How to correctly relieve pressure from HPFP ?

[ruedisrs6]

A question to all the experts in this forum who had to replace parts of the high fuel pressure system,

I recently wanted to replace the cam follower of the HPFP on my 2008 RS6 4F. As a first step, I wanted to relieve the high pressure of the high pressure fuel pump. I looked in the official manual and it states to remove the 5A fuse 1 of the second row on the fuse holder on the right side of the dash board (have a left hand drive car since I am in Germany) during idle and watch the rail pressure in block 140. I did this and waited and waited and waited and nothing happened, the rail pressure stayed at around 40 bar. So, I ended up opening the relay box on the left side of the windscreen cowl and manually removing the fuel pump relay. The engine immediately stopped and the fuel pressure in the rail was totally gone.
But, you can imagine that 1) I do not want to open the relay box every time I want to relieve the high pressure from the HPFP and 2) want to avoid the engine to stall because on no fuel supply (danger of cat damage).
Is there a fault in the wiring of my car (there are some undocumented specialisties with the RS models anyway ...) or do I overlook something? How do you relieve the high pressure for repairs? Is there a way to do it with VCDS (have done this with basic adaption on the AUK 3.2 engine of my wife's car many times)? I know, the hard way is simply to use a cloth and open the connecting nut on the tubes of the fuel pump. But as I said, if there is a more elegant way, I would appreciate it, if someone can share this information with me.

I really like this very active forum with so many experts around, many thanks and sorry for my English in need of improvement,

best

Ruedi

[Pinkfoot]

Nothing wrong with your English Ruedi.
I can't answer your question but welcome.

[Jim Haseltine]

My info also says fuse 1 in fuse holder 2 (brown). Fuses are numbered from left to right.

[ruedisrs6]

Dear Jim,

thanks for the reply. Fuse no. 1 is the one to the left in the brown holder and the only one with 5A. To add to the confusion, I looked at the circuit diagrams and found that the "activating" pin of fuse 1 should connect to pin 2 of the fuel pump relay via a brown connector in the relay box in the windscreen cowl. I disconnected the brown connector and used a multimeter to check for a connection between pin 2 and the fuel pump relay. However, I did not find one .... In fact I checked all pins of all connectors in the box and found no connection. I have no idea from where the fuel pump relay is activated. Next, I will look for a connection with the ECU. And I will use a long cable to test where the pins from fuse 1 are going to. This is not the only issue that makes me crazy with this car ....... I can tell a multiple page long story about the heavy grey smoke on start caused by a faulty one-way valve in the oil module and the nerves any money this cost me two years ago.

Does removing fuse 1 work in the car(s) you know ?

Best

Ruedi

[Jim Haseltine]

I can't comment - sorry. My car has been off the road for a year now - it started with the intent of dealing with oil seal and turbos but has expanded to all sorts of other things that needed to be done. I'm taking advantage of the lock down (and retirement, coincidentally) to do a deep clean of my garage so I've not touched it for a couple of months. Which considering the current low cost of fuel is more than a little annoying.

[ruedisrs6]

Does someone has experience with the pressure relief from the high pressure fuel pumps or can at least confirm that pulling the fuse 1 on the fusebox does work or not work ?

Many thanks.

[douglas96]

Does someone has experience with the pressure relief from the high pressure fuel pumps or can at least confirm that pulling the fuse 1 on the fusebox does work or not work ?

Many thanks.

Hi,

I have done this a couple of times on my car, quite some time ago now though (I replaced the solenoid valves on the vacuum reservoir, under the upper inlet manifold and the N249 valve which cured a fault code that was coming up on VCDS).

I followed the same procedure, removing the same fuse (although I'm in Britain and have a RHD car) and watching the fuel rail pressure on VCDS, it just remained the same and didn't go down at all - so in the end I just held a rag over the bleed screw and let the pressure off that way. However after putting everything back together (or so I thought), when I went to start it up the engine just turned over - it didn't fire. Obviously I wondered what I had done to cause that, and then it dawned on me that I hadn't put that fuse back in - I put it back in and it fired and ran as normal. So the fuse does do something, but obviously not what ElsaWin says it does, at least on our cars.

I wonder whether anything changed in the production run regarding this - my car is a model year 11 (B). What is your cars model year?

[ruedisrs6]

Mine has manufactured in July 2008. Interesting to hear that yours does also not follow the official procedure ...

[ruedisrs6]

Hi douglas96,

so, I purchased a 1-hour access to ERWIN and checked the official procedure there. Interestingly, ERWIN does not copy the information in ElsaWin of removing fuse 1. It simply recommends your approach to "held a rag over the bleed screw and let the pressure off that way". Of course, it recommends to wear protecting glases .So it seems that this is the official way to do it. Nevertheless, I would still additionally remove the fuel relay, simply to avoid that the fuel pump is activated by opening the driver side door.
I tried several constellations of removing fuse 1 (engine running; engine off; ignition on, but engine not running). In my case fuse 1 does not do anything, beside throwing an error when removed with engine off and then restarting it. Very strange, and unfortunately not correctly documented behaviour.