PD Lift Pump Pressure

GoremanX

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2001 Audi A4 Avant quattro w/BHW TDI & 01E 6-speed
I recently put a TDI-specific in-tank pump in my 2001 Audi A4 with transplanted BHW to replace the old high pressure pump. Before starting the engine, I ran basic setting 035 to run the pump for 30 seconds and prime the system with the engine off. I have a 15 psi fuel pressure gauge on the line to monitor pump activity. To my dismay, when I ran basic setting 035, the gauge got pegged way beyond its max! The gauge ended up being damaged as a result and now rests at 6psi all the time.

So I switched out the gauge for a 200 psi diagnostic gauge and ran basic setting 035 again. Lo and behold, the gauge comes right up to 90 psi!!! That's the kind of pressure I'd expect to see from a gasser pump, not a diesel lift pump. Am I doing something wrong???

This is the pump I ordered and installed:
http://m.ebay.com/itm/231347988398?_mwBanner=1
 

1854sailor

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Bentley doesn't have a pressure spec for the in-tank pump, but the Tandem Pump's minimum output pressure should be 7.5 bar or ~109 PSI.
 

GoremanX

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2001 Audi A4 Avant quattro w/BHW TDI & 01E 6-speed
Make sure you've got fuel pipes correct way around. There's a one way check valve in the tandem pump. Top feed, bottom return
Good call, I did have them reversed coming out of the filter. However I'm still getting 80+ psi when I run basic setting 035.
 

GoremanX

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2001 Audi A4 Avant quattro w/BHW TDI & 01E 6-speed
Bentley doesn't have a pressure spec for the in-tank pump, but the Tandem Pump's minimum output pressure should be 7.5 bar or ~109 PSI.
The engine isn't running yet. I just want to be sure I have the right inlet pressure before I start anything. I'm pretty sure the consensus is that the lift pump should put out about 8 to 10psi. 80+ is a very far cry from that. This brand new tandem pump wasn't cheap, I don't want to break anything in it. Also, 80+psi in the fuel filter canister sounds like a recipe for trouble.
 
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Franko6

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I'm wondering why your fuel return line hasn't blown off...

The fuel coming in from the lift pump to the fuel filter is what is 8-10 psi. From there, the tandem pump raises the pressure more. At Idle, it's about 25 psi. At 2500 rpm, it's more like 100 psi, but it needs to be read with a dampened gauge, as it is very erratic pressure. The test point for the tandem pump on the arm that goes to the fuel return line. There is a plug with a copper washer on it.

I'm guessing that the return line is plugged at the in-tank fuel sensor/ pump unit. The incoming fuel used to have a check valve, but over time, it proved to be more trouble than worth. We drilled several of those out in older model ALH's. However, the return line has another one-way valve, but that one I think is more necessary. If it isn't there, your fuel filter will drain back to the tank. The valve can clog, and might be the first place to look.

Test by removing the return line that goes from the filter back to the tank. Pressurize the line to see it if flows. Remove the return line from the top of the in-tank fuel pump. It's got a plastic tab lock that you push in to release. Check for clog by blowing back through the line as it is removed from the fuel filter. Blow out one way valve at the lift pump. I hope that will take care of the problem.
 

GoremanX

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The fuel coming in from the lift pump to the fuel filter is what is 8-10 psi.
My gauge is right before the inlet to the fuel filter under the hood, so I'm only measuring fuel pressure from the in-tank pump.

Test by removing the return line that goes from the filter back to the tank. Pressurize the line to see it if flows.
It does not. There's a mere trickle from the return line after the fuel filter when I run the in-tank pump, and the pressure on my gauge just rises and rises. So it looks like I have a blockage somewhere, either because I have something installed backward or some more serious issue.

What regulates the pressure from the in-tank pump? Is it a restriction in the tandem pump? Or at the in-tank pump itself? Could I have somehow damaged the check valve in the tandem pump when I pressurized the system with the lines installed backward? Surely I'm not the first idiot to install the lines backward after a fuel filter change...
 

GoremanX

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I'm totally not winning here. There's no restriction anywhere in the inlet line or the fuel filter. But when I turn on the in-tank pump, pressure rises to 80+ psi and almost nothing comes out of the return line (which I've unhooked immediately after the filter). I don't know what else to check. I don't want to start the engine with the fuel system this way, and this is literally the last thing preventing me from starting the engine after over a year of hard work.
 

GoremanX

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2001 Audi A4 Avant quattro w/BHW TDI & 01E 6-speed
Before taking off the tandem pump (which is brand new and has never been used, I bought it and installed it a year ago), I found my old one and tried blowing through the fuel inlet. There's a small amount of resistance, and then I can blow right through without too much effort.

I tried the same thing on the new installed pump (using a new piece of fuel hose), and I can't blow through it at all. It's plugged right up.

I removed the installed pump, and sure enough, I can't blow through it. I tried some compressed air at 50psi, but it won't go.

The new, bad pump is a Pierburg. The old, still-working pump is a Bosch. The construction of the 2 seems very different in many ways. I wonder if the Pierburg has a more delicate check valve that somehow got jammed when I ran the in-tank pump with the fuel lines reversed.

I only bought a new one because the tandem pump is a pain to change on an Audi A4 and I didn't want to have to ever deal with it for the life of the car. Now it looks like I wasted my time and money. I'll just put the old pump back on and run with it for now.
 

GoremanX

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I decided to investigate further. The check valve on the Bosch pump is at the return line outlet. The check valve on the Pierburg pump is at the inlet. I jabbed a small screwdriver in there and felt a click. Now the Pierburg pump behaves exactly like the Bosch one; when I blow in the inlet, there's a small resistance and then I can blow through with minimal effort. So I guess I did jam the check valve when I pressurized the system with the lines reversed. A little surprised that 80+ psi of fuel pressure wasn't enough to dislodge it, though...
 

GoremanX

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2001 Audi A4 Avant quattro w/BHW TDI & 01E 6-speed
So I re-installed the old Bosch tandem pump, hooked up all my lines, started up basic function 035 again... AND IT DOES THE SAME THING!!! Pressure in the feed line cranks up to 80+ psi, almost nothing coming out of the return line! Am I on crack? How is this possible? I've checked every possible line and component! Has anyone ever actually checked the feed line pressure while running basic function 035?!? Maybe this is how it's supposed to happen when the engine isn't running?

To top it off, one of the small bolts on the tandem pump stripped the threads at about 8.3nm of tightness. Now I gotta pull the engine out again just so I can drill out that bolt hole and put in a thread insert.

I HATE TANDEM PUMPS ON LONGITUDINAL ENGINES!!!
 

GoremanX

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2001 Audi A4 Avant quattro w/BHW TDI & 01E 6-speed
Phew! Got the bolt hole fixed without pulling the engine. I stuck a too-long bolt into the hole, turned it until all the threads were completely stripped, then forced a tap in there without drilling first (no way I could fit even an angle drill back there). Used a ton of WD-40 for lubrication. The tap was really tough to turn, I was worried about ruining the new threads at some points, but with enough pressure it finally worked through to the bottom of the hole and made perfect threads for the insert. This probably wouldn't have worked on a cast iron part, thank goodness for soft aluminum! Cleaned out the hole thoroughly, threaded insert went in smoothly, and now I have a perfect bolt hole once again.

What a nightmare this simple task is turning out to be... meanwhile, still can't figure out why I'm seeing such high fuel line pressure, or which part of the fuel system is supposed to govern the pressure in the first place.
 

GoremanX

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2001 Audi A4 Avant quattro w/BHW TDI & 01E 6-speed
Ok, here's what I've done:

STEP 1
- unhooked feed line from the tandem pump, and put it into a bucket
- run basic setting 035 in VCDS
- in-tank pump runs for 30 seconds, pours into my bucket
- pressure never climbs, stays at 0 psi the whole time
diagnosis: there's zero restriction on the feed line all the way to the tandem pump, including through the fuel filter

STEP 2
- unhooked return line from the tandem pump
- attached a MightyVac vacuum/pressure gun to the end of the line
- pumped the MightyVac furiously for 10+ seconds
- pressure never climbed to more than 1 psi, if that
diagnosis: there's minor, irrelevant restriction in the return line, probably caused by the check valve at the in-tank pump which is supposed to prevent the line from draining itself while the engine is off. This includes the fuel temperature sensor and the thermo-T on the fuel filter.

So there's nothing wrong with my feed or return lines. This means one of two things:

- the tandem pump itself is supposed to regulate the feed line pressure, mine isn't doing it properly (neither the Bosch nor the Pierburg)

or...

- the lift pump is supposed to have some kind of bypass when the pressure reaches a certain maximum level, and mine isn't working right or doesn't exist
 

GoremanX

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2001 Audi A4 Avant quattro w/BHW TDI & 01E 6-speed
Finally figured it out after much research.

On most 2.0 PD TDIs, the fuel pressure in the feed line is regulated by the in-tank pump. It self-limits to about 10psi with a pressure release valve. None of these engines were used on Audi A4s.

PD TDI Audi A4s regulate the fuel pressure in the feed line "somewhere else". Could be in the fuel filter, could be in a diverter valve for an external fuel cooler, it depends on the model and engine. The in-tank pump, however, DOES NOT self-regulate. It'll build up pressure as high as it can as long as there's restriction. It typically maxes out around 90psi or so.

So I need to find a different solution.
 

GoremanX

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