Yanmar 2GM lift pump failing

[Wotname]

^^ OK, I don't really know BUT...
Some years back I took the stock mechanical fuel pump off my 2GM20 and set it up on the bench and measured how much fuel it moved per stroke. Even after factoring in the different in head between the bench setup and the boat, the pump was moving at least 5 times more fuel than the engine burned at normal cruising revs.
Sorry I can't recall the actual numbers

[Dulcesuenos]

I've had diaphragms go bad on lift pumps on 2gm20f's,, never ever had one cause a runaway situation. Are you sure there isn't something else going on in your system?

[boatbod]

A 2GM does not return much fuel at all. Regular pump will be fine.

[lonesoldier0408]

Originally Posted by Dulcesuenos

I've had diaphragms go bad on lift pumps on 2gm20f's,, never ever had one cause a runaway situation. Are you sure there isn't something else going on in your system?

A puncture of the diaphragm sucks oil from the crankcase, causing a runaway. As the failure is described as, "Rare." I would think a defect in alignment, causing the stroke to be longer, on that instal, not back pressure. As for electric, good idea, works on my, "Gas converted to Diesel Mazda truck." The issue is, you've added another device to fail. It does prevent, "Runaway." It also makes, purging air from the system, child's play. I installed an injection pump, "bypass line," for filter changes. Too easy.

[lonesoldier0408]

Originally Posted by Dulcesuenos

I've had diaphragms go bad on lift pumps on 2gm20f's,, never ever had one cause a runaway situation. Are you sure there isn't something else going on in your system?

You were lucky, they drew air from the crankcase first, causing non-start. A puncture under way would draw oil causing a runaway.

[Dulcesuenos]

Originally Posted by lonesoldier0408

You were lucky, they drew air from the crankcase first, causing non-start. A puncture under way would draw oil causing a runaway.

If that happens, Can you still shut it down with a fuel shut off/ kill lever? Or ?

[Wotname]

Originally Posted by lonesoldier0408

A puncture of the diaphragm sucks oil from the crankcase, causing a runaway. As the failure is described as, "Rare." I would think a defect in alignment, causing the stroke to be longer, on that instal, not back pressure. As for electric, good idea, works on my, "Gas converted to Diesel Mazda truck." The issue is, you've added another device to fail. It does prevent, "Runaway." It also makes, purging air from the system, child's play. I installed an injection pump, "bypass line," for filter changes. Too easy.

Originally Posted by lonesoldier0408

... A puncture under way would draw oil causing a runaway.

Please explain this process.

If indeed it was possible to suck oil from the crankcase with a punctured diaphragm (which I doubt), the oil would still be delivered to the injection pump which controls the amount of fuel going to the cylinders. As well, the fuel cutoff control is after the fuel pump. So hows does a 'runaway' occur?

I could believe that diesel might drain into the sump with a punctured diaphragm and that is not a good thing!

[sailorchic34]

The primary reason for the fuel return is the excess fuel flow helps cool the injectors. That is the case with all mechanical injectors. Though it does act as a poor mans fuel polisher too.

[jimbunyard]

I too was curious about the runaway situation, but could only come up with fuel being pumped into the crankcase and when enough was in there, it got pushed out through the crankcase vent, and then ingested somehow through the intake... Seems a little farfetched, but still possible.

Regarding what caused the problem, is it possible that there has been a part number change or update that allows using the same pump for different model engines? Could it be that there should be a spacer plate between the pump mounting and the block that is missing?

[Hunter33]

I have a 1977 hunter33 with the 2gm have had no problems yet but good to know in the future,

[lonesoldier0408]

Originally Posted by Wotname

Please explain this process.

If indeed it was possible to suck oil from the crankcase with a punctured diaphragm (which I doubt), the oil would still be delivered to the injection pump which controls the amount of fuel going to the cylinders. As well, the fuel cutoff control is after the fuel pump. So hows does a 'runaway' occur?

I could believe that diesel might drain into the sump with a punctured diaphragm and that is not a good thing!

The oil system is under pressure and a vacuum is filled by any, greater pressure. The leaking turbo seal is, a more often cause of runaways. The next step, after the engine kill lever, is blocking the intake, with an airtight seal.

[Wotname]

Originally Posted by lonesoldier0408

A puncture of the diaphragm sucks oil from the crankcase, causing a runaway. ..........

Originally Posted by lonesoldier0408

.......... A puncture under way would draw oil causing a runaway.

Originally Posted by Wotname

Please explain this process.

If indeed it was possible to suck oil from the crankcase with a punctured diaphragm (which I doubt), the oil would still be delivered to the injection pump which controls the amount of fuel going to the cylinders. As well, the fuel cutoff control is after the fuel pump. So hows does a 'runaway' occur?
..............

Originally Posted by lonesoldier0408

The oil system is under pressure and a vacuum is filled by any, greater pressure. The leaking turbo seal is, a more often cause of runaways. The next step, after the engine kill lever, is blocking the intake, with an airtight seal.

Let's try again. My question was straightforward. Your answer did not address the question; in fact, your answer is ludicrous. The oil system has nothing to do with the fuel pump diaphragm other than one side of the diaphragm is open to the crankcase sump. However it is mounted above the oil level in the sump and there is no appreciable pressure in the crankcase.

Even if by some miracle, pressurised oil could get into and then through the puncture in the diaphragm, it would not get uncontrollably into the combustion chamber.

So unless you can provide a rational explanation for the claim that a puncture in the fuel diaphragm while underway would cause a runaway, I would suggest it is incorrect.

And let's not worry too much about leaking turbo seals on a 2GM -

[boatbod]

The only way I see potential for a runaway is if (lots of) diesel overfills the oil pan and starts bypassing the rings.

[Steveschr]

Regarding my experience with the runaway, the evaporation point of diesel is something like 140F a temprature easily reached in the crankcase at cruising speed. My assumption of what happened is that some fuel was forced through the ruptured diaphragm into the crankcase, boiled off and entered the cylinders through the breather. The high rpms were accompanied by lots of smoke and knocking. The crankcase wasn't over full and I didn't detect much of a fuel smell in the oil. It wouldn't take a lot of fuel to raise rpms considerably on that little engine. Try spraying some wd40 into the intake and watch it go. I was able to shut it down with the engine stop only. Sort of died down slowly and I was expecting to need to choke the intake.

As to electric pumps, I have a Walbro FRB-20 installed and I believe it was running when this all happened. Clearly the pressure from the electric pump would force fuel through a leaky diaphragm. It was running because I had been having what seemed like intermittent fuel starvation or air issues. I now belive these were caused by the displaced outlet valve in the lift pump. Clearly I can abandon the mechanical pump but when working it seems inherently more reliable than an electric pump.

Some had suggested it might be a misalignment or pump stroke issue. That's an interesting thought but I can't figure out how it would raise the pressure on the outlet side of the pump enough to lift the valve and the plate. Can you elaborate?

It's certainly possible I had two pumps with identical defects but it seems odd nobody else ever heard of this. I would think they'd make many pumps with the same defect. Of course it's still the only believable explanation with a theory that makes sense....

I'm planning to dissect the current lift pump at around 75 hours to see if the issue has reappeared.

I really do appreciate this forum and all the responses.

Steve

[Wotname]

Originally Posted by Steveschr

Regarding my experience with the runaway, the evaporation point of diesel is something like 140F a temprature easily reached in the crankcase at cruising speed. My assumption of what happened is that some fuel was forced through the ruptured diaphragm into the crankcase, boiled off and entered the cylinders through the breather. The high rpms were accompanied by lots of smoke and knocking. The crankcase wasn't over full and I didn't detect much of a fuel smell in the oil. It wouldn't take a lot of fuel to raise rpms considerably on that little engine. Try spraying some wd40 into the intake and watch it go. I was able to shut it down with the engine stop only. Sort of died down slowly and I was expecting to need to choke the intake.
..................

Now there is a rational explanation of how the puncture might cause a runaway!
While unlikely, this explanation is rational and therefore possible unlike the idea of the oil system being involved.

As for the cause of the puncture, here is another wild A'd guess.

You mentioned there were two part numbers; a superseded one and its replacement. Maybe the old one was prone to bending (and occasionally breaking) the retaining plate and the new one was improved. Now I realise you understand your second failure was on the later part number pump but it wouldn't be the first time a box contained the "wrong" item, even if sealed from the factory - especially when they look alike. Were the two pumps marked in any way except for the packaging?

By the way Steve, welcome aboard CF; nice first post and thread. You got us all thinking with this one and that is always a good thing