priming fuel system troubles

[Alan Wheeler]

Post No.8.
Sorry, you just have a simple misunderstanding of my comment. The low pressure lift pump side of the IP does not force fuel through the IP to the injectors on it's own. The lift pump simply supplies fuel to the body of the IP. It is used to cool and lubricate the IP as well as be the supply to the injector pumping side. The injector pump itself, is a series of pistons that push fuel into the injector line at very high pressure. Give or take, about 300Bar. The pressure over comes the seat tension in the injector itself and the injector opens to allow the does of fuel through. The lift pump can not generate 300Bar of pressure for one, and two, the pump pistons has a valve system to allow the fuel in and keep it there. The rack determins how much fuel the piston is going to pump to the injector. The return line is a bleed from each injector and normally daisy chains off each injector. It is from a different side of the injector and allows the unused dose of fuel back to the tank. It must remain without any back pressure or it can stop the injectors from lifting. The return line is often led back to the tank, but in fact does not need to be. It is often commonly plumbed back into the low pressure suction side of the fuel supplying the lift pump. Usually connected at the Filter intake.
The above scenario is for the older more common Fuel injection systems. The modern common rail systems are completely different.

[drh1965]

Thanks Alan. That is how I thought it worked. I was relatively sure it was semantics.

[MidLandOne]

Originally Posted by drh1965

Thanks Alan. That is how I thought it worked. I was relatively sure it was semantics.

The key is that the engine has to be rotating (either running or being cranked) for the injector pump to pump. As it is a positive displacement pump the lift pump cannot pump through it to the injectors and then leak off back to the low pressure side.

The comments before were of the sense that this could happen with the engine stopped (one example - "If the engine isn't running, the ALL fuel returns to the tank through the return.") - that is not so, to get fuel flow on the injector side of the injector pump the engine has to be rotating.

[MidLandOne]

I am interested in learning (in fact I asked you for an explanation in an earlier post as I could not understand how fuel got through a stationary injector pump to which you essentially just said that it did, which was not particularly helpful). So in the interests of trying to learn this is a typical injector pump (those on common rail engines may be different, I would not know) - Robert Bosch type VE Diesel injection pump .

Now maybe there is some way through the porting that allows fuel flow through the injector pump when it is not operating. Another has said no, and looking at the attached description it would seem not but you say it does flow through yours. A confusion to me is that if there is no flow possible through the injector pump when it is stationary, then how does venting take place when one uses a manual priming pump, does it vent air/excess fuel befor or after the injector pump.

So how about an explanation from you or someone as to how it does that and if there is flow through the stationary injector pump by what route through the porting does it take? I'm willing to learn.

[MidLandOne]

Just to be able to be a bit more specific regarding my questions in my last post I have done a bit of checking.

For my own engine if one operates the manual priming pump it only primes as far as the fuel filter and air/excess fuel is vented to waste through the bleed screw on top of the filter mount. To bleed the injector pipework one has to release the banjo connection at the injector and crank the engine - the manual priming pump cannot pump through the statinary injector pump (I did not recall that until just now looking in the manual - the engine has always self primed).

I have also looked at references to several other engines on the internet and they seem to be the same with respect to manual priming.

QUESTION 1: Is this typical for non common rail engines?

QUESTION 2: Do some non common rail engines allow priming flow through the stationary injector pump (it seems so in the case of drh1965's engine?)?

QUESTION 3: If answer to Q2 is "Yes" then could someone explain the flow path through the injector pump or if a different pump design is used?

QUESTION 4: Are common rail engines different in respect to the responses to any of the above questions?

Thanks.

[Alan Wheeler]

Q1=A=Yes. At least in the size of engine we get to play with. "No Ties" has worked on much bigger stuff than I and maybe able to comment further. Pat may and a couple of others may also have. But normal Earth working size engines are the biggest I have got to play with.
Q2=A=No. umm....if I understand your question correctly. There is a bleed screw on the side or top of the IP. By pumping the lift pump, you should be pumping fuel and any possible air right through the lines and filters and air will bleed through that bleed screw. Once the air is gone, you nip up the bleed screw and as we say here in Kiwi Farming talk, "ya kick'er in the guts Trev".
If a leak is in the line between lift pump and tank, it is possible air can be drawn into the line. But that is very noticable in that you are constantly re-bleeding the IP.
Common rail systems are completely different. Much more like the fuel system on most modern day Petrol(gas) cars. A Tube (Rail) connects all the injectors. The injector is opened electricaly and the time opened controls the amount of fuel delivered. The pressure are scarily high. In the thousands of Bar in pressure.

Darryl, to answer your PM, I don't believe the Injector pump was ever removed. At least that is how I read it. The Oil pump was removed and refitted. Yes it could still be possible that fuel lines have been removed and refitted, but the fault described is very much the kind of fault that air in the injector line causes. Being that the original poster seemed to have done everything correctly in bleeding all the way to the IP and not so after the IP, I would naturally assume the problem will be in the injector line. If the fuel lines had been removed, one would assume air was in the IP and most likely will end up in the Injector line. Some pumps can handle air, like rotary pumps, so In line IP's can tolerate a little air and manage to get going although it's a ruff start and others simply will not fire at all.

[Alan Wheeler]

Perhaps if I add one more little snipet of information. Fuel is not pumped in a continuous cycle from tank through pump and back to tank. The return line is there for the injectors. The injector seat is lifted by fuel pressure. Let me go back a few steps.
The fuel Rack lifts and drops the IP piston. This allows more or less fuel to be pushed through the line. The pressure in the line rises and eventually overcomes the seat tension in the Injector and the seat lifts, allowing fuel to flow out the nozzle. As fuel flows the pressure drops away to the point where the injector seat closes once again and seals off the nozzle. However, this opening and closing is carried out by a simple yet complicated system inside the injector. OK, so now there is some remaining fuel sitting in the injector, that didn't have enough pressure left to squeeze through. That fuel has to be bleed off, or the next pump of fuel "locks" the injector shut. So the bleed off has to go somewhere. You don't want it in the bilge, so returning it to the tanks is what happens. Hence the name, return.

[MidLandOne]

Originally Posted by Alan Wheeler

...if I understand your question correctly.

You did .

Thanks for the response Alan.

John

[Westie]

Thanks everyone for all the suggestions. i am no diesel mechanic either (obviously!!) and really appreciate you folks taking the time to help. We did everything that was reasonable to ensure no air was in the system as I said before. I found a fellow by the name of Gerald Tobey at Blueridge Diesel in Salem Virginia when I was searching for parts. This guy is a REAL gem. I would say that if it wasn't for him I probably would have replaced the injection pump, injectors etc. as a few mechanics here had suggested. Gerald listened to the engine on the cell phone and talked us through the problem. Amazing man!! Below the injector pump on this engine is a small access plate that when removed exposes the "?rack? ". Sorry if the terminology is not correct! Basically a metal rod that slides back and forth as the throttle is advanced etc. Below this assembly on the side of the block is a fitting with double adjusting nuts, a hose that runs from this fitting to the top of the oil filter housing. I thought this was a by-pass in case the oil filter plugged. Not! When the throttle is advanced to full open position it is actually past full throttle to assist with the start. This fitting has a "plunger" within and this plunger is pushed out to contact the governor assembly within the block as oil pressure is developed - apparently pushing the throttle back to full open position as a maximum and preventing the engine from overrevving. I think!! Anyway, as there was little oil pressure before, the previous owner must have adjusted this plunger all the way in to get the engine to run. Would have been nice to know that!! Now with 40 psi of pressure developing as you turn the engine over before starting, the plunger extended out, pushed the governor and "rack" back to idle position and prevented a start. Gerald somehow knew this from listening to it - on the phone no less! Adjusting the double nuts to limit the plunger travel against the governor and - presto!! - an almost new engine!! Ha! Total cost was $171.03 including $80.64 for overnight shipping costs!! The replaced oil pump has this baby purring like new. This seems so simple once you understand. A manual would certainly have made this job a lot easier but none were available - surprise - Gerald has found one and is trying to get it for me! I wonder how many guys like me have been talked into new parts or even a new generator over simple things like this?? I can certainly tell everyone that Gerald's phone number has been moved to the top of my list and if I ever need to replace a genset or repower - Blueridge diesel is where I'm going!!

[DeepFrz]

Wow, Westie, great post. Thanks for posting your findings. Its great that your gen. is running like new. Congratulations.

[Charlie]

Wow Westie:

That is an amazing story. I'm glad it is fixed. I never would have thought to check the govenor. That is an amazing mechanic.

[Alan Wheeler]

exposes the "?rack? ". Sorry if the terminology is not correct!

Your spot on and so is the rest of your post. No one has suggested that, because as you said, not something anyone in their right mind would have played with. Really dumb on the part of the PO. In fact, down right stupid. Fancy adjusting the fuel to start the engine when the oil pressure is dangerously low.
The area you played with is the Cold start system for your engine. It allows an extra squirt of fuel to get going. If it was your main engine, you would normally activate this sort of thing by pushing your throttle hard forward and then back to the idle position before you turn the key. The oil pressure then releases the rack when the pressure builds. In a genset, this is usually activated by a start solenoid.

[drh1965]

Originally Posted by Alan Wheeler

Perhaps if I add one more little snipet of information. Fuel is not pumped in a continuous cycle from tank through pump and back to tank. The return line is there for the injectors. The injector seat is lifted by fuel pressure. Let me go back a few steps.
The fuel Rack lifts and drops the IP piston. This allows more or less fuel to be pushed through the line. The pressure in the line rises and eventually overcomes the seat tension in the Injector and the seat lifts, allowing fuel to flow out the nozzle. As fuel flows the pressure drops away to the point where the injector seat closes once again and seals off the nozzle. However, this opening and closing is carried out by a simple yet complicated system inside the injector. OK, so now there is some remaining fuel sitting in the injector, that didn't have enough pressure left to squeeze through. That fuel has to be bleed off, or the next pump of fuel "locks" the injector shut. So the bleed off has to go somewhere. You don't want it in the bilge, so returning it to the tanks is what happens. Hence the name, return.

Alan: I did some fuel system work this weekend and wanted to clarify something that you said above. On my Universal M18, there is not a continuous loop when the engine is not running under normal circumstances. However, at the base of the Injection Pump, there is a knurled knob that when unscrewed, allows fuel to flow through the path I described above (mostly). The part linked below is screwed in to the base of the injection pump. When opened, the nipple allows fuel to bypass the injection pump by being connected to the bleedoff/return line on the injectors. I proved this to myself by disconnecting the return line at the tank. As soon as the knurled knob is opened, the fuel pump clicks faster and fuel comes out the return. This is actually a great bleeding system IMO.

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