I changed oil couple of weeks back in my 2004 XJ-8 using a Mann filter. All went well.

I could not remember when I had last looked at my air filter. Bad sign. Took off the airbox top and removed the K&N. Geeez. I must have forgotten to service the K&N for probably two years. That's not like me but stuff happens. I tapped out at least two tablespoons of dirt, grit and grime...maybe more. Blew out the rest with my compressor then washed the filter and let it dry. Oiled per specs. Funny, some critter had made a nest in the airbox at one time...lots of birdseed in the box. Vacuumed out and sprayed with Lysol in case the vermin had left some kind of cooties.

Removed the MAF sensor and cleaned it with appropriate chemical. Removed hose to throttle body and sprayed CSC TB cleaner inside. The amount of black stuff that ran out the TB was crazy. Opened the 'butterfly' and sprayed further back into the TB. More crud. Kept this up until run out of cleaning fluid was clear. Saturated a clean shop towel with TB cleaner and wiped the reachable interior of TB clean. Buttoned everything up and the performance of the 4.2L had increased appreciably. So, I purchased new PCV valve (by Uro?) and installed it. Out for a 10 mile test drive and all was good.

Then wife started the car a day or so later and plumes of white smoke flowed out of the exhausts. Ok, last thing I changed was PCV valve. Swapped back to the OEM. Same white smoke. Ran her hard (car not wife) and accelerated 5-7 times and white smoke seemed to have quit. Next day more smoke but not like a mosquito fogging truck. Taking a closer look at the hose from the airbox to the TB, noticed bottom of hose at TB was not seated properly and hose clamp was askew. It had slipped off due to my lack of diligence when replacing the hose/clamp the first time.

The moral of my story is: I took my time the second time around, used a long flat head screwdriver and made sure hose to TB was snug all the way around. No smoke. Accelerated hard 5-7 times, no smoke. I got on the nearest highway with 70mph 'posted' speed and ran her up to 85-90mph. Wow. The XJ-8/ X350 is running so much better better and accelerates with no hesitation whatsoever. I now need to find a way to keep chipmunks or field mice out of the airbox.

BTW, my 1989 Jeep Wrangler had the same critter problem. I used galvanized 1/4" hardware cloth to cover the air intake behind the driver's side headlight. I tuck it on with 'Goop" adhesive and a couple of self-tapping sheet metal screws. I'll take a look in a month or so and see how that worked. Cheers!