I previously posted about the issues with my 2005 Cayenne Turbo and the history with poor running and percieved fuel issues which led me to replacing both pumps, filter and PCV. As it turned out the main issue was the Crankshaft position sensor as @hopsis kindly pointed out, so with that fitted all seemed good! (see thread https://rennlist.com/forums/cayenne-...n-t-start.html )

Unfortunately there is still some strange things going on, specifically when the tank gets down to 25% it starts intermittently stalling. As soon as I pull over, select park and restart it fires up and runs again for a few minutes, then cuts out again! Once I fill the tank over 25% the stalling stops, although sometimes it does some odd things. Today while trouble shooting, and after filling the tank (took 75l so nothing wrong with fuel gauge or senders), it seemed to be running perfectly, but I pulled over, put it in park and the revs slowly rose to 1800rpm and sat there, if i selected a gear it would drop to normal idle, bus. as soon as I put it back in Park it ran back up to 1800rpm!

After driving home and pulling up in the carport, it had stopped doing this and was idling normally. It idled normally for a couple of minutes then appeared to start misfiring on a couple of cylinders and dropped revs, although it didnt stall.

I also tried isolating the fuel pumps one at a time by removing relevant fuses and then testing, this also induced odd behaviour - in both cases the car ran for a bit, stalled and then when I tried to restart just turned over without firing! As soon as I went back to both fuses in it would start and run.

So basically its drivable with the tank at more than 25%, but still a bit fluffy at times. Under 25% fuel level and it will start stalling regularly.

I have run the LAUNCH X431 on it fairly often in trying to trouble shoot, sometimes it throws no errors, othertimes i will get a few DME errors which seem to me more symptoms than anything, so Misfires on cylinders, P0300, 301, 302, 303 etc It did throw a P0507 Idle Speed Control , Above Upper Limit today - which I am guessing is when it was idling at 1800rpm.

I dont think its actually a fuel issue as even when it stalls it starts instantly once I select Park and turn the key off and then turn to start, if there was a loss of fuel pressure I would expect it to turn over for a couple of seconds to pressurise again? I am wondering if it might be an intermittent fault in the DME unit itself (i really hope not!!).

I am really running out of ideas for troubleshooting, and driving with only ¾ of a tank in use is not optimal where I live! Any ideas appreciated.

 

You've got a plumbing issue inside your fuel tank. Somewhere there is either a quick connect, that isn't connected or a pickup that was secured to the bottom of the tank has come loose - possibly while wrestling with the hoses during the pump replacement. I would get down to a quarter tank, then open your fuel rail and rig up an extension hose to pump out the rest of the tank.

If my VAG wiring logic applies, you would have to pop the cover(s) off of the fuel pump relay(s) and gently push their contacts closed with a folded matchbook cover or something equally gentle, turn the key to RUN and pump out the fuel. If it aerates and spits, that is your problem. If it pumps out in a steady unbroken stream until nearly the last drop, then you are good. I became adept at pump installs on a Audi D2 A8 - akin to the Apollo/Soyuz docking with your eyes closed - and rigged up a threaded hose barb (in place of the fuel pump assembly banjo bolt on top of the tank) and six feet of low pressure gas resistant clear plastic tubing in lieu of the output line on top of the pump assembly to test for a good install. But you could also rig something up at the fuel rail. Audi's method of confirming proper installation was insane - drive around with a gas can in the trunk in case it cuts out at around 1/3 tank. I would always pump it empty, add two gallons and pump out all but maybe a cup of it. Good to go.

When I replaced the pumps in my 04 CTT, the fuel tank hoses were a many headed hydra. It would be too easy to miss something that only reveals itself once the fuel level is low. Does anyone have a workshop image of the inside of the fuel tank to share with the class?

Good luck,

Brian C.

 

There are quite a few of those images in prior threads discussing the fuel pumps and fuel tank. Let me see if I can quickly locate one on my computer,

Here is a PDF version of the fuel system from the factory manual. Sorry it's in PDF format. I'm sure if you search on fuel pumps - several of the many threads will have the diagram in them - I know because I put it there.

 

Thanks Don, you're a mensch.

 

Thanks deilenberger and BrianC72gt its taken me a bit to get to, but I pulled everything out again and checked it all and could find no issue, i already had the diagrams from when I changed the pumps, filter & PCV. I followed the suggestion to put a clear hose on the outlet of the PRV and then jumpered each fuel pump relay socket, both pumps were functioning perfectly with strong, unairated flow.

Next step I think is to get a pressure gauge and check pressure at the fuel rail.

The car has got worse, tank is well over ¼, nearer 60%, and now its idling like a dog, popping, missing, and nearly stalling. Still only throwing the occasional symptom fault, like low idle speed, lean idle, and cylinder misfires.

Its like every time i eliminate one issue another one arises! Thank god I have another 4WD with a big old chevvy diesel with mechanical fuel injection!! Starting to really appreciate the simple designs!

 

OK, update on my problems, I have been unable to resolve the issues when tank is under ¼ full, got a pressure gauge and everything checked out at the rail, so I give up on that one for now!

I changed the valve cover gaskets out as I was getting a bit of oil in the spark plug sockets and also leaking onto the exhaust, I also had heard that can affect idling. It made no difference to the idle, but it needed doing anyway!

I found a couple of small vacuum leaks, I have repaired both and now it seems to idle a bit better so I have ordered new hoses and will replace all of the brake vacuum lines to see if that helps even more.

Latest problem is that all the switches on the steering wheel have stopped working, so the gear change buttons and all the ones for the entertainment system. I suspect a fuse but so far haven't found a fuse for the steering wheel switches.

Its all a bit one step forward, two steps back!

 

Curious if you every made any progress on resolving your 1/4 tank issues? I'm having similar symptoms.

Thanks!

 

in my case it turned out to be the plumbing of the pumps in the tank, when I replaced them I got part of the plumbing incorrect, once i found that and fixed it the problem was resolved.

 

Interesting... I also did my fuel pumps myself. Wondering if I may have triggered the same issue. I'm realizing your last update was two years ago, so if this is foggy, no problem, but do you recall the issue with the plumbing? Had you connected the new fuel pumps/fuel filter to each other incorrectly? Or had you tangled/moved one of the in-tank hoses during install?

Thanks!

 

it was something to do with the in tank hoses and how i had hooked them up, as you say, pretty hazy on the details now!

i think left hand side of tank, and towards the front, when i got a. better look i either missed a connection entirely or had it wrongly hooked together.