Heavy Misfire at idle
10-22-2018 | 01:58 PM
#1
Thread Starter
1st Gear
Joined: Apr 2018
Posts: 1
Likes: 0
Heavy Misfire at idle
Hey guys, currently having a hard time with my 08 Cayenne 3.6L. It is having a heavy misfire at idle and when revved the misfire goes away. I've read out the low pressure fuel system pressures which at idle varied by over 60 psi so I replaced both low pressure fuel pumps, fuel filter, and regulator, no fix. I then read out the high pressure system and found it is at 9 bar when it wants 39 bar, pressures are even at rev. I replaced the high pressure fuel pump, no fix. I replaced the high fuel pressure sensor because I found the vehicle idled like normal with it unplugged but after I replaced the sensor it still misfires. I tried to unplug the MAF because I know it can do weird things, no change. I've tested to see if there is a leaking Injector but there is not. Does anyone have anything else to try besides removing the intake manifold and inspecting all components? I have no clue why the fuel pressure is so low at idle but fixes it self at rev with everything I've replaced. Thanks guys!
10-22-2018 | 04:29 PM
#2
Banned
Joined: Mar 2011
Posts: 10,084
Likes: 1,164
From: Spring Lake, NJ, US of A
What do the codes tell you? Read with a Porsche specific code tool? I assume you have a code reader since you read out the high-pressure side of the injection system. I'm sure some codes are stored somewhere.
The fact that the low pressure side varies by 60PSI really indicates something is wrong with that system. The HPFP requires a certain constant input fuel pressure to work correctly.. but it's possible that it's erratic behavior is the cause of the wide variation of input side pressure.
Unfortunately, since you can't read the fuel pressure via the high-pressure sensor if it's unplugged...
It almost sounds as if the ECU is causing the HPFP to vary in pressure - not reacting correctly to the pressure sensor. And when you unplug it - the HPFP goes into a default constant pressure mode, resulting in improved idle.
Very odd - is there a control circuit going to the HPFP? If so - what happens if that's disconnected?
The fact that the low pressure side varies by 60PSI really indicates something is wrong with that system. The HPFP requires a certain constant input fuel pressure to work correctly.. but it's possible that it's erratic behavior is the cause of the wide variation of input side pressure.
Unfortunately, since you can't read the fuel pressure via the high-pressure sensor if it's unplugged...
It almost sounds as if the ECU is causing the HPFP to vary in pressure - not reacting correctly to the pressure sensor. And when you unplug it - the HPFP goes into a default constant pressure mode, resulting in improved idle.
Very odd - is there a control circuit going to the HPFP? If so - what happens if that's disconnected?
10-22-2018 | 07:28 PM
#3
Instructor
Joined: Oct 2017
Posts: 233
Likes: 8
From: Colorado
Don't know on the V6, but on my 4.5L V8 Turbo, I had the hose come loose from the charcoal canister purge line into the manifold, and the car ran extremely rough! Felt like rod knock combined with multiple misfires... Plugged the hose back in, and it was fine! Check all the "vacuum" hoses.
