Hi
I have recently experienced a strange phenomenon on my 04 Cayenne S. Under acceleration, the engine power seems to "pulse", it is not significant like a misfire, and there are no warning lights, but it feels as if I was pulsing my foot on the throttle by +-5%. It occurs under hard acceleration and seems more significant going uphill.

Under normal driving conditions, it is perfectly smooth.

Maybe fuel pump losing pressure ?

Any ideas ?

 

I once had a vehicle do that. It ended up having moisture around the spark plugs. I had just detailed my engine compartment...

 

I noticed this the first time on a long journey, and it has occurred again recently on a long journey, so any moisture would have had lots of time to dry up. I have read possible O2 sensor problems causing this kind of symptom on a BMW forum. My Cayenne has 212000 km so it is possible that O2 sensors need replacing (I have read they should be replaced at 250000km)
Still would like to hear from others with ideas.

 

Here's an update on this situation.
Turns out this pulsing phenomenon is also known as "surging" and could have many causes including faulty coils , MAF sensors or Oxygen sensors.

My Cayenne S went into Porsche Centre Johannesburg for a major service in August, part of which included the replacement of all 8 coils (due to cracks). (There was no mention of any issues with surging in their diagnosis during the service btw.)

I hoped the coil replacement would stop the surging , but it didn't .

I decided to replace the MAF sensors. I have attached a pdf (2004 Cayenne S MAF Sensor Replacement.pdf) with some pics of the process. It seems to have solved the problem, as she is running much better than before in general . I'll have to get out on a long uphill stretch to know for sure, as this is when the symptoms were most noticeable.

Interesting to me was that the idle speed dropped from around 800 rpm to 400 rpm after the change and then rose up to (a correct) 580 rpm after pulling the ECU fuse for a few minutes.

I have also purchased new pre-cat O2 sensors as these are also due for replacement (she has 221000 km on the clock now) . From what I have read these are more of a challenge to replace, so I'll delay this as long as possible.

I hope the info will be of use to somebody

 

I recently experienced a similar surging and also hesitation but it turned out to be the fuel pump.

 

So it turned out that changing the MAF sensors did not solve the surging problem, so it was on to the O2 sensors (and then the fuel pumps if that failed).
Porsche Center Johannesburg quoted 6 hours labour for the job and after reading some other accounts of the task on this and other forums, I was prepared for the worst.

I purchased 2 oxygen sensor tools on ebay in preparation.
Turns out the job on my 04 Cayenne S is not really that difficult, its awkward, but quite doable, (I 'm an aeronautical engineer not a professional mechanic so I consider my spanner swinging skills mid level).

The repair manual I used said to remove the wheel and wheel housing liner, but I found this unnecessary as the entire job can be done from above ,just removing engine covers and the airboxes.

With these removed you can actually reach the oxygen sensors with your hands. The problem is you cannot actually see them. I used a USB endoscope to get a view of what I was doing. The attached pic of the sensor was taken using the scope.

I've attached pictures of the oxygen sensor tools and a picture of the right side sensor. The problem is pointed out by the yellow arrow. The wiring exiting the sensor bends sharply immediately after the sensor as the sensor is very close to the engine block, this means there is not enough space to thread the tool over the sensor, so the nice chrome tool was totally useless for the job. I managed to thread the black tool along the wire of the left sensor and remove it. On the right sensor I resorted to cutting a standard 22mm spanner short (about 150mm) to make a "special tool". That worked perfectly.

So long story short, the job took about 2 hours and best of all the surging is now gone !!