Fuel pressure rough idling and won’t rev?

Tiny
GF
  • MEMBER
  • 2009 PORSCHE CAYENNE
  • 4.8L
  • V8
  • 4WD
  • AUTOMATIC
  • 135,000 MILES
Hi, so I have 6 second cranking time before the car starts. Once starts it’s rough idling and won’t rev over 4000 rpm.

Plugged in topdon, live data shows fuel high pressure at around 4500 mbar. It should be 10x that at 40000 mbar at idle at least.

Swapped out HPFP for China one but no difference. Getting 5 bar pressure at tank side. It all points to hpfp but it’s £800 and am not 100% sure that’s the issue.

Any ideas?
Saturday, November 8th, 2025 AT 1:36 PM

11 Replies

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 54,137 POSTS
Yes, does the engine make a ticking sound at idle from the area of the high pressure fuel pump? If so the problem can be the camshaft lobe is worn out which will cause low pressure. The pump will need to be removed and the lobe inspected to confirm, also I have seen problems with the lift pumps inside the fuel tank starve the HPFP. If you have a check engine light on the fuel pressure sensor on the fuel rail on the engine can be bad and cause the pressure dump. Lets run the codes to see if any are present.

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

Please go over this guide and get back to us. Also, upload a short video in your response of the problem, so we can see what's going on.
Was this
answer
helpful?
Yes
No
Sunday, November 9th, 2025 AT 12:30 PM
Tiny
GF
  • MEMBER
  • 4 POSTS
Thanks Ken, I can’t say I can hear a ticking from the HPFP. I did a pressure test of the low pressure feed to the HPFP and it’s 5.5 bar holding strong so happy that the feed to the HPFP is sufficient. I also ran the starter for 20 seconds and I’d delivered probably 2 litres into a petrol can.

See error codes attached. Suspicious of hp fuel pressure sensor tho although replaced it 3 months ago.
Was this
answer
helpful?
Yes
No
Sunday, November 9th, 2025 AT 5:13 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 54,137 POSTS
What is the error code I don't see it? It is good that the lift pump is working well, but it does sound like the HPFP is out or the lobe on the camshaft is worn and not working the HPFP correctly. I would remove the pump and inspect the cam lobe and if okay replace the pump. But, please type out the code number here first.
Was this
answer
helpful?
Yes
No
Monday, November 10th, 2025 AT 9:11 AM
Tiny
GF
  • MEMBER
  • 4 POSTS
Hi, sorry maybe photo of codes didn’t send. Codes are P1026 P1031. I have done a compression yes too. All cylinders are around 225 psi except cylinder 1 which is about 190psi. I’ll take pump off tomorrow and take photos of lobe if I can. Thanks for all your help. I’ve been working on this off and on for 6 months!
Was this
answer
helpful?
Yes
No
Monday, November 10th, 2025 AT 10:54 AM
Tiny
GF
  • MEMBER
  • 4 POSTS
Hi Ken, just remembered something. When I put the China pump on a few months back, I didn’t fit the white plastic stopper that holds the clutch onto the end of the crankshaft. It was broken from what I remember but stupidly thought it was just there to help with assembly. I have been reading that it is very much needed! I also remember the remaining part of the plastic had slipped further into the hollow end of the crankshaft :(. I just left it in there. I think that might have been the problem all along! I will take the pump out tomorrow and remove what remains of the plastic stopper.
Was this
answer
helpful?
Yes
No
Monday, November 10th, 2025 AT 12:50 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 54,137 POSTS
Yep, I think you found the problem, the pump must have the stopper on it, also I would get a an OEM pump for it, (they barely work as it is) for the best results if the camshaft lobe is okay. Try uploading a short video that should work.
Was this
answer
helpful?
Yes
No
Tuesday, November 11th, 2025 AT 9:36 AM
Tiny
GF
  • MEMBER
  • 4 POSTS
Hi Ken, so the plastic stopper arrived today. I fitted it but still no joy! Only getting 5600 mbar pressure at rail. Even swapped back to the old porsche HPFP but the same result. I am getting a P1031 fuel high pressure sensor stuck at start - limit exceeded error. After Googling this error I see a few results, one is pointing towards O2 sensor. Confiused! :(
Was this
answer
helpful?
Yes
No
Thursday, November 13th, 2025 AT 6:06 AM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 54,137 POSTS
Failing High-Pressure Fuel Sensor (MOST COMMON)

The sensor at the end of the fuel rail sticks or reads incorrectly during cold start.

So I would replace the sensor which should fix the problem, did you notice the camshaft lobe when you had the pump off? Here is the location of the sensor and I will email it to you as well. Check out the images (below). Please let us know what happens.
Image (Click to make bigger)
Thumb
Was this
answer
helpful?
Yes
No
Thursday, November 13th, 2025 AT 10:55 AM
Tiny
GF
  • MEMBER
  • 4 POSTS
Thanks Ken, I replaced the high pressure sensor back in May. I suspect the replacement was faulty though but only realising this now. I have the old one off the car but can't see anywhere online on how to test its resistsnce (if thats how you test it) put ohms meter accross all 3 pins in every variation but get no reading on my multimeter. I have raised a warranty replacement that should arrive next week. Will let you know how I get on. I have had that inlet manifold off and on about 3 times now lol! Thanks again for all you help. I feel a fix is near!
Was this
answer
helpful?
Yes
No
Thursday, November 13th, 2025 AT 11:02 AM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 54,137 POSTS
Yep, you are on the trial, it will be difficult to test the sensor unless you test it in operation since that is when it fails, unless it has an open circuit which most do not. I would get a new part because the old one sounds defective. Please let me know.
Was this
answer
helpful?
Yes
No
Saturday, November 15th, 2025 AT 8:29 AM

Please login or register to post a reply.

Related Engine Low Power Content

Fix an Engine with Low Power!
VIDEO
Fix an Engine with Low Power!
Instructional repair video
Symptoms of Low Engine Power
GUIDE
Symptoms of Low Engine Power
The power your car's engine is producing is important. When you step on the gas you need your car to accelerate, mainly to keep you out of trouble when getting onto the...