Hi I’m new to the site, not sure If I’ve posted this in the right place, if not please excuse me.

my names Adam.
I own a 2003 cayenne 4.5 turbo petrol (not the s)
car has 110,000 miles on the clock
quantum remap (500bhp) (750nm torque)
great service history

ive owned the car a few months now, so far replaced

water pump
thermostat
t-piece water pipes
coolant temp sensor
expansion tank + cap
auxiliary water pump
alloy coolant pipe upgrade under the manifold
plus a few other bits and bobs water related.

The car was only boosting to around 0.4 bar, had a few boost leaks that I fixed,
changed the Bosch diverter valves as they were leaking, and a few other bits and bobs

the car started to boost like the remap requires, not sure if actual pressure as I haven’t monitored it, the factory gauge on the car maxes at 0.8 bar

car ran absolutely spot on for weeks, no issues what so ever

I got in the other day to drive home, car felt normal as always, until I wanted to join on to the motorway.. I accelerated onto the slip road about half throttle, the car was in 3rd gear, it accelerated fine untill it reached around 3500rpm, the whole car started to shake, best way to describe it was it felt as though I had a buckled wheel, or I was driving down a cobbled road fast I immediately pulled over to check my wheels and other external parts.

so far
wheels have been checked, they are balanced fine, no buckles ect, tyres are good
all bushes/ball joints/ mounts have been checked, everything seems tight
there was a split cv boot on the front right axle, I replaced the axle to be safe
Cardan Shaft centre bearing seems ok, no play

at this point I contacted a friend who has been a Porsche mechanic for some time, upon explaining he immediately said it would be my transfer box at fault.. I was sceptical about this as the gear shifting ect is super smooth, I insisted he take the car for a test drive, he then said to me “no it isn’t your transfer box” but he isn’t sure what it is

since then I tried to gently drive the car and recreate the issue, it happens in any gear, only on half to full throttle, at the same Rev range, once it starts to shudder/vibrate, if I back off the throttle slightly the power comes back in briefly and then drops off again, I assumed from there it was an engine misfire

I replaced spark plugs, checked coil packs, did a diagnostic scan, no codes are present on any module in the car

i remembered at that point having an issue with a Subaru Impreza I used to own, jerking under hard acceleration, it turned out to be the boost pressure sensor faulty, once replaced it was fine..

so I unplugged the boost pressure sensor on my Porsche to try it, the one located in the plastic y pipe that connects to the throttle body.. I took the car for a drive, obviously the car puts its self into a safe mode, the boost is restricted to 0.4 bar again, but when I floor the car it flies through the Rev range with zero shuddering or vibration, it feels silky smooth again Just underpowered..

i have ordered a replacement sensor in the hope this cures the issue, but in the mean time I was wondering if anyone on here had an input or something similar they had experienced themselves Incase the sensor doesn’t fix my issue

many thanks in advance and my apologies for the long read

adam

 

I had a similar problem on my 2004 CS. I had a shudder and lack of power, and also surging when coming off power on the highway. Also no fault codes.
Problem on mine was primary oxygen sensors. Replaced them and problem went away.

When in "safe mode" the ECU runs "open loop" which means it ignores feedback from many sensors and runs at default values, so it indicates that it is probably a sensor issue

I found out after I had already replaced my sensors that an OBD2 mode 6 analysis can identify some sensor issues ie deteriorating O2 sensor , before any fault codes appear.
Most technicians don't even know what mode 6 is.

 

Thanks for the reply, I did consider o2 sensors, I’ve watches the live data and they seem to be switching ok but it’s still a possibility, tonight I did some tests, I vacuum tested the diverter valves, they have less than 50 miles on them but one has completely failed already, I have the original bosh ones that were taken off, so I’ve replaced the damaged one with an original, the car now drives 99% better and the vibration/stutter is minimal, my conclusion is that possibly when 1 diverter valve failed the turbo on one side was delivering full boost, and the turbo on the other side was bleeding off boost causing and imbalance of the banks making the engine shake.. now I’ve replaced with the factory Bosch it’s improved significantly but is still slightly present, so I’m thinking perhaps the Bosch valve being so old is leaking slightly compared to the other side which is new, causing the same imbalance effect but not as harsh... I’ve orderd 2 new valves to rule out but it would seem it’s definatley due to an engine misfire, I mite just replace the sensors like you say for peace of mind..

thanks again for for the input

 

Have your fuel pumps been changed? You are right about at the age and mileage where they give up the ghost. Mine at 105k miles was running on one pump, the other was already dead, and the remaining one was starting to die. Simple check: remove the cover to the engine fuse panel located up by the firewall (driver's side in LHD models). If memory serves, fuses 13 & 14, both 15 amp, run the two fuel pumps. Pull one, start engine and let it run. Replace the first and pull the other fuse, start the engine and let it run. The engine should run fine on either pump. If it dies on either one, then that pump is bad and the other will die soon. Replace them both and the fuel filter while you are in there.

Good luck,

Brian C.