Intermittent, Random Misfire XC90 v8 Topic is solved

[PuzzledLlama]

Hey everyone,

I could use some help.

I recently bought a 2008 XC90 that had been almost completely rebuilt. New transmission, new coil packs, new timing belt, the list goes on and on.

Pretty soon after I got it, it threw a random misfire code without any change in performance. It'd throw this code once every 100-200 miles.

Things got progressively worse. Now when it throws the code, it actually feels like a misfire (increased engine vibration and loss of power), and the "Slow down or shift up" message appears. The engine hasn't ever stalled on me, but I have the car parked until I can figure this out.

The current codes it throws are random misfire as well as codes for each individual cylinder. Occasionally it'll throw a code that led my mechanic to suspect the engine crankshaft sensor. I'm not sure what code that was, but I went ahead and had them replace that sensor.

It didn't help. My mechanic (who specializes on Volvos), has looked over the engine a couple times without any success. He inspected the grounds with the coil packs.

His latest theory is that when the new transmission was put in, that they somehow damaged the flywheel, and it's got enough of a warp or wobble to cause random misfires. Although possible, that sounds like an expensive long shot to me.

Any other ideas of what might be causing this? Is there other info that I can supply to help?

Thanks in advance!

[Masscomguru]

I just went through this on mine. Same thing as you described. Mine ended up being the rear passenger side coil ground post. It felt tight but when I went to loosen the nut, the nut and the clip spun with little effort. I tightened it back down and have not had that issue since.

The key things for me to track it down was that even though it was showing random miss fires, I had always had a P0352 and P0354. Both of those coils are tied to the grounding stud I had to investigate.

[PuzzledLlama]

Thanks for the quick reply!

It sure seemed like a grounding issue to me, but I needed the extra nudge to dig into it. I normally do my own maintenance, but thought, man, wouldn't it be nice to hire it out for once. Haha

The mechanic said he pulled off all of the grounds and cleaned them, but I just pulled off the cover and looked, and they haven't been touched for years. They were covered in a mixture of oil and dirt, and that was preventing the ground.

I'll post back on this thread if the issue comes back, but I'm pretty confident after seeing the gunk in between the connectors (and using a wire brush to carefully clean out everything), that this issue is resolved.