My wife's 850; Goldie as we call her had a misfire over the weekend so we left the car at my mother-in laws as I didn't want it being driven and having all that unburned fuel dropping down into the new synthetic oil I'd just fitted. The OBDI system didn't or couldn't detect properly that there was a misfire at all (it's a Siemens or Fenix system as fitted to 10 valve Volvos) I removed the cap & leads to find a very pitted set of contacts in the cap. I already had put aside an aftermarket cap a couple of years ago and was sitting on my desk looking for a new home. I also had a spare set of leads from a motor that had low miles (about 75K miles) and had been sitting in the toolshed unused for a couple of years.
One of the contacts on the old cap had a severe pitting mark (white powder) on the inside close to one of the contacts so I chose to swap the cap over with the new one. I used the Bosch black shield back on top of the aftermarket cap and I used the used leads even though the ones in the car were probably OK. Result one very happy motor with no more backfiring.
