Cayenne S 957 4.8 High fuel pressure question

[TomBelgium]

Hi, 

I am trying to figure out if my high pressure fuel pump is faulty. I noticed longer cranking when starting the engine, misfiring when it just starts (or very rough idle the first seconds). After revving a little bit it goes back to normal and the rough idle is gone. This only happens when i leave the car parked for a couple of hours. I already got p1023 and p1026 (reffers to high pressure problems)

The high fuel pressure (in the rail to the injectors) loses pressure very quickly when i turn the engine off. It goes from the 40 bar it had when idling to around 4 to 5 bar in some hours. 

It seems like the 4 to 5 bar are is the pressure that the low fuel system is providing so my guess is that the HPFP is leaking the high pressurized fuel back to the low pressure circuit (or mixing). Thats why it never goes to 0 bar but keeps its pressure from the low pressure circuit. 

Am i correct or is it normal that the fuel injector rail/ High pressure circuit loses its pressure when i turn the engine off? Otherwise i have another problem.

[TomBelgium]

Anybody?

[Loren]

Each fuel pump must deliver at least 1.8 liters of fuel during an operating time of 1 minute.

The measured result in milliliters delivered is multiplied by 3. For example: After pumping for 20 seconds, around 750 ml of fuel is measured. Multiplying the result by 3 gives a delivery rate of 2.250 liters per minute. The pressure gauge shows a fuel pressure of 2.5 to 2.7 bar during the measurement.

 

If your fuel pump(s) (individually) do not meet this test - then that fuel pump is bad.

[lewisweller]

4 hours ago, TomBelgium said:

Anybody?

I doubt anyone here will put there neck on the line to definitely confirm but from what I read 90% the hpfp is faulty. Your story seems the same as other who have had the same issues but been in denial for a while that it needs replacement, even some pumps that had only been on the car for 4000km failed. 

I don't know how how else you can test for the failure yourself, bench testing is ideal but who has that setup in there house? 

Porsche have exchange units and may give some good will if you plead nicely. 

 

 

[lewisweller]

12 minutes ago, Loren said:

Each fuel pump must deliver at least 1.8 liters of fuel during an operating time of 1 minute.

The measured result in milliliters delivered is multiplied by 3. For example: After pumping for 20 seconds, around 750 ml of fuel is measured. Multiplying the result by 3 gives a delivery rate of 2.250 liters per minute. The pressure gauge shows a fuel pressure of 2.5 to 2.7 bar during the measurement.

 

If your fuel pump(s) (individually) do not meet this test - then that fuel pump is bad.

Tombelgium

Loren has given the low pressure fuel pumps flow here just in case there was any confusion. Of course the low pressure pumps flow must be within spec or the hpfp cant do its job. 

As you see 4 bar at rest in the fuel line this confirms the pressure of the low pressure fuel pump (at least one of them anyway) is good and within spec. Once you rule out the low pressure side it only really leaves the hpfp and injectors as possibles I think. 

[Loren]

Yes, you can not skip steps and hope to diagnose the issue. Even very experienced techs always go through the steps - so not to cost you more money than need be.

[TomBelgium]

Yesterday i took my HPFP out and opened it. I tought that it could be some worn o rings. And indeed, the o rings on the check valve were cracked, so all pressurized fuel leaked back to the low pressure fuel system. So today i bought some new o rings in a specialized store. Put my HPFP back in and tried to start the car. It took as long as before only now the rough idle was gone. Unfortunalty i got a check engine light, the p1023 and p1026 errors and a lack of power above 2000 rpm. Seems like my pump is completely dead now. But the obd shows the the high pressure is maintained when the car is turned off. So i fixed the leak but that might have broken the (weak) pump. Tommorow i am going to order a new pump at Porsche... 

[TomBelgium]

Update, didn't want to spend more time on fixing the pump so I bought a replacement for 900 bucks. Porsche told me they already replaced it a couple of years ago. I tried to explain that it is not common for a quality car to have 3 different HPFP in just 100k kilometers but they didn't agree. My former daily was a "boring" Skoda Superb diesel, I only had to do regular maintenance, and it had even more kms on the clock than the Cayenne. I am selling my Cayenne now because I also heard about the cylinder issues. My dealer reassured me that only the pre facelift 4.5's had cylinder issues. But I spoke around 10 cayenne S 957 owners and they all had the cylinder issue. Also mine had the engine replaced at 40k kms, right now I am just waiting for it to break again. 
I also drove a BMW 850i V12, I always heard people telling about that they know somebody (friend of a friend of a friends cousin etc.) who had an 850i and it should be rubbish. It never let me down, not once I had anything unusual to repair. But with the Porsches, just open a topic in an owners club or on any car forum and you get dozens of owners who have or had an engine replaced, cylinder issues, bad bearing in the engine etc. It was my first Porsche and I didn't expected it to be cheap or whatsoever, I just expected it to be reliable but they just are not. I really hope that Porsche does an effort to bring the quality back like it was in the 80s and 90s. 

[lewisweller]

16 hours ago, TomBelgium said:

Update, didn't want to spend more time on fixing the pump so I bought a replacement for 900 bucks. Porsche told me they already replaced it a couple of years ago. I tried to explain that it is not common for a quality car to have 3 different HPFP in just 100k kilometers but they didn't agree. My former daily was a "boring" Skoda Superb diesel, I only had to do regular maintenance, and it had even more kms on the clock than the Cayenne. I am selling my Cayenne now because I also heard about the cylinder issues. My dealer reassured me that only the pre facelift 4.5's had cylinder issues. But I spoke around 10 cayenne S 957 owners and they all had the cylinder issue. Also mine had the engine replaced at 40k kms, right now I am just waiting for it to break again. 
I also drove a BMW 850i V12, I always heard people telling about that they know somebody (friend of a friend of a friends cousin etc.) who had an 850i and it should be rubbish. It never let me down, not once I had anything unusual to repair. But with the Porsches, just open a topic in an owners club or on any car forum and you get dozens of owners who have or had an engine replaced, cylinder issues, bad bearing in the engine etc. It was my first Porsche and I didn't expected it to be cheap or whatsoever, I just expected it to be reliable but they just are not. I really hope that Porsche does an effort to bring the quality back like it was in the 80s and 90s. 

Welcome to the porsche cayenne. Always full of surprises .......not all of them good ones. 

You'll find most people have a soft spot for their Porsche where character, heritage and performance out weighs the breakdowns, major failures and cost. 

I'm at the end of my rope and can't wait to sell. 

 

Bring on the backlash of the purists! Haha

Edited July 29, 2017 by lewisweller

[TomBelgium]

Yeah, I am selling my S  too after just one month of ownership. Maybe I buy a 3.6 cayenne (VW VR6 engine) but for sure I don't buy another Porsche with an Engine built by Porsche. 
A lot of people say a BMW X5 is nothing against an Cayenne and I agree in terms of driving experience but at least I don't have to worry about engine failures when I go on holidays with an X5. With the Cayenne I would never be relaxed taking it for long road trips. Such a shame, I expected more from Porsche and I was really happy the first days I had it until I had the HPFP failure and especially now with all the people I talked to who say Porsche engines are way worse than everybody thought.