Yanmar Fuel Priming Woes

[SoggyHalyard]

I have a Yanmar 4JH2TE. When I get air in the lines, I can never get the injectors primed manually. I can the engine secondary filter primed easily using the bleed screw but it seems like I can't get past the high pressure pump. I use both the manual lever on the lift pump, and also I have an inline low pressure electric pump. Neither get fuel past the high pressure pump. In the past, the only way I get the injectors primed is by using the starter with the injector nuts backed off.

I can't find any reference of this situation in the manual. Does anyone else have this problem?

[Cheechako]

Can you start it? Just do so and crack the nut on the injectors.

Using the starter with the injector nuts backed off is the only other way I have ever done it.

[SoggyHalyard]

Yep, I get it started no problem. I just like the idea of using my low pressure fuel pump to prime the injector lines more than cranking the starter and couldn't find any reference to why that wouldn't work.

[donradcliffe]

The HP injector pump is a separate pumping system. You first bleed up to the inlet of the HP pump, then crack the injector lines at the injectors, open the throttle wide, and crank until you see solid fuel spurting at the injectors. If the engine will start after you get solid fuel to the HP pump inlet, the injectors will usually self bleed.

More importantly, why is there air in your fuel lines and where is it coming from?

[SoggyHalyard]

Thanks for that explanation. So no manual priming of the high-pressure lines. Makes sense given my efforts.

Can anyone explain why you can’t prime through the high pressure pump?

As to how I found air in my lines, when the boat was new to me, I ran one of my fuel tanks dry. More recently, I had my injectors, professionally serviced.

[Cheechako]

Originally Posted by SoggyHalyard

Yep, I get it started no problem. I just like the idea of using my low pressure fuel pump to prime the injector lines more than cranking the starter and couldn't find any reference to why that wouldn't work.

Yeah, The fuel injection pump is, of course, high pressure, but you could fill the lines I suppose with low pressure if the outlet in the high pressure pump is open to each line. But you're still going to want to bleed the air after I think. I dont really know the intricacies of the high pressure pump, but imagine it has to close the outlet to the fuel lines in order to build high pressure then cycle each one open? If so, some are blocked when you try to use the low pressure to fill the lines.
Have I got that about right ya'll?

The lines seem to fill so fast when you cycle the engine that I just never saw a need for it, although I often had an electric pump in the fuel line.

[Wotname]

Originally Posted by SoggyHalyard

Thanks for that explanation. So no manual priming of the high-pressure lines. Makes sense given my efforts.

Can anyone explain why you can’t prime through the high pressure pump?

..........

Cheechako has is right.

Think of it this way.
The low pressure side operates at a few psi (less than 10) which is provided engine lift pump.
The high pressure side operates at greater than 20,000 2,000 psi.
Obviously the low pressure side has to be keep separated from the high pressure side as the low pressure side could not withstand pressure.

It is kind of similar to how the air intake of a combustion engine is keep separated from the combustion chamber pressure by the inlet valve during the compression stroke. Once the piston starts to compress the air in the cylinder (compression stroke), you can't try to push air into the chamber from the low pressure (atmospheric side) via the inlet manifold; the inlet valve prevents it. This is not an exact explanation of the injection pump but the principles are similar.

EDIT - Correction - The high pressure side of a mechanical injection pump is around 2,000 to 2,500 psi

[leecea]

I think with a mechanical system the high-pressure pump is opening and closing valves to control the injectors, so the low pressure pump can't force fuel through to the injectors.

Coincidentally, I recently had the high pressure pump fail on my car. In that case, I could limp it to the dealer as long as I used very little throttle because it was able to run (barely) off the low pressure pump. Car injectors are controlled electronically.

[Cheechako]

Originally Posted by leecea

Cheechako, I think you're right. I'm not an expert, but I think with a mechanical system the high-pressure pump is opening and closing valves to control the injectors, so the low pressure pump can't force fuel through to the injectors.

Coincidentally, I recently had the high pressure pump fail on my car. In that case, I could limp it to the dealer as long as I used very little throttle because it was able to run (barely) off the low pressure pump. Car injectors are controlled electronically.

I always figured they were similar to a rotary or piston type of hydraulic pump, which I understand a bit. But able to send pressure to multiple outlets.

[Wotname]

Originally Posted by Cheechako

I always figured they were similar to a rotary or piston type of hydraulic pump, which I understand a bit. But able to send pressure to multiple outlets.

There are a couple of common types.

Typically the inline type has a seperate piston pump for each cylinder i.e. 2 cylinders, 2 identical pumps; 3 cylinders, 3 pumps etc. Each pump is actuated by a separate cam on the camshaft. The metering of the fuel is achieved by having the inlet side of each pump connected together by the fuel rack i.e. the rack moves the metering orifice of each pump together.

The rotary type has only one pump and a rotating vane to deliver each pulse of fuel to the correct cylinder - all mechanically driven.

I dunno about common rail pumps though - they are magic - operating at pressure around 30,000 psi with electronically operated valves.

[SoggyHalyard]

30,000 psi! Jeeze! Thanks for the explanations, y'all. This makes much more sense.

[donradcliffe]

Originally Posted by SoggyHalyard

Thanks for that explanation. So no manual priming of the high-pressure lines. Makes sense given my efforts.

Can anyone explain why you can’t prime through the high pressure pump?

As to how I found air in my lines, when the boat was new to me, I ran one of my fuel tanks dry. More recently, I had my injectors, professionally serviced.

If you have a functioning fuel system, once you bleed the low pressure side, you won't have to do it again. However, if you have to bleed it often, you have air leaking into a part of the system which is at a vacuum.

[bony]

Whenever I'd bleed my yanmar 2qm15 I could never get it done by bleeding the injectors alone, it just wouldn't work. I found that by cracking the two fuel line nuts on top of the injector pump was the best way to bleed the lines.

You can find a diagram of the said two nuts at reply no. #6 in this old thread -

https://www.cruisersforum.com/forums...15-155140.html

I'd first bleed the air out of the filter housing, then crack and bleed the two fuel line nuts one at a time. After that the engine would start without me having to bleed the injectors at all. I believe in my engine a pocket of air is trapped at the injector pump which can't be bled out by just cracking the injector nuts alone.

Just thought I'd mention this in case it's of use to somebody.