Out of curiosity, would a faulty injector cause a somewhat rough idle even before the computer shuts off fuel supply to that cylinder?

Yes yes and yea While the computer would not shut it off it would send signals to the fuel side to extend the injector dwell (amount of time open) as the O2 sensors would see a lean condition on this bank. IF the injector faults out enough to cause a cyl miss fire code. Otherwise it stores a pending fault in the memory.

Poor idle, loss of power, stumbles, etc all driveability issues raise their ugly heads. It would act just like a bad/dead plug when totaly dead otherwise it may crap out intermittently. Most just do not flow what the other injectors are flowing so there is all kinds of weird symptoms. One example would be wide open throttle (WOT) performance struggles. Like going up a hill, drops a cyl, stumbles, then picks back up, kinda stuff.

Well I spent some time listening to the injectors. I'm not exactly sure whether a bad injector will sound drastically different or whether the difference will be subtle. That said, I think I may have noticed a subtle difference in sound on the fifth injector. I can definitely hear the difference when the computer shuts off the fuel supply (stops clicking).

On a separate note, changing the DISA o-ring seems to have decreased the frequency that the p1351 code comes up, but it's not gone. I have also noticed that the code pops up primarily at idle. I can drive on the freeway for a long time with no code.

I'm torn between tracking down the oil leak that has gotten somewhat serious and seeing whether that might be impacting the code, or whether I should take the time to swap the injector from the fifth cylinder to the fourth cylinder to see if the code follows it.

Question about the injector swap: should I get a set of o-rings for reinstall or reuse existing o-rings?? Anything else that I should have on hand if I take the injector rail off?

Injectors o-rings aren't that expensive and like the DISA o-rings, after so many heat and cool cycles, the o-ring has a deformation that will only cause another problem.

Update:

I replaced the CCV. The PO had replaced only the hose from the bottom of the CCV to the dipstick tube and the others were very brittle.

I also changed the throttle body gasket while I was in there.

And the ICV.

Finally, I found some bad vacuum hose section leading from the auxiliary fan pump to the back of the manifold, so I replaced those as well (the small 1/8' style lines that connect to the plastic tubing).

None of these things appears to have made a real difference. The idle is still rough, even when there is no CEL or pending code, which makes me believe it is misfiring almost all the time now. But the issue seems only to present itself at idle. Never any problems when the car is in motion.

I think next I will do a compression test and then swap the injector and check the fuel pressure.

This problem is really annoying.

I read on another thread by SarTech that he had almost the same issue I am having and he never found a definitive solution, but that when he did an engine flush and changed to a slightly higher viscosity oil his issue seemed to subside. I may put that on my list as well.

As always, if anyone has insight it would be appreciated.

Update:

I conducted a compression test. These are my results:

Cylinder 1 = 160
Cylinder 2 = 180
Cylinder 3 = 170
Cylinder 4 = 175
Cylinder 5 = 140
Cylinder 6 = 170


Cylinder 5 has lower compression than the rest (and that's where I have my issue). I think I will try this next and see if it makes any difference.

https://www.youtube.com/watch?v=_Vo4fm0JDPY

Add a tea spoon of engine oil to the #5 cal through the plug hole and see if the compression does increase (rings).
Next up: leak down test. Tons of internet videos on how to. All you need is access to a air compressor and one of the leakdown testers. Real inexpensive as tools go.
This test will determine if the cyl seals, valves, etc.
there are many reasons for poor compression readings in one hole. It could be something simple like rings washed down by too much fuel. Does the oil off the dip stick smell like gas?
Other thoughts.....
if the Disa valve has been replaced in the past, there isn't the possibility of the shaft coming loose and making its way down the intake to one of the cycles.
Valve Burt/ bent not closing due to a valve spring, etc.
Here is hoping all end up simple. :thumbup:
Install a new/ known good injector and see if your issues go away.

After using the upper end cleaner (mentioned in the video I posted earlier) in each cylinder, my idle smoothed out a lot and I don't feel a distinct misfire anymore. It's not perfect, but the X5 has 170K. Also, it hasn't thrown a code since the cleaner. I think I might do the cleaning process again, and see if things improve even more, but it appears my code/misfire problem is solved. If it returns, I'll be sure to update this thread.

Well unfortunately my misfire is back. Still the fifth cylinder with fuel cutoff.

This led me to do a leak down test. The fifth cylinder is leaking into the fourth and sixth cylinders (about 30% or so). Oddly, however, the fourth and sixth cylinders aren't really leaking into the fifth cylinder (they both have a loss of less than 10%).

This leads me to believe that the head gasket is the fault.

It seems odd to me that the fifth cylinder is leaking into the others, but the others aren't really leaking into the fifth cylinder. Has anyone experienced this before?

My X5 has also developed the same problem on the same cylinder,
So will be following this thread closely