazatlan, I replaced the fuel lift pump, which upon disassembly had a crack in the diaphragm.

My question is: "What would be the symptoms of the same failure in an operating engine?"

I suspect that mixing even a small amount of diesel fuel with lube oil would thin the oil enough to wipe all the bearings in the engine. Worse, if the oil sump filled with fuel, the engine could suffer catastrophic failure from attempted compression of the liquid.

Am I being paranoid or is these legitimate concerns? Has anyone seen such an event?

Thanks in advance.

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Old 05-01-2008, 17:36   #2
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On Ford 2402E

On my old (now scrap) Ford 2402E the only "symptom" was that it would not start.

I believe that it is possible for the diaphragm to be cracked so that no damage is visible with no diesel fuel in the oil, but the pump will still not work - especially if the fuel needs to be lifted some height.

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Old 05-01-2008, 17:41   #3
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checking the oil just prior to starting the engine EVERY TIME can not guarantee heading off failure but it sure will go a long way.
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Old 05-01-2008, 23:11   #4
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Basically, ypu would not be able to start the engine, or if the engine was running, it would most likely stop. So filling to the level you discovered with a fully operational engine is rare. However, small leaks can still allow the lubricating oil to be contaminated. So regular inspection as suggested above is prudent.
Diesel is a good lubricant in itself. So major damage is not the norm in a short term situation. But long term, seriouse damage can occur. It is actually the cam surfaces that tend to get the biggest hiding. These are highly polished hardend steel surfaces that normally have no bearing shells or rollers to take the wear. Diluted lubricant will not protect those surfaces for long.
A majorly over full crankcase such as you encountered will most likely blow seals and you would have excessive oil leaks from everywhere. And if it did start, the build up of lubricant in the upper head area would quickly overcome the suction vent and you would most likely end up with a runaway. A litle scary when that happens. Hydraulicing the engine would not really be possible as the dilution would have to get high enough to flood the cylinders and fluid would have come out places like the dipstick and filler by then I am sure.

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Old 08-06-2009, 14:23   #5
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Is it possible to have a worn or tired lift pump that means the engine won't rev to maximum?
It runs at 2500rpm for hours but push the GM 30 Yanmar to any higher revs and she does not like it. The filters are clean the tank is clean the pipes are clean.

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Old 08-06-2009, 15:55   #6
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The lift pump would not effect the high rpms of the engine.....unless air is getting in?
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Old 08-06-2009, 15:59   #7
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Diesel fuel is, I believe an oil. Unlike gasoline it really isn't a solvent for oil. From the one diesel class I took, they said it had very high lubricity. Now, given the formulations of modern oils, I certainly not suggest mixing it or using it as a lubricant. But it may not be as tragic as it could be.

Try mixing a half a cup of diesel and a half cup of your engine oil.

I'd certainly use the opportunity to change my engine oil, but having run oil cooled cars most my life, I am just a stickler for oil changes.

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