On some GM models (4.2 inline 6 cyl. LL8 VIN code S – PRO LL8), water intrusion will cause premature failure of the coil and spark plugs— it’s not a coil problem!
If a customer complains of a rough idle and/or the MIL/SES light flashes on after a rain shower or car wash, this may indicate water intrusion.
Check for possible DTC codes P0300 through P0306 and inspect the coils and spark plugs. If upon inspection of the coils, a white chalk-like build-up on the inside and/or outside of the spark plug boots is detected, the coils have been damaged and must be replaced—Standard hi-quality UF303 coils should be installed. It is also advised that the spark plugs be replaced.
Water can seep past the engine cam cover and collect below the coils in the spark plug wells of the cylinder head resulting in a possible misfire. To prevent this condition from reoccurring, the AIP seal must be replaced with a replacement seal that redirects the water away from the engine cam cover.