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Fuel pump testing sequence

Old 10-10-2018, 05:44 PM
  #16  
FredR
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Originally Posted by merchauser View Post
that is partly what I was looking for. the cut was near razor clean, and not completely thru, so I assume, depending on smoothness of the road,
acceleration and pitch of the car, and cornering, the delivery would be varied. at times the cut could be near closed, and other times more open.

I am trying to discern, is whether fuel pressure to the rails was also varied???

FWIW: surprisingly, the in tank hose still feels supple??
The small head developed by the in-tank pump will be lost through the fracture -the hose cannot hold any pressure once it is breached
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Old 10-10-2018, 06:46 PM
  #17  
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Once the hose on the in-tank pump splits anything entering the variable-width-due-to-movement-and-big-bumps-in-the-road crack in the hose bypasses the intake filter.

So, debris sucked through the crack, or pieces of the 'mini-hose' itself get sucked into the main pump.

If your in-tank pump works then:
- thank your favorite deity/deities.
- buy the repair kit from Roger and use it.
- reverse the main fuel pump and see if you can it to 'spit up'
- mark your calendar to service the in-tank pump in 5 - 7 years(*) to inspect the hose.

As for gestation period: my data suggests 9 years, almost to the day, for an in-tank pump to spit crap into the main pump with E10 gas.

(*) Unless data collected by the Rennlist masses 'beta testing' this repair suggest otherwise. This repair hasn't been available for all that long. It should work. Greg thinks it will work. But, only time will tell.
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Old 10-10-2018, 07:20 PM
  #18  
merchauser
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[color=left=#222222]Once the hose on the in-tank pump splits anything entering the variable-width-due-to-movement-and-big-bumps-in-the-road crack in the hose bypasses the intake filter.[/color]
thanks dave

[color=left=#222222]So, debris sucked through the crack, or pieces of the 'mini-hose' itself get sucked into the main pump.[/color]
I am confused about what is happening here, because you have the in tank pump picking up fuel thru the filter, and the hose is on the other side of the pump.
so the in tank pump is just pumping fuel back into the tank? is that correct?

then the main pump is sucking from the tank, but what effect does that have on the fact that there is recirculating fuel in the tank?? does this diminish the fuel
pressure at the rails?

I have ordered all needed bits from roger, and since I had the original 28 year old green pump, I decided to get a new one. so everything should be fine in a few days
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Old 10-10-2018, 10:19 PM
  #19  
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Originally Posted by merchauser View Post
I am confused about what is happening here, because ...
The in-tank pump's purpose is - essentially - to prime the main pump. Fred, DrB, et. al. may remember the exact figure, but the in-tank pump supplies IIRC 5 PSI max. This does not increase, in a meaningful way, the output pressure of the main pump. If you model this as incompressible fluid flow the main pump intake pressure is meaningless because of the pump design. If, however, you model this, realistically, as compressible fluid flow - and gas is actually more compressible that water, but not much - then there is a tiny increase in main pump output pressure.

Keep in mind that rail pressure is actually controlled via the fuel pressure regulator at the point where fuel is returned to the tank and via the two pressure dampeners. The pump supplies 'naturally' 52-58 PSI. The regulator and dampeners keep the steady-state working pressure at 48 psi. So, any tiny decrease in main pump pressure doesn't effect working pressure. The 'head room' in pressure does have a use but that's getting off topic and is a longer discussion. (BTW, all of this is covered in the Probst book that I'm sure I provided a link for a while ago.)

Last, these values are for 87+, older 928s have different working pressures.
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Old 10-11-2018, 02:04 AM
  #20  
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FWIW, my car never had an in-tank pump, just the external unit, and it worked fine for 10 years in Saudi, at up to 120F temps. It also provided a higher pressure than L or LH systems, being CIS running at 75psi. In my hands it did this with an S4 pump that isnt usually called on to produce more than 48psi as above. It might be useful to know where you are, so that others in your area with your model might advise their experiences with or without an in-tank pump.
jp 83 Euro S AT 57k
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Old 10-11-2018, 09:24 AM
  #21  
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[color=left=#222222]Keep in mind that rail pressure is actually controlled via the fuel pressure regulator at the point where fuel is returned to the tank and via the two pressure dampeners[/color]
when these fail, do they tend to decrease or increase pressure and/or volume?
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Old 10-11-2018, 10:49 PM
  #22  
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I doubt volume would be affected unless the leak was large, but pressure in the rail will go down, but not much. The biggest effect of a leak will be to richen the mixture as the leaked fuel is sucked into engine.
jp 83 Euro S AT 57k
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Old 10-11-2018, 11:41 PM
  #23  
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Originally Posted by merchauser View Post
when these fail, do they tend to decrease or increase pressure and/or volume?
The dampeners consist, internally, of a diaphragm connected to a vacuum source. There may or may not be a spring involved. I've never dissected one. (Although, I do have a dead one on-hand and might do that.) When vacuum connected the diaphragm restricts fuel flow (and as a result fuel pressure since fuel is relatively incompressible.) As above the purpose of the dampeners is to reduce pressure in the rails during steady state engine operation.

The regulator also has a check valve that ensures one-way flow through it. It also has a mechanism - perhaps the same check valve, perhaps diaphragm related too(*) - that maintains pressure after the engine is stopped. (* again, haven't dissected one, but have a dead one on hand.)

Failure of the diaphragms results in fuel being sucked into the intake via the vacuum lines. This will cause super-rich engine operation at low-loads (idle, part throttle.)

Failure of the regulator's check valve results in immediate loss of rail pressure when the engine stops. This leads to hard-starts and vapor lock.

There's also the possibility of damage to the housing and external fuel leaks. But, those things can be mashed pretty good and not leak.

There are possibly other failure modes that I don't remember or never knew about.

The most common and only mode I've encountered is the busted diaphragm and external damage. However, failure of these parts does not seem very common so far.

Originally Posted by jpitman2 View Post
I doubt volume would be affected unless the leak was large, but pressure in the rail will go down, but not much. The biggest effect of a leak will be to richen the mixture as the leaked fuel is sucked into engine.
jp 83 Euro S AT 57k
Yes. Remember that the dampeners actually reduce rail pressure during steady-state operation. Steady state fuel pump pressure is only seen in the fuel rails when manifold vacuum is lost. In this context I will ignore transient increases in rail pressure due to the first derivative of manifold vacuum. Thus failure of the dampener's diaphragms will result in a fuel leak to the vacuum source, but will not - I strongly suspect - result in an easily measurable decrease in steady-state rail pressure.
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Old 10-12-2018, 07:12 AM
  #24  
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^^^^^^^ thank you dave

is it possible to replace the in tank hose without detaching the wires?? wishing......hoping...…
something about hot wires submersed in gas makes me uneasy.....
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Old 10-12-2018, 07:23 AM
  #25  
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Originally Posted by merchauser View Post
^^^^^^^ thank you dave

is it possible to replace the in tank hose without detaching the wires?? wishing......hoping...ů
something about hot wires submersed in gas makes me uneasy.....
Yes but note it does not take much to detach the cables. On the other hand they are relatively easy to solder. Cables submersed in petrol are no issue. The bits that spin are made of plastic [non sparking] and the motor is a sealed unit cooled by the flow of fuel around it.
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Old 10-12-2018, 07:32 AM
  #26  
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Yes but note it does not take much to detach the cables. On the other hand they are relatively easy to solder. Cables submersed in petrol are no issue. The bits that spin are made of plastic [non sparking] and the motor is a sealed unit cooled by the flow of fuel around it.
what I was looking for. thanks fred
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Old 10-12-2018, 09:21 AM
  #27  
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DO NOT disturb the electrical leads between the two sections of the intank pump.

It is easy to replace the in-tank hose section without damaging the leads.
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Old 10-12-2018, 11:42 AM
  #28  
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[color=left=#222222]DO NOT disturb the electrical leads between the two sections of the intank pump.[/color]=left
=left
[color=left=#222222]It is easy to replace the in-tank hose section without damaging the leads.[/color]
I like this idea a lot better!!

do I need to apply soap or WD to help the hose onto the fittings?
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Old 10-12-2018, 12:20 PM
  #29  
Rob Edwards
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No WD on rubber, get some proper assembly lube. I like P80-THX.

https://p80lubricants.com/request-a-p-80-sample-kit/
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Old 10-12-2018, 01:03 PM
  #30  
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Originally Posted by merchauser View Post
what I was looking for. thanks fred
My post was advising to be very careful with the cables as they break off very easily- the flip side is that they are not too difficult to solder back on if they do inadvertently get disturbed.
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