Diesel engine bleeding: -- question

[russells]

Hi all - There's something that I can never quite understand when bleeding a diesel engine: When bleeding the engine, we usually bleed at the lift pump (which is the low pressure side) and often this will suffice. If not, then the fuel lines at the high pressure side of the injector pump are loosened and the engine is cranked over until fuel flows, after which all should be OK.
Now, my question is - what is the difference between performing all of the above and simply cranking the engine until the air purges out? (I understand that some engines are 'self bleeding'). I can't quite see though, why my hand pumping the lift pump is not the same as what the engine does anyway when cranking. Why won't the air lock simply move out of the system as the engine is cranked?
regards
Russell

 

[SamSalter]

Because the diesel is incompressible (or virtually so) and air is compressible.
When the air gets to the high pressure pump it just compresses and decompressed and doesn't move along the line.
The same will happen anywhere along the line ( even on the low pressure side) if there's not an exit for the fuel/air. By bleeding you introduce this exit; reduce back pressure and allow the air to move along the line and out of the system. Once you get to the high pressure side there's not enough pressure from compressed air to open the injector (against the spring)
(As the diesel is incompressible, the pressure created is enormous compared to the compressed air and will easily overcome spring pressure in the injector.

sam

 

[Bav34]

Sam's the man

The other thing that you must consider is that the water you pump into the engine is actually expelled by the exhaust.

If the engine isn't firing whilst you're trying to prime it,you are actually filling up the whole exhaust system with salt water which then makes its way into the cylinders.

Water doesn't compress so guess what happens!

 

[SHUG]

Sam and |Bav 34 have given the whole story ...almost. You might find that when you start the engine it runs roughly. With self-bleeding systems this should clear after a few minutes with older systems you may have to "crack" the unions at the injectors to get rid of residual air.
I have had bad experience of water getting into the cylinders and a couple of bent con-rods as evidence. So if the engine doesn't start first time, switch off the saltwater intakeand get bleeding.

 

[angelsson]

Hi all - There's something that I can never quite understand when bleeding a diesel engine: When bleeding the engine, we usually bleed at the lift pump (which is the low pressure side) and often this will suffice. If not, then the fuel lines at the high pressure side of the injector pump are loosened and the engine is cranked over until fuel flows, after which all should be OK.
Now, my question is - what is the difference between performing all of the above and simply cranking the engine until the air purges out? (I understand that some engines are 'self bleeding'). I can't quite see though, why my hand pumping the lift pump is not the same as what the engine does anyway when cranking. Why won't the air lock simply move out of the system as the engine is cranked?
regards
Russell

Previous post spot on why air compresses and needs an 'exit'.

There is of course a bleed screw on the injection pump HP, which should be used before slackening an injector or two. Cranking the engine continuously will burn out starter and not help you battery either. I have an engine mechanical pump, which can make bleeding labourious, so have set up a 12v electric pump which both acts as a standby pump, and to assist bleeding following filter changes.

 

[VicS]

Now, my question is - what is the difference between performing all of the above and simply cranking the engine until the air purges out?

I used to have a Fiesta diesel car. That's all you did with that after changing the filter as there was no lift pump ( and no rubber bulb like the current Peugeot)

I used to fill the new filter with fuel before fitting and then crank and crank until it fired. With luck and a wide open throttle once it fired it would keep going. The secret was to keep it going once it fired until all the air was purged.

Even with the Pug I only use the bulb to bleed the filter.

 

[ChiPete]

Hi All,

Can anyone tell me if the Yanmar 3YM20 is self-bleeding?

I'm having a nightmare bleeding after replacing the filters. From reading the thread, I now understand that I can only bleed as far as the primary fuel filter - despite loosening the injector nuts on the block and operating the lift pump for tem minutes, nothing came out.

If the engine isn't self-bleeding, to summarise what I think has been said:

1. Bleed the low pressure side with the lift pump.
2. Loosen each injector nut in turn and crank engine until fuel is ejected from the nut. Repeat for each cylinder.
3. Once all three injectors have fuel, crank engine until it starts then open raw water sea cock.

Isn't there a danger when, with the injector nut loosened, diesel is going to get sprayed around at high pressure?

Any assistance would be hugely appreciated.
Cheers,
Pete

 

[Martin_J]

ChiPete... Remember, with the lift pump the engine crankshaft may have to be turned a little to allow the pump to work..... If the engine already has the lift pump at the top of it's travel then however much you pump it will do very little.

Others may explain this a little more clearly soon.

The injector nuts should not need loosening in order to get fuel out at the engine filter bleed screw when using the lift pump by hand.

 

[earlybird]

Hi All,

Can anyone tell me if the Yanmar 3YM20 is self-bleeding?

I'm having a nightmare bleeding after replacing the filters. From reading the thread, I now understand that I can only bleed as far as the primary fuel filter - despite loosening the injector nuts on the block and operating the lift pump for tem minutes, nothing came out.

If the engine isn't self-bleeding, to summarise what I think has been said:

1. Bleed the low pressure side with the lift pump.
2. Loosen each injector nut in turn and crank engine until fuel is ejected from the nut. Repeat for each cylinder.
3. Once all three injectors have fuel, crank engine until it starts then open raw water sea cock.

Isn't there a danger when, with the injector nut loosened, diesel is going to get sprayed around at high pressure?

Any assistance would be hugely appreciated.
Cheers,
Pete

The GM series isn't self bleeding, for my 2GM20, I seem to remember loosening the injector unions and cranking the engine over with the speed control advanced, and decompressed until fuel emerged.
Keep the seawater cock closed
The lift pump won't purge the high pressure side.
The fuel onlly dribbles out of the loosened joint.

 

[prv]

The GM series isn't self bleeding, for my 2GM20, I seem to remember loosening the injector unions and cranking the engine over with the speed control advanced, and decompressed until fuel emerged.

That's what I do, but turning it on the handle rather than the starter motor to save the battery / starter windings / etc.

Pete

 

[CaptainBob]

I had this problem last week. Turns out that my engine self-bleeds at the injectors and the manual lift pump actually only is effective at a very specific point of the engine's rotation.

I'd sat for about an hour pumping and got nothing at all. Quick call to Beta Marine about my BD 722 (20 HP) Beta and they told me the above and suggested I remove the impeller, pre-fill primary and secondary fuel filters with fuel, and use the starter motor to crank the engine over for a maximum of 5 seconds at a time, followed by a wait until the starter motor was cold again. They said I "did not need to bleed the injectors unless I really wanted to".

I did what they said and within a few goes then engine was firing perfectly.

 

[ChiPete]

Thanks to you all for your speedy replies. OK, I'll try Captain Bob's method to start with (less faffing trying to get the unions undone), if that fails, then crack the unions and turn over by hand (if poss) or starter motor.

I'll let you know how it goes!
Cheers,
Pete

 

[SamSalter]

I'm talking about a GM here 'cause that's what I've got. The YM may be similar but I've no experience.

The bit about the lift pump needing to be on the low part of the cam is right. If you get very little throw or nothing seems to be happening, turn over the engine 1/2 a turn or so and try again.

I've got 3 bleed screws: one on the primary filter (separate from the engine, between tank and lift pump); one on the secondary filter (on the side of the engine and a Yanmar part, between lift pump and injection pump and one on the injection pump. Bleed them all in turn starting with primary, secondary then injection pump. All with the lift pump. Can be a pain. Some people put a squeeze bulb somewhere before the lift pump (the kind on an outboard motor fuel line) and find it a lot easier.

Bleed in turn until raw diesel and no bubbles come out and nip up the screw.

Usually not necessary to bleed the injectors. I've done it just to be sure while the engine is running. Slack off each injector pipe in turn and nip it back up.
If your uneasy about doing it - don't. Do it with the engine off.

 

[prv]

I'm talking about a GM here 'cause that's what I've got.
[...]
The bit about the lift pump needing to be on the low part of the cam is right. If you get very little throw or nothing seems to be happening, turn over the engine 1/2 a turn or so and try again.

I've been meaning to ask about this. I've never got the lift pump on my 2GM20 to do anything at all by hand, whatever position the crank is in. I can flip the little paddle up and down a reasonable way, but it doesn't feel like it's doing very much and it doesn't cause any noticeable flow of fuel. Can they break that way, or am I just not being patient enough, or what?

I can bleed the engine fine by turning it with the handle.

Pete

 

[SamSalter]

you may not be moving the lever far enough. There is a slack portion in the movement and at the end of the flow it gets a little tighter. But be careful, don't be too aggressive.

The pump does have a very small capacity, so it does take a while to fill up a filter. Which is why many people fill the filters before installing them. Cuts down the number of strokes before fuel seeps out the bleed screw.

The lift pump has a "rubber" membrane that moves up and down as you move the lever. This sucks fuel in and along the line through 2 non-return valves. Lift pumps usually fail at the membrane. Then you would see your oil level rise and the oil would smell of diesel.

sam
C&C 26. Liquorice
Ghost Lake. Alberta

 

[CaptainBob]

How can operating the lift pump help you bleed the first filter, between diesel tank and lift pump?

My setup is:

Fuel tank -> CAV type water trap filter -> Lift pump -> Primary filter -> Injectors

If I opened the bleed screw on the CAV filter and operated the lift pump, I'd be sucking air into the system.

Only way I can think of to do it is to fill my fuel tank (which is below the CAV) right to the filler cap so the fuel level is above the CAV, then open the bleed screw and let gravity do the work.

Or am I missing another way?

 

[prv]

Only way I can think of to do it is to fill my fuel tank (which is below the CAV) right to the filler cap so the fuel level is above the CAV, then open the bleed screw and let gravity do the work.

Or am I missing another way?

Rubber bulb in the line between tank and filter. That's what I have, and it's great for filling the system as far as the engine lift pump, even when the hoses are completely empty from rebuild work.

Pete

 

[DownWest]

Slightly earlier someone was worried about loosening the injector nuts and fuel spraying out at high pressure. This is not a problem, it just ouzes out from the treads and back of the nut. The real risk is testing an injector by pulling it out and reconnecting to the HP pump and turning the engine. Putting one's fingers near the nozzle risks it blowing diesel through the skin. Very bad news for your health.
It is a quite normal injector test, but you need to know what you are doing.
A

 

[Heckler]

I used to have a Fiesta diesel car. That's all you did with that after changing the filter as there was no lift pump ( and no rubber bulb like the current Peugeot)

I used to fill the new filter with fuel before fitting and then crank and crank until it fired. With luck and a wide open throttle once it fired it would keep going. The secret was to keep it going once it fired until all the air was purged.

Even with the Pug I only use the bulb to bleed the filter.

In the earlier rotary injector pump it was as Sam says, you could bleed to the pump and then sometimes you had to bleed to the injector. In the case of the Pug with a Bosch VE pump there is an intermediate integral pump which can actually suck fuel from the tank and internally pass it to the hi pressure plunger. The bulb is in case the internal rotary lo pressure pup cant prime itself, there is a lo pressure relief valve which sends excess lo pressure fuel back to the tank along with entrapped air. Nowadays on the later Pugs, the in tank electric pump pumps to a hi pressure pump which then pumps to the hi pressure rail which is connected to the electronically actuated injectors. The hi pressure pump has a bleed off back to the tank, as does the lo pressure side this allows the system to bleed it self. Not necessary to have a bulb on these.
Stu
PS
a link to see how the VE pump works http://www.cs.rochester.edu/u/jag/vw/engine/fi/injpump.html, this is the pump that is fitted to the MD22

 

[SamSalter]

Capt. Bob,
Yeh, you're right! Now I think about it my tank is above the primary filter so that one get's done by gravity. It's been a while!
sam