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For some time now I have had an issue where as soon as the fuel tank gets to exactly 25% the car starts stalling and I have to pull over and restart it - and it will go another few hundred metres and then do the same thing. Prior to this I had recently replaced both fuel pumps, filter & NRV and have double checked everything a couple of times to no avail.

 

This week the symptoms suddenly changed, I was running near ¼ tank because I was going to pull the tops of the pumps and try and find the cause, and twice, as soon as it got down to 25% the car stopped and would not restart until I added some fuel to the tank.

 

Today I started to troubleshoot, first thing I noticed is 2ndry (right hand side) pump is not running when I open the drivers door.

 

If I remove 2ndry RHS pump fuse (pos 13) & relay (pos 19), the car starts and runs fine.

 

if I remove primary LHS pump fuse (pos 14) & relay pos 6/1), the car wont start. 

 

Tried swapping the relays, no difference.

 

With Primary pump disabled by removing fuse & relay, car wont start or run, but if I remove the secondary pump relay and jumper it, the pump starts running and the engine starts and runs!

 

So it seems that something is preventing the secondary pump from running, both when the drivers door is opened, and when the tank is at or below 25%. But its not the fuse or relay?? It almost seems like a logic issue! None of the things that should cause the pump to run are doing so.

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Mmmm...must be an unusual problem! No suggestions yet. I am going to run it down to ¼ tank again and then see if by jumpering the relay on the 2ndry pump, whether I can get it running again.

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So today the car stopped as soon as the fuel level dropped to ¼ tank - as it always does. I pulled relay 19 and used my made up jumpering wire - now its running again and also below ¼ tank on the gauge.

 

Its showing an error as I would expect, P2362 Fuel Pump relay 1 final stage. No signal/communication.

 

So it seems to confirm that there is some electrical/wiring fault between the DME & the relay which means even when the DME tells it to run, the signal doesnt get to the relay.

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Regulation of Fuel Quantity
As a result of the on-demand regulation of fuel flow capacity, fuel heating in the tank is reduced. This is achieved by the DME control module switching on fuel pump 2 only as required, while fuel pump 1 is always activated. With the following switching criteria, fuel pump 2 is additionally switched on to increase flow capacity. At engine start and then 2 seconds run-on (with a hot start 5 seconds run-on). With a calculated fuel consumption of > 50 liters/hour the pump is switched on, and at < 45 liters/hour switched off again. With a fill level of < 10 liters in the tank, it is switched on and with > 15 liters it is switched off again.

 

Fuel Pressure Regulator
The pressure regulator is also built into the fuel tank below the right-side tank flange. Fuel pressure, which is present right up to the fuel injectors, is about 4.0 bar from idle to wide-open throttle. The quantity injected is thus dependent on intake manifold pressure, which is measured by the DME control module when calculating injection duration.

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Thanks Loren, its somewhat confusing about Pump 1 and Pump 2. I have seen different 'official' quotes of which runs when and why. Some places quote 25lt or ¼ tank as the switchover to the other pump. No doubt mine cuts out at exactly 25lt. Its further confusing because Porsche just label the pumps 1 & 2 whereas many talk about them as primary and secondary.

 

After thinking about what you quoted I thought, if thats correct I am following a red herring by jumpering the relays, I need to go back to where the whole problem first raised its head, which was after changing out the pumps.

 

So I decided to pull the covers off and remove the pumps, starting with the left hand side, I got the pump out and then thought the other thing I have never got my head around is what and where the sucking jet pumps that people refer to are located. The left hand side of the tank had very little fuel in and when i felt around I realised the sucking jet pumps are way forward, under the front seat and in fact the tank is shallower in that section so the left hand side sucking jet pump was high and dry!

 

I went back to all my notes and pictures and found a really good hand drawn diagram from another thread that clearly shows the left side sucking jet pump is plumbed to the right hand fuel pump and vice versa. These connections are the only ones that are interchangeable and when I put the new pumps in, somehow I missed this vital piece of info and had plumbed the left hand sucking jet pump to the left hand pump and vice versa! 

 

When I pulled the cover off the right hand pump the reasons for my problems were obvious - the tank looked to be ⅓ to ½ full - the incorrect plumbing had caused the fuel to be pumped across to the right hand side of the tank!

 

So I plumbed it all correctly and then got the car running and it ran happily with well under ¼ tank without any jumpering etc. 

 

Thanks Loren for persisting with me, it definitely got me on the right track!|

 

For anyone else having the problem of the car cutting out at ¼ tank of fuel this is worth checking, its not such a difficult mistake to make! (thats my claim anyway!)

 

I have attached the image that helped me understand how it works in case anyone else finds this thread.

 

The only thing that still seems a little odd, but I haven't bothered trouble shooting yet, is I still dont hear the pump running when i open the drivers door anymore. 

 

 

pressure from pump.png

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No need to apologise, I already had the drawing and another image - I just assumed like all the other connectors that it was impossible to connect incorrectly, so I had never checked or even considered that I had done something wrong and was blissfully unaware that they sucking jet pumps went to the opposite side fuel pump. I am just very grateful you replied a couple of times and gently pushed me in the right direction!

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