Hi All,

2008 Cayenne S with 172k miles on the clock. Same one I have posted several DIY's on before here on RL, but not for a while.

Any comments or suggestions on the below would be most appreciated

Suddenly started to get some very rough running and misfires in almost all cylinders. Car runs worst at idle speeds but smooths out about at higher revs.
Also had a "1023 fuel pressure too low during starting" code.

Plugs, coils and battery are definitely all good. So I started looking into fuel pressure and this is what I am seeing (durametric logs below)



High Pressure Side:

• The High Pressure side seems to be tracking the set point pretty well
• After engine shutoff the high pressure actual values stay very steady between 5,5 and 5.7 Bar and will remain that way for several hours

Low Pressure Side:

• The Fuel tank level is currently at 41 liters (also measured with Durametric)
• According to the Porsche service manual
• Pump 1 and 2 run for 10 seconds on starting the engine
• Pump 1 should stay on all the time for values >15 Liters and <60 Liters
• Pump 1 and 2 run for tank levels < 10 Liters
• Pump 2 is always active when the level is > 60 Liters

Therefore in my case I would expect to see both pumps run for the first 10 seconds (this does happen) and then only pump 1 operate after that (this does not happen as pump 1 shuts down and 2 remains on)

I then tried pulling fuses 14 (Pump 1) and Fuse 13 (Pump 2)

Pull Fuse 14 and start car:
Car runs rough for 10 seconds with both pumps and as soon as pump 1 stops car stalls (even extra throttle input could not keep it running)

Pull Fuse 13 and start car:
Car runs for rough for 10 seconds with both pumps and when pump 1 stops car does not stall (although still runs a bit rough)

My next step was to insert a pressure tester in stream at the LP fuel line fitting on the bulkhead, but I cannot find an adapter with the correct threads.
Failing this I will check for fuel volume out of each pumps by triggering them at the relay.

Question: should the HP side still be able to track the set point values if the volume or pressure from the low pressure side was inadequate?


The below durametric logs are with both fuel pump fuses in place (i.e. both pumps are allowed to run)

 

Interesting, if I remember correctly, when I start the car, fuel pump activation was "1" for both pumps and shortly pump 2 went to "0".

As I think about the system, I think your pump 1 is bad and upon start, the system defaults to pump 2 because output from pump 1 is faulty..

Unless mine is failing opposite yours...

I'll see if I can run your tests tomorrow to compare.

 

I would also guess that if you add 25L of fuel that the car will run normally as you'll get above that one pump's threshold so the one that's still working will be the one pumping.

 

Thanks @GlenC & @Petza914

I had seen some posts where one of the pumps had been jumped to always stay on and also other which said that removing one fuse or relay would force the other pump to stay on and in some cases make the car run better.

I can confirm the following:

I re-ran the tests this morning removing fuse 13 & 14 one at a time with ignition shut off between each test.
I did the same process with both fuses in place but removing the relays instead. (as expected the behavior was the same and followed the same patterns)

When Fuse or Relay for Left Pump (Fuse #14) is pulled, engine always stalls when durametric chart shows the left pump activation going to zero.
This does not make sense as

1. if the Fuse was already out it means that only pump 2 (Right side pump could have been running for the first 10 seconds when both pumps were turned on). Therefore switching off pump 1 should have no impact as the fuse was not installed anyway. (
   This just not a logical behavior and is making me wonder if durametric has pump 1 & 2 labels interchanged??
2. the car runs equally rough for the first 10 seconds whether both pumps are running, just the left or just the right (only smooth running is in the exception of during a completely cold start where the rich mixture is running to pre heat the CATS)

During cold start Nominal Fuel Flow Rate ave = 32g/s (both pumps running)
During warm start Nominal Fuel Flow Rate ave = 25.5g/s (both pumps running)
During warm start Right Pump (fuse #13) removed Nominal Fuel Flow Rate ave = 23.5g/s (left pump running)
During warm start Left Pump (fuse #14) removed Nominal Fuel Flow Rate ave = 22.4g/s (right pump running)

My understanding is that the Cayenne in tank fuel pumps should be capable of delivering "at least" 3 liters per min of fuel so I did the conversions on my numbers below:



Not enough difference in the values for the 3 warm start scenarios. However there is a big difference with the cold start value and the fact that it runs pretty smoothly during that process. (although I am aware the mixture mapping is also different which will also contribute as a deliberate richer condition is implemented at this cold start stage)



If anyone can share their fuel flow rate, pump activation values and pressure values as a reference point, that would be highly appreciated.
If I were to assume for a moment that the durametric labels for pump 1 & 2 were interchanged then it would point to a faulty "pump 1 / LHS" 1 as the stall occurs when "pump 2 / RHS" is shut off.

See results from testing this morning below:

 

I am wondering if we are seeing the ECU signal to pumps 1 & 2 and not their status.
Yours is operating different than mine, in that at start, I activate fuel pump 1 & 2 for about 5 - 8 seconds then fuel pump 2 goes to state "0"

So you pull fuse 14 and start.... ECU sends signal to pump 1 & 2 (pump 1, fuse 14 not running) car starts and then falters and stops. With pump 2 fuse out (13) car will run.
I wonder if the ECU doesn't default to backup pump through activation signal .....
Seems like your pump 2 is faulty, maybe on volume. Has enough pressure to start but not enough volume to run.
Are you getting high pressure fluctuation error code?

Download of data to Excel

 

Thanks so much for sharing your data @GlenC

I just completed the flow tests on both pumps (one at a time) by disconnecting the fuel line ahead of the HP Pump and connecting a hose to flow into a container for measurement.

Remarkably both pumps delivered just over 1 liter in 15 seconds (4l/min) which is well above the minimum requirement.

What really strikes me about your graph vs mine is the stability in your HP setpoint. The only time mine was so stable was during the cold start early this morning.
This makes me wonder if the set point is trying to compensate for something the car is detecting elsewhere or what...?

added pics for reference..

 

I uploaded my data if you want to compare.

 

Thanks for the file @GlenC

It is really weird that your pump activation values and sequence seem to be the opposite of mine.
We have the same car (2008 Cayenne S 957). I am using Durametric software program version 6.5.6.5. (with Durametric Pro cable)

The behavior of my HP Fuel Setpoint is still really confusing. It seems like it is trying to compensate for something and bounces all over the place while yours stays very stable.

I also forced both pumps to stay on for one of the tests using the relay jumper but that made no difference to the roughness.

Given that the pumps have a good flow rate I am wondering:

• the pressure regulator in the tank?
• the high pressure fuel sensor in the rail (although those readings stay very stable when the engine is off)
• the high pressure pump (although it can deliver the required high pressure, is it still possible that it could be delivering fluctuating pressure)

 

Remember, I have an issue with the shutting off at ½ tank. My pump sequence may be altered, however my understanding is the left pump is the default "always on" pump.

Pressure regulator could be a cause. Since you tested good flow with both pumps, I would rule out fuel filter.

Since I had to replace the fuel filter and now have possible pump problems, I may just replace both pumps and pressure regulator. Much easier now than on a road trip in the middle of nowhere or no cell service.

 

@GlenC you are correct about still needing to verify pressure on the Low Pressure pumps. I spent hours trying to find a suitable T fitting for the pressure tester I had but without success.
In the interim I've been looking at the sensor below which is on the LP fuel line just ahead of the HPFP. On the Panamera parts catalogue in the same location (albeit with a different P/N) it is labeled clearly as a pressure sensor, but in the Cayenne parts catalogue it is just referred to as a sensor.
I'm pretty sure it must also be for pressure monitoring (not temperature as one online source called it). It is easily visible on the bulkhead above the rear of the engine.

However I cannot find anything on the Durametric actual values that report on this sensor (I'm continuing to search).

 

That could be the fuel flow or consumption sensor.

 

@GlenC I should have mentioned I tested with that sensor connected and disconnected and the Fuel Flow and Fuel Consumption values were still coming in live on the actual values.
Disconnecting it also made no difference to the previously mentioned rough idle issues and the HP values and curves remained very similar.

 

Looks as if it's a temperature sensor in the fuel line

 

@GlenC I think I finally found a fitting to hook up the low pressure fuel line test tomorrow. Hopefully that will either confirm or rule out the low pressure pumps....

I pulled the lambda and fuel trim values again tonight also and the car is definitely running very lean (no big surprise there).

 

A pair of pumps is a couple hundred $$ and a couple hours to swap in. You've probably spent more time (and more $$, depending on what your time is worth) diagnosing this.