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  #76  
Old 03-31-2008, 09:33 AM
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That's what I mean. Once that 3 way banjo is disconnected at the secondary filter head, if the now open port on the head is not plugged, the supply side cannot build pressure.
Quote:
Originally Posted by Brian Carlton View Post
The banjo at the secondary.........not the banjo at the check valve.........
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  #77  
Old 03-31-2008, 09:56 AM
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Originally Posted by funola View Post
That's what I mean. Once that 3 way banjo is disconnected at the secondary filter head, if the now open port on the head is not plugged, the supply side cannot build pressure.
I'm not following.

If the three way is open, you've got one line from the check valve, one line from the injectors (high side) and one line to the cigar hose.

Why won't the low side build pressure under this condition?
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  #78  
Old 03-31-2008, 10:19 AM
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Maybe I do not understand how the priming pump is plumbed. I thought it is in series with the lift pump. If true, fuel has to go from priming pump/lift pump to secondary fuel filter, then to IP. If the filter head banjo port is left open, supply pressure to pump cannot build.

Do you have a detailed diagram of the fuel system? It would really help me visualize how it is suppose to work.

Quote:
Originally Posted by Brian Carlton View Post
I'm not following.

If the three way is open, you've got one line from the check valve, one line from the injectors (high side) and one line to the cigar hose.

Why won't the low side build pressure under this condition?
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  #79  
Old 03-31-2008, 11:07 AM
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Quote:
Originally Posted by JimSmith View Post
I believe the original design pump is a style of pump called a positive displacement pump. The pressure attained by the pump is a result of the rate at which you operate the pump, which you should be able to notice as you increase the stroke rate - the resistance goes up. Each full stroke pushes the same quantity of fluid, thus the positive displacement label. The increased resistance comes from forcing more fluid against the system resistance in a given time
You find with the old style pump achieving the pressure to pop the check valve takes a bit of effort, but it is not something that will give you a strain or cause you to break out in a sweat. I have no idea what the new pump is as I have never installed one. Jim
I believe you have incorrectly mixed some definitions and cause/results descriptions...
If you will change your example to a hand air pump ( also positive displacement) it will be clearer. On a positive displacement pump one stroke puts a certain amount of air into the tire. Assuming good check valve the SPEED at which the strokes are made does not affect the tire pressure reading ( ignore the heat/air expansion effect...which will go away when the temperature stabilizes)...increase.
A system in which the speed of input is working with something else... perhaps an output ( or leak ? ) of a certain size which causes the input rate to need to exceed that output by a certain amount... would make a positive displacement pump output look like the effects which are usually reserved for something like a fan or impeller output...
The pressure goes up because the input increases but the leak or specified output stays about the same.

A positive displacement pump outputs a certain amount per stroke.

The pressure attained by a positive displacement pump is not changed by the RATE of stroking.

The pressure in the overall system may be increased but that is NOT a function of the fact that it is a positive displacement pump.. it would happen with a pump in which the output was variable due to rpm also... typical of impellers, fans, etc....
The difference in pressure comes from increasing the amount of input compared to the leak or specified output... but this is not caused by the fact that a positive displacement pump is being used.

Since we are being technical here.... the pressure inside the positive displacement pump temporarily is increased only due to the normal design of pumps.. that they are usually working in conjunction with a check valve and it is usually a bottleneck in terms of flow. If you visualize an open to the air cylinder with a piston in it... which is a positive displacement pump... you can see that only at bullet speeds could you measure an increase in the pressure ahead of the piston...and that only microsecond in duration.
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  #80  
Old 03-31-2008, 11:26 AM
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Quote:
Originally Posted by funola View Post
Maybe I do not understand how the priming pump is plumbed. I thought it is in series with the lift pump. If true, fuel has to go from priming pump/lift pump to secondary fuel filter, then to IP. If the filter head banjo port is left open, supply pressure to pump cannot build.

Do you have a detailed diagram of the fuel system? It would really help me visualize how it is suppose to work.
I don't believe that the screw that secures the three way banjo will extend into the secondary filter. This cannot be the situation, otherwise the return system would be directly plumbed with the supply system and the IP would never build pressure.

It must be possible to build pressure with the three way return banjo fitting removed from the top of the secondary.
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  #81  
Old 03-31-2008, 11:44 AM
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That's why it is so confusing without seeing a diagram. Is the 3 way banjo mounted on the filter head for convenience and not connected to the fuel filter plumbing wise? Since it is mounted at the highest point on the filter head, I thought it was for bleeding air from the IP and the filter -back to the tank. If true, there must be another check valve in the filter head. Anyone got an extra filter head I can have or buy? I really like to take a closer look at one.
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  #82  
Old 03-31-2008, 12:05 PM
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Quote:
Originally Posted by funola View Post
That's why it is so confusing without seeing a diagram. Is the 3 way banjo mounted on the filter head for convenience and not connected to the fuel filter plumbing wise?
That is my understanding of the setup. From an engineering perspective, I don't see any other possibility.
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  #83  
Old 03-31-2008, 01:50 PM
Dionysius
 
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From my inspection here is the entire low pressure fuel circuit.

The three way banjo is only a mechanical mount point with no plumbing connection into the filter. If I am wrong I will dance a jig.

Now get ready for a journey my friends...........

Consider you are a molecule of diesel fuel about to leave the Fuel Tank which is vented to assure your continued ability to leave. Here is the path:

Vented Fuel Tank
Fuel Tank Screen Filter
Fuel Supply line
Primary filter
Check valve under the Priming Pump
Priming Pump pressure stroke can kick you in the ass here if stroked
Lift Pump check valve
Lift Pump kicks your ass here if cranking is enabled
Enter Secondary filter outer annulus
Squeeze through Secondary filter porous medium
Enter Secondary filter inner cylinder
Exit the Secondary filter via the brown tube to IP
Arrive at IP
Either get pulled up into the High Pressure side and be combusted or continue as excess back through the IP Check Valve
Making it through the check valve means you had 7psi plus of pressure to defeat the spring
Continue to the three-way banjo bolted to the secondary filter
Join with your brothers which were survivors from the Hi Press circuit and did not enter the cylinder of hell fire
Enter the cigar hose
Enter the metal return line to the tank
Enter the vented tank
Receive accolades from your brethern back at the tank for having survived yet another cycle
Wait to be called again and test your luck.



Now guys.....please tell me if you agree or disagree because I am trying to learn and these threads can be frustrating as direct tit-for-tat feedback is not often given.

I will never be offended except when I cannot improve my knowledge base.

Also go check my new thread on bleeding procedure since it is a derivative from this thread as suggested by Leathermang. I also want your ideas there.
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  #84  
Old 03-31-2008, 01:56 PM
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I think in your description you should say... like when going out the bottom of the fuel tank,, go through the 46 mm locking nut and then the 6 inch rubber fuel line to the metal line which then proceeds as you have described.... some of those things will give others ideas as to how they can address intermediate situations... I am not sure of those sizes... just giving the idea....it can sure help when someone starts under the car.... sorta like the great ' how to keep your VW alive for the complete idiot' ..by Muir... it told all those practical things ...
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  #85  
Old 03-31-2008, 02:56 PM
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Please look at post #48. How do you explain when I had a 15 psi pressure gauge plumbed to the 3 way banjo port on the fuel filter head and as I cranked the engine, I saw pressure rise of 2 to 3 psi? Was I high on crack and seeing things?

"Quote:
Originally Posted by funola View Post
That's why it is so confusing without seeing a diagram. Is the 3 way banjo mounted on the filter head for convenience and not connected to the fuel filter plumbing wise?

That is my understanding of the setup. From an engineering perspective, I don't see any other possibility."
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  #86  
Old 03-31-2008, 03:18 PM
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Quote:
Originally Posted by funola View Post
Please look at post #48. How do you explain when I had a 15 psi pressure gauge plumbed to the 3 way banjo port on the fuel filter head and as I cranked the engine, I saw pressure rise of 2 to 3 psi? Was I high on crack and seeing things?
Yep, that's a problem. If fuel escapes to the bypass lines via the secondary filter, there must be some type of flow restriction at that banjo bolt to allow a very slight amount of fuel to pass during normal operation..........otherwise pressure could never build in the IP.

Naturally, if you've got no flow, the reading on the gauge will be the full line pressure to the fuel pump.

It's a strange system........for sure...........but, that location cannot have the same pressure as the supply line to the IP during normal fuel pump operation.

What it does tell you, however, that pumping the hand pump is a very inefficient process because the flow is quite low and a good bit of that fuel could go right out the bypass line at the secondary. This would make an excellent argument to plug that port (as you have done with the gauge) when trying to bleed a system that is air bound.
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  #87  
Old 03-31-2008, 03:58 PM
Dionysius
 
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Quote:
Originally Posted by funola View Post
Please look at post #48. How do you explain when I had a 15 psi pressure gauge plumbed to the 3 way banjo port on the fuel filter head and as I cranked the engine, I saw pressure rise of 2 to 3 psi? Was I high on crack and seeing things?
Funola: I thought I would get some attention here. It would appear, from your experiment, that there is a small diameter relief port plumbed into the filter head connecting to the three-way but without the casting in my hand I am at a loss.

I am not going to disable my car to examine it but someone out there must have either a diagram or an actual casting.

The only reason to do this that I can see is to offer an early relief point for air bleed. My money says that if you plug this orifice you will not notice any difference. Why would a designer offer an early return path and shoot himself in the foot. Beats me.

This is how we learn together folks. I am happy now. I danced a jig.
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  #88  
Old 03-31-2008, 04:18 PM
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Dionysius, Maybe I was seeing things? When I have time, I will pull the 3 way banjo and wipe the filter head "banjo port" clean of fuel and have a look and see what's really in there.

Quote:
Originally Posted by Dionysius View Post
Funola: I thought I would get some attention here. It would appear, from your experiment, that there is a small diameter relief port plumbed into the filter head connecting to the three-way but without the casting in my hand I am at a loss.

I am not going to disable my car to examine it but someone out there must have either a diagram or an actual casting.

The only reason to do this that I can see is to offer an early relief point for air bleed. My money says that if you plug this orifice you will not notice any difference. Why would a designer offer an early return path and shoot himself in the foot. Beats me.

This is how we learn together folks. I am happy now. I danced a jig.
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  #89  
Old 03-31-2008, 04:29 PM
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I was in frantic mode fearing I had a bad IP and tried all the suggestions given by forum members and tried so many different things that I forgot about this which I reported in post # 48

"I then put the check valve back in and cranked more, pressure went up to about 8 to 10 psi and it fired up!!! More later."

There is no mistake about that one. That 3 way banjo port on the filter head IS connected to the filter and not just a deadheaded mount for the 3 way banjo.

Dionysis, dance the jig some more for us.
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  #90  
Old 03-31-2008, 05:07 PM
Dionysius
 
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OK I'm a dancin'

Now some sage one please tell us what is the purpose of that orifice.

Maybe to act as a "fuse" in the event of total blockage of the Primary Filter so as not to deadhead the lift pump.

I am scraping the bottom of the bucket guys. Anybody from MB out there??

Anybody improve on my theory.
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