Gasoline direct injection question (relief valve)
 

I've been tracking down a fuel pressure problem with my '08 Cayenne 3.6 for about a week and have touched way more parts of this thing than I really wanted to now :)

Anyway, I had been having odd hesitation and stumbling issues off idle for a while and it finally got serious. It will idle and putt about just fine but any acceleration leads to heavy stumbling and while it will eventually get it to speed it's really undriveable.

I tossed my PORII on it and got code P2293 (fuel rail pressure). Running the live data stream the fuel pressure at idle is about 500psi but pulling away fro m idle gets up to maybe 800 psi then starts bouncing around between just 80psi and 800.

Through searching in a hurry I was convinced it was the high pressure fuel pump so I ordered one up and replaced it...no dice. Then I found some having the same issue with the low side pumps so I did the fuse pull trick and found it would not run at all on just the primary (drivers side) pump. So I ordered both low side pumps, in tank regulator and the filter. After replacing those still no change. I then decided all that was left is the high side pressure sensor and the pressure relief valve so I ordered both and installed the sensor but the valve is on a slow boat due late this week. In the interim I pulled the intake and inspected the injectors (looked fine but I replaced the teflon seals and all orings then reinstalled).

*I did find a broken actuator shaft on the vacuum operated varioram assembly so at least that wasn't a total waste.

So now here I sit waiting until Friday or Saturday for the part and I'm wondering if it would be bad form to mechanically block the relief valve just to test if that solves the problem. I'm not sure if the high pressure pump is internally (electrically) regulated or if it relies on this valve for normal operation on the high end. The risk would be over pressurizing the the high side and popping something like a fuel rail or injector.

The valve is a simple mechanical poppet that diverts pressure back into the low pressure supply line but I can't say for sure if it's part of the normal cycle.

Yea I know I basically threw parts at this rookie style but live and learn...anyway, here is the valve, thoughts? If not this what else could it be? Blockage in the low side pickups or lines? I'm running out of parts to replace :) Thanks and apologies for the novel. Here's the valve (which I have removed, cleaned and inspected but it's not something you'd see the issue on unless it was an oring which it wasn't).

http://forums.pelicanparts.com/uploa...1596594383.jpg

Offhand, I would tend to think "over-pressurization" will never cause full throttle WOT stumbling.

Low FP pressure problems usually only occur under high load..
So forget about "too much" line pressure. (i.e. that relief valve)

Put a fuel pressure gauge on the rail schrader valve, same as a bike pump, and measure it directly!
(And forget about what the lying computer says.)
It will be par for the course I'm just guessing.

As a stupid mechanic from the internet, I suspect the throttle body is gunked up.

There is a throttle bypass mixture circuit.
The Mass Air Flow/Manifold Absolute Pressure/combo of the two might be off.
(I honestly don't know what the modern pcars cars use)
Some "carb cleaners" will damage the delicate MAF wire. Don;t spray on that wire. Be warned. Use the approved ones.

Another possibility might be hard mechanical problems.
Valves are sticking or worn.
EGR is stuck open.
A vacuum leak from a hose. Use propane and see if idle speed changes.

Another possibility might be low system voltage at idle may confuse the computer.
Car computers don't function below 9V.

Start from the basics:
Battery. Alternator charging amps at idle and 3,000 rpms.
Those are known good.
Move onto the next set.
It's a huge list. One step at a time. It's all practice until the Olympics.

I appreciate the reply but this would be under-pressurization. These GDI engine run almost 2k psi (which makes testing pressure tough without special tools).

I had a tsi 2.0 that was extremely picky with fuel. It would hesitate and run like crap unless I put quality premium in it. It was to the extreme that I could fill up and notice when the tankers dumped regular in the premium tank.

Change the fuel filter, and possibility of knock sensor?

My apologies. I misread the original post. I thought the problem was idle misfire.

Start with fresh fuel obviously.

There may be shyte in the fuel tank blocking the fuel tank screen occasionally.
Full tank vs. 1/4 tank will make this more obvious.

The fuel pump itself might be weak.
Or it might be getting weak voltage.

Total power failure would indicate total system failure..ie...not some single injector but the entire delivery system.

Change the fuel filter.
Change it again.
Make sure the battery and alternator is supplying the 13-14V juice at the FP connector.
Then go from there.

Thanks guys, I can flow/pressure test the low side pumps/system easy enough (and no doubt I should have done this right off the hop but hey).

Assuming that checks out I was looking for thoughts on blocking the relief valve to test the theory that it's bleeding off prematurely.

If its purpose is just for emergency relief at abnormally high system pressure I would feel comfortable blocking it off for a test (until my part arrives and I can replace it altogether). But... if the system is designed to use this valve regularly to limit the high end of the pressure scale I'll not try it as I would hate to expose the downstream components to pressures that might damage something...I'll just wait for the part.

My guess is that it's just an emergency/abnormal-conditions safety valve and the pump regulates itself internally but I wouldn't be shocked to find I'm wrong.

Not the end of the world to just wait for the part, I have other vehicles but the impatient child in me can't help but want to know in advance so I can try something else in the interim.

make sure to check all vacuum hoses for leaks. on my wife's car (a 2009 cayenne 3.6) this led to the varioram not working and fuel pressure error messages.

My '08 Cayenne started running bad one day and I was barely able to limp it home. I changed both pumps filters/reg in tanks as I always had a noisy fuel pump since i bought it. Well the new in tank fuel pumps did nothing. Car would start but not take any throttle under load. Ended up being my high pressure mechanical pump. Bought a new one on Ebay and that was the problem. Pain to change, but it sounds like you already changed it to no avail.... Sorry, I do not have any more knowledge other than after I changed it and was checking codes, I realized I could monitor fuel pressure on my scan tool and it was very high once replacing the pump (I can't recall how high, but I do remember being shocked at how high it was).

Thanks, I just ran out and got a cigar to perform a smoke test (I have a high volume hand pump already) and pumped a full cigars worth into the intake and couldn't find even a hint of a leak. I'm still not 100% convinced there isn't one but this narrows the likelihood.