Fuel Pump: Is it Redundant for PD Engines?

Hello everyone,

The tandem pump (diesel/vacuum) in the PD engine performs a similar function to, for example, the vane pump in the VP 37, by drawing fuel from the tank and providing several bar of pressure at its outlet to supply the high-pressure section.

According to the VAG-SSP (Technical Information System) regarding the introduction of the PD (Pumpe-Düse) engine, the tandem pump is the only fuel supply pump in the fuel system.
Now, the Polo 9N and, as far as I know, at least the Golf 4 with PD engines are equipped with an additional electric fuel pump that (in my case) consumes about 55 watts of power and is connected in series with the tandem pump - as if the latter wasn't providing enough fuel flow.

According to SLP, in the Passat with the AJM engine (from 1999 onwards), the fuel pump is only installed in all-wheel drive vehicles, and its relay is connected between the starter and the alternator, presumably so that it only runs during starting.

So it seems that, at least in the earlier PD designs, the fuel pump was either completely absent or not constantly operational.
However, in my car, the pump relay gets just as warm after each trip as the 109 relay or the relay for the X-contact, which means the pump is apparently running constantly.

I'd like to temporarily shut down my pump (by pulling the fuse) out of curiosity, but I'd like to know what problems might potentially arise as a result.
Therefore, my question to everyone:
Does anyone here know what experiences or (initially overlooked) necessities led VAG to implement the standard use of a fuel pump for PD engines?

Hi!

It's conceivable that the pump, for example, compensates for pressure drops and thus maintains a stable pressure. This way, for instance, the risk of air causing problems in the system is minimized, as the air is essentially 'pushed through' without the booster pump having to activate first. Otherwise, I only see it performing supporting functions.

But something else: Why do you want to disconnect it? Shouldn't you be happy that you have one and that it works? Remove unnecessary baggage?

Greetings.

Isn't that just the standard variable vane oil pump, like the one that was installed in my Golf 2, for example? Exactly for the reason that m3nx mentioned...

m3nx wrote:

But something else: Why do you want to disconnect it? Be happy that you have one and that it works. Remove unnecessary baggage?

That would also be an aspect .
Seriously: out of curiosity -> what is this thing for, or what happens if it doesn't work?

And to potentially prevent future problems (speculation mode ON):
If the pump improves the starting behavior, but once the engine is running it doesn't matter whether it's running or not, then you could, for example, fix it in a way that it only runs when the engine is revved up. This would extend its lifespan, rather than wearing out after perhaps 100,000 km, and thus pave the way for numerous workshop visits due to poor starting performance.
(Speculation mode OFF)

The pre-lift pump in the tank is only there to supply the tandem pump with fuel that is free of air bubbles. As far as I know, it generates a pressure of 0.6 bar.
Your car will probably run fine even without a pre-lift pump.

Your car will probably run even without a pre-lift pump.

Depending on how full the tank is. My Golf2 would only fill up to just below half a tank because the pressure to the main pump was insufficient, and it was drawing in more air.

In terms of PDEs, I would skip this 'test.'

Hi,

"If a pump is deactivated by pulling a fuse, I would also expect to see an issue with the flow resistance of the deactivated pump, because the other pump would then be drawing fuel through it. Therefore, I would probably leave it as is..."

Best regards,

Jan.

Jan6K wrote:

If a pump is deactivated by pulling a fuse, I would also expect to see a problem with the flow resistance of the deactivated pump, because the other pump would then be drawing fuel through it.

Hi Jan,

I was also thinking about that... I suppose I'll start by monitoring the pump's power supply with an LED.
Perhaps I'll discover some more interesting switching strategies than just "on as long as the engine is running"...

Hello Ulf, today I replaced the diesel filter on a VW Golf (AJM engine) and installed the new filter without any diesel in it. Then I turned the ignition on and off several times (5 times) and then started the engine.
Lo and behold, the engine started immediately, just like always.
Thanks to the pre-filter pump, all the air is flushed out of the filter.