My Second 1.6 FSI

Good Evening Mac,

On the subject of "no sensor on the low pressure fuel delivery pipe to the HPP to monitor the pressure generated by the lift pump".

Just an idea, IF the HPP does not act like a valve then the value in the fuel rail shown by measuring group 140 with engine off is the pressure in the low pressure fuel delivery pipe? My FSI just now with engine off.

View attachment 107412

@Radian

I have a memory of fuel rail pressure of about 60 bar at idle. It does increase with engine revs up to 99 bar. The top end of the value range is actually 120 bar but by a quirk of VCDS the display field seems limited to two digits, that is it maxes out at 99 bar.

Andy
I've emailed Ross-Tech to see if the limitation of 3 digits, (99.9 bar), for rail pressure is in VCDS or the ECU.
Mac.
 
Okay, with advice from Mac this is what I have done.

Posted a picture of hot engine running. (you can see 100bar if no decimal place FYI)

Posted a picture of hot engine off.

Posted a picture of hot engine off (after turning engine for a split moment)

Posted a graph of the hot engine off, followed by a start-up then running for a short time. The arrow shown is the low point the revs dropped to after starting the engine. This was nearly a full stall. (For those that don't know I am having hot engine start troubles)
Getting an idea of the performance of the electric lift pump may be difficult. For a hot start, the high pressure pump need 5.2 bar from the electric pump to prevent vapour bubbles forming in the line to the HPFP. The increase in pressure, from 3.8 to 5.2 bar is achieved by closing the bypass valve that normally recirculates some of the electric pumps output. I suspect the bypass valve, (N290), only opens when a hot engine is actually spinning, when the pressure seen in Group 140 will be the HPFP output, so no help in checking the 5.2 bar out of the electric pump.
It may be that if the N290 is unplugged, it will be closed, (I'm guessing it's normally closed, powered open).
If this is the case, with the N290 unplugged, and the engine cold, and some time after it last ran, (to allow high pressure retained in the rail to dissipate), Group 140 might just, (or should be), at it's higher pressure, ideally 5.2 bar.
Looking at ads for aftermarket pumps, they seem to quote it as a 3.8 bar pump, so unlikely to give 5.2 bar. So genuine is the way to go.
Here's the location of the N290, (credit to @Andrew for the pic).
Mac.

index.php
 
More really useful information!

These two parts are number 1 and 2?

The one with the electrical connector being number 2?

I already purchased a new oem pump so let's hope thats all thats needed here.

See screenshot
 

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Last edited:
More really useful information!

These two parts are number 1 and 2?

The one with the electrical connector being number 2?

I already purchased a new oem pump so let's hope thats all thats needed here.
Item 1 is the bypass valve, which Audi call a "Fuel Metering Valve"
It is always open, dropping the electric pump pressure to 3.8 bar, except for starting a hot engine, when it's, briefly closed, to give 5.2 bar.
I think the idea behind the bypass, (return to tank), is to keep the pump cool and lubricated, the flow of fuel through it, via the bypass does that, but the flow through the injectors, on it's own, would not be enough.
I may try unplugging the N280 later in the week, assuming all goes well at tomorrow's MoT 🤞
Mac.
 
Last edited:
More really useful information!

These two parts are number 1 and 2?

The one with the electrical connector being number 2?

I already purchased a new oem pump so let's hope thats all thats needed here.
The Bosch part in the pic is the rail pressure sensor, (G247), not the lift pump, (G6). I think the electric pump is p/n: 8Z0919051, but double check.
Mac.
 
I am now thinking that the description of the pump as 3 bar, is correct, and that 3 bar is the pressure with maximum flow rate. Maybe the title "Fuel Metering Valve" is more accurate than I thought. The bypass flow rate, through the Fuel Metering Valve really is "metered" so that the flow rate is low enough to achieve the 3.8 bar pressure, and with the bypass closed, (for a hot start), there will be almost zero flow, causing the pressure to momentarily increase to 5.2 bar.
@VVVVVV who replaced the pump on his FSI a few weeks back, (to cure hot non starting), has sent me details of the pump he had fitted:
PIERBURG 702550540 or
PIERBURG 702550610
His mechanic also found a small leak in the fuel pipe between the lift pump and the HPFP.
Mac.
 
I am now thinking that the description of the pump as 3 bar, is correct, and that 3 bar is the pressure with maximum flow rate. Maybe the title "Fuel Metering Valve" is more accurate than I thought. The bypass flow rate, through the Fuel Metering Valve really is "metered" so that the flow rate is low enough to achieve the 3.8 bar pressure, and with the bypass closed, (for a hot start), there will be almost zero flow, causing the pressure to momentarily increase to 5.2 bar.
@VVVVVV who replaced the pump on his FSI a few weeks back, (to cure hot non starting), has sent me details of the pump he had fitted:
PIERBURG 702550540 or
PIERBURG 702550610
His mechanic also found a small leak in the fuel pipe between the lift pump and the HPFP.
Mac.
Elring Parts, (UK's Pierburg Distributor), confirm the correct pump for the BAD Engine is Pierburg 7.02550.61.0
Mac.
 
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