Tandem Pump pressure under load?

martin33100

Veteran Member
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Oct 13, 2015
Location
UK
TDI
VW T5 2.5
Hi guys, just wondered what kind of pressure a PD tandem pump should deliver under load, I have read somewhere it's around 7.5bar at 2000rpm.
I had my T5 2.5 dyno'd a week ago as I had fitted a new ex manifold and now the AFR is very lean.
At full load and higher RPM the fuel pressure drops to around 4bar but I never looked at it in detail when the AFR was fine so I don't know if this is low or not?.
One of my digital gauges picks up tandem pump fuel pressure but I may double check it with a gauge.
 

[486]

Top Post Dawg
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Mar 1, 2014
Location
MN
TDI
02 golf ALH
How is inlet pressure to the tandem pump? If it is going into vacuum you can't rightly blame the tandem pump itself. Might be time for a bigger in tank pump, filter and lines.

it should be around/above 100 psi rail pressure, yes. Otherwise you don't get good element filling and it stops making power.
 

martin33100

Veteran Member
Joined
Oct 13, 2015
Location
UK
TDI
VW T5 2.5
How is inlet pressure to the tandem pump? If it is going into vacuum you can't rightly blame the tandem pump itself. Might be time for a bigger in tank pump, filter and lines.

it should be around/above 100 psi rail pressure, yes. Otherwise you don't get good element filling and it stops making power.
I will disconnect the tandem pump feed hose and force on the in tank lift pump via vagcom and measure what's comeing out.
I am guessing it should hold this pressure when under full load?, it holds 100psi when reved and when driving normally but it drops off when you are on full load.
I am going to put on a oil filled gauge to monitor the pressure, currently using one of the digital gauges on my dash but even though there is a snubber valve fitted the fuel pressure still changes very quickly.
 
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Festa

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Aug 14, 2016
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South east uk
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Vw t5 van
Might be time for a bigger in tank pump, filter and lines.
What would you suggest? any recommendations?

The problem with this 5 pot engine is the restrictive nature of the port so they don't flow well :( It's also a fact that the single fitting of the injector to the head is a weak spot as the injector rocks in it's bore and so suffers from fuel in oil.
Have you sent away another oil sample for testing before looking else where Martin?
I know you have another tandum pump that you could try :)
 

martin33100

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Location
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VW T5 2.5
What would you suggest? any recommendations?

The problem with this 5 pot engine is the restrictive nature of the port so they don't flow well :( It's also a fact that the single fitting of the injector to the head is a weak spot as the injector rocks in it's bore and so suffers from fuel in oil.
Have you sent away another oil sample for testing before looking else where Martin?
I know you have another tandum pump that you could try :)
Yes I do have my original tandem pump but the current one that is fitted is the one I got from you.
The OE VW one is LUK but you can buy the Pierburg ones for around £270.
The fuel pressure under load may have always been slightly low but I honestly can't rember what it was, I only started looking at it recently after my van was dyno'd and had a very lean AFR, the only part that has been changed is a new ex manifold made of slightly thicker material.
 

[486]

Top Post Dawg
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Location
MN
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02 golf ALH
What would you suggest? any recommendations?
I'm talking out my ass, captain. Never worked on any PD motor.
Just trying to help along the diagnostic process, as the guy with his hands on the car's really the only one that can find his own issues.

I'd ditch the tandem pump and run an EFI pump in the tank from a gas motor golf/jetta run into a bosch 044 to bump the pressure up, like fords used to do.
External pressure regulator holding 100-150 psi in the rail, if one isn't already integrated into the return.
but that's my woefully uneducated solution to a problem he may well not even be having
 

martin33100

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Location
UK
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VW T5 2.5
I'm talking out my ass, captain. Never worked on any PD motor.
Just trying to help along the diagnostic process, as the guy with his hands on the car's really the only one that can find his own issues.
I'd ditch the tandem pump and run an EFI pump in the tank from a gas motor golf/jetta run into a bosch 044 to bump the pressure up, like fords used to do.
External pressure regulator holding 100-150 psi in the rail, if one isn't already integrated into the return.
but that's my woefully uneducated solution to a problem he may well not even be having
I ran a Bosch 044 on my Westfield track car but the fuel pressure was only 4bar, also a tandem pump is required to run the vacuum system for the brake servo and turbo boost control.
 

[486]

Top Post Dawg
Joined
Mar 1, 2014
Location
MN
TDI
02 golf ALH
I ran a Bosch 044 on my Westfield track car but the fuel pressure was only 4bar, also a tandem pump is required to run the vacuum system for the brake servo and turbo boost control.
My thought is feed the 044 with another pump in the tank.

All the 044 sees is inlet and outlet pressure, no atmospheric reference, so it's gotta just add that 4 bar or thereabouts atop what you feed it.
Electric vacuum pumps are easy to find in the junkyard. Heck, some mk4's even came with them mounted down by the steering rack.
 

martin33100

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Location
UK
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VW T5 2.5
If the fuel system is working correctly then the Tandem pump should have no issues in supplying a good fuel pressure under load.
When my van was first mapped 15 months ago by Bobby the AFR was spot on, now it is very lean and all I have changed is my exhaust manifold for one made of thicker material but the ID may have gone up by 1mm.
I am sure my van held better fuel pressure under load before but I can't be sure what it was.
 

Festa

Veteran Member
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Location
South east uk
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Vw t5 van
If the fuel system is working correctly then the Tandem pump should have no issues in supplying a good fuel pressure under load.
When my van was first mapped 15 months ago by Bobby the AFR was spot on, now it is very lean and all I have changed is my exhaust manifold for one made of thicker material but the ID may have gone up by 1mm.
I am sure my van held better fuel pressure under load before but I can't be sure what it was.
When was the last time you changed the filter? the fuel isn't as good as it used to be:( it could just be blocking up and restricting flow
 

martin33100

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Location
UK
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VW T5 2.5
When was the last time you changed the filter? the fuel isn't as good as it used to be:( it could just be blocking up and restricting flow
The fuel filter was changed Jan 2017, I will check the flow over the next few days.
 

[486]

Top Post Dawg
Joined
Mar 1, 2014
Location
MN
TDI
02 golf ALH
If the fuel system is working correctly then the Tandem pump should have no issues in supplying a good fuel pressure under load.
When my van was first mapped 15 months ago by Bobby the AFR was spot on, now it is very lean and all I have changed is my exhaust manifold for one made of thicker material but the ID may have gone up by 1mm.
I am sure my van held better fuel pressure under load before but I can't be sure what it was.
there you go, as said, you're the only one that can do the diag

stick a pressure gauge on the tandem pump inlet
go for a rip
no inlet pressure when loaded means supply issue (filter, the inlet sock in the tank, lift pump, gelled fuel, etc...)
small pressure drop when loaded means issue is elsewhere
 

martin33100

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Location
UK
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VW T5 2.5
It may have been a faulty tandem pump leaking oil into the fuel return, cut the old filter open and it was full of oil.
 

[486]

Top Post Dawg
Joined
Mar 1, 2014
Location
MN
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02 golf ALH
fuel in video does not look oil contaminated, one would think it would be in the return fuel to the tank if it were in the fuel rail, not knowing the details of the fuel loop.

maybe algae in tank?
 

martin33100

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Oct 13, 2015
Location
UK
TDI
VW T5 2.5
fuel in video does not look oil contaminated, one would think it would be in the return fuel to the tank if it were in the fuel rail, not knowing the details of the fuel loop.

maybe algae in tank?
It does look like oil and carbon in the filter, the fuel return from the cyl head runs back through the filter because it has an in built thermostat. This thermostat doesn't allow the fuel to return to tank when the fuels is cold in winter.
As it's been cold recently the fuel return would have been going back through the filter and back into the tandem pump hence the oil in the filter.
I had a spare tandem pump so swapped them yesterday.
I know a guy that owns a VW garage and he said it's actually quite common to get this problem with failing tandem pumps if the vehicle has a 4 connection fuel filter with built in thermostat.
 
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