Testing Tandem Pump

S

spike

Well-Known Member
A friends A2 TDi (2004 model) has got starting problems, with all the symptoms of a leaking fuel filter drain plug. First start of the day and most times it cranks over (and over) but does not fire up. He's started to keep the tank at least half full and parks 'nose down' which helps a lot.
I've not personally checked everything but have been assured the battery and starter are good and there are no fuel leaks or smell of diesel but the symptoms suggest an issue with fuel supply while cranking.
There are a couple of tests which don't sound too difficult but was wondering if anyone had already tried them -

Tandem pump fuel pressure while cranking
Pressure test fuel line from tandem pump to fuel tank
Vacuum test fuel supply line.

I'll add more posts with the test details

Cheers Spike
 
Testing Tandem Pump fuel pressure
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Test port thread is M12 x 1.5 Picture shows a banjo bolt but there seems to be plenty of room for a straight fitting for pressure gauge hose
 

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Pressure testing fuel supply line for leaks
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Same test port is used on tandem pump
Will try and rig up my push bike track pump to pressurise the system
Need some method of blanking off the two lines at the fuel tank (may turn up some blanking plugs to insert into the quick connectors)
 

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Last edited:
WANTED TDi fuel tank sender unit (the float level switch module that goes inside the tank)
Looking for an old or non working sender unit to have a play with as part of my investigation into poor starting on my brother in-laws car.
He's had the car checked over, a new fuel filter and the tandem pump changed by a VAG specialist but it's still reluctant to fire up after standing for a while.
I've advertised in the 'WANTED' section but realised if anyone has a broken unit sitting around they won't be looking there
Happy to pay a few quid plus postage for it

Cheers Spike
 
Hi Spike,

As far as I know, the A2 doesn't have a non-return valve on the line into the pump. Therefore there is nothing to prevent the fuel level dropping if there is a small leak in the pump, allowing in air when the pump is not operating. The pump doesn't pump air as efficiently as it pumps liquid. You might think about rebuilding the pump with new seals/gaskets - I had a similar problem with my Lupo some time ago and a pump rebuild fixed it. You could also try adding a Lupo non-return valve (165201542B) to the line in.

RAB
 
RAB said:
It would only make sense for the non-return valve to be in the inlet line, not the return line.

RAB
Click to expand...

Still have the tank fuel level sender module on the bench so checked I'd not got the supply and return connections mixed up.
The check valve is definitely fitted in the return line so the fuel can return uninterrupted to the tank but not allow 'back siphoning' from the tandem pump cavity via the supply line when the car is stood for long periods.

Cheers Spike
 
Hi Spike,

But if there is an air leak in the pump (air is less viscous than diesel and so there could be air leak when there is no fuel movement and no fuel leakage when there is), fuel will drain down the inlet line. If the check valve was in the inlet line, this couldn't happen. The Lupo 3L non-return valve is in the inlet line.

RAB
 
Here is the location of the check valve on the Lupo 3L:

100_1518.JPG

Leftmost tube, between the two clips. Note the sensible location of the fuel filter!

RAB
 
Decided to cut open the fuel tank level sender unit to see what's inside the integral return line. I used a Dremel with a cutting / grinding disc so the sectioning is not very neat but it shows a red rubber flap valve supported on 'star shaped' ribs.
I had wondered if the return line had been blocked with debris but there is no question the 'valve' is an integral part of the design.
Just need to work out the 'Logic' of how and why it works

Cheers Spike
 

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depronman said:
Is it in the feed or return legs of the fuel system ?


Sent from my iPhone using Tapatalk
Click to expand...

It's in the return line.
Before I cut it open I did a basic test by blowing down the tube to simulate the fuel return. It required a bit of pressure to blow down the tube but there was no flow while sucking on the tube.

Cheers Spike
 
Ref post 3, I made a couple of blanking plugs to seal off the quick connect supply and return lines at the fuel tank. This should simplify pressure testing the lines between the fuel tank and tandem pump as part of the troubleshooting process relating to fuel drainback and poor starting after parking up for several hours.


Cheers Spike
 

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