I have a 99 S70 and I decided to change the spark plugs w/o changing the coils and one coil managed to go out in the process causing a misfire on cylinder #1. Yesterday I changed all 5 coils and the misfire is still present in cylinder 1 just not as bad. Do I need to change the plugs again? Could they have gone bad since they had to work harder? 143,000 miles on the car.

 

check the gaps on the plugs, also check the connector to the coil in question - look for cracks/corrosion in the wiring itself. If you are getting a specific code, ie P0301, try swapping the coil for #1 with a coil from say #3... If you still get an 0301, its not the coil - so consider replacing the plug or the connector.

 

The gaps don't need adjustment since they're Bosch as noted on Amazon before purchasing. I asked some people and they said it's probably the plugs. They probably fouled out after driving around with the misfire for 2-3 days.

 

while the plugs do ship pre-gapped, I still check since they can be off - I believe the '99s use .028 for NAs and .03 for turbos. I'd be curious to what you find - ie cracked insulator, blackened tip, a loose or soft connection to the plug cap etc.

 

Funny story. We took out the 6 day old spark plug and it cracked clean in half. Put in a brand new one and BAM. Problem fixed.

 

$2.49 and you're back in business. thanks for the update.