It all started with a P0101 - Mass airflow sensor.
This started in early January. I used the reader at AutoZone to read it and continued to drive it. It ran well and I had a lot of things going on between work and kids. As I remember it, the idle eventually degraded some. It was due for an oil change, so I opted to replace the MAF sensor at the time I did the oil change (132k). I purchased a remanufactured Cardone MAF unit from OReilly’s.
The morning after changing the MAF, I noticed that the idle had gotten worse and it seemed to miss when driving at speed. This time the code reader showed both the MAF code and a misfire on cylinder 3. I stopped back at AutoZone and they sold me ‘the best plugs they sell’ - NGK Iridium.
When I pulled the existing plugs out, cylinder 3 was obvious… the ground electrode was bent and touching the center electrode! It looked as if it had been dropped on the ground before being installed. I replaced the plugs (4/5), expecting this would do the trick. The misses at speed were gone, idle better, but still not right. I also noticed the turbo - now it had a pronounced change in tune between 1k-2k RPM when the engine was cold.
After reading multiple threads about ‘misfire on cylinder 3’, I grew concerned that I may have major damage… so I got desperate and took it to an independent mechanic that had been fair with my buddy on his BMW. Over the phone, he sounded like he could help. My first clue that he wasn’t a Volvo expert should have been when he didn’t appreciate my comment about the T6 transmission issues - and why I opted for the T5.
While he dug in, I called a local independent that specializes in Volvo. When I explained my trouble and what I’d done, he told me that it might be as simple as my spark plug. That he’s seen a number of Volvo’s struggle with the NGK plugs and that I really needed to install Bosch FR7DPP+.
After three hours of working on my ride, the mechanic informed me that to the best of his knowledge, I must have sucked something in the intake when I did the MAF swap. In turn, this damaged the turbo impeller (hence the change in sound), damaged the plug, and probably damaged an exhaust valve. He dismissed any suggestion that it might be a spark plug issue and acknowledged the fact that it ran good at speed but still didn’t idle quite right - even after installing an OEM MAF. Given the expense of pulling the head, etc. he figured I’d probably prefer to drive it as is. He provided me the compression test results (cylinders 2 & 3) saying that, while cylinder 3 was quite different from 2, he didn’t think that was it - cyl2=115psi, cyl3=80psi. I declined the OEM MAF and was happy to get it back - as, at this point, I wasn’t confident that he was going to help me and I was afraid of how many more hours he may spend trying.
I immediately stopped and picked up a Bosch plug - hoping that would be the silver bullet. The plug made things better, but it didn’t solve my problem completely. The CEL is usually on for MAF and misfire on cylinder 3. On Monday, I drove to work and noticed that the CEL had gone off. By the time I got home Monday night, it was back on.
So here’s where I’m at - I currently have the reman’d MAF installed with 5 good plugs (I believe
4 Volvo installed as part of 120k dealer service and 1 Bosch). It seems to idle well from cold (February in MN) but after sometime, the idle can get choppy. It runs well at speed and I haven’t noticed any changes in performance. I agree that the damaged plug sure points to something of significant size going through the system… however, for the life of me I can’t figure out how it happened or what it could have been.
A few other notes :
I changed the air filter along the way, because I thought it couldn’t hurt.
I suspected the PCV system, as there was oil around the ignition coil for cylinder 3. The glove test on the oil fill port demonstrated that there is suction. I originally thought I must have spilled some when I filled during my DIY oil change.
The top of piston 3 looked like it had some sludge on it. The mechanic said he thought it looked ‘oily’ when he assessed it with the borescope. It’s definitely not as clean as piston 2.
Prior to pulling the plugs, I swapped ignition coils without changing the symptoms or fault codes, so I think they’re all still good.
I tried cleaning the factory MAF unit by removing it and cleaning it with MAF spray - letting it dry really well before re-installing it.
With that as my story, I’m curious if anyone has been down this path. Does the theory of passing and object through and damaging an exhaust valve sound reasonable? Do the results of the pressure test suggest anything? Is there anything more I can try or should I just get used to it, knowing something happened?
2006 XC90 AWD 2.5T
