I replaced the plugs and coils at 87K as preventive maintenance before 100K. Now I have repeat coil failure 3 times on only #8. I have used new, good old and new oem. Nothing seems to fix it. What is my problem. I used new Bosch oem equivalent plugs. Why everything works on 7 out of 8. I have a 2004 S-type 4.2L v8.

 

What are the exact symptoms leading you to say the coil is bad? Are you getting a P0308 code? If so:

If the computer has set a misfire code, please be aware "misfire" wasn't the best terminology that could have been used. Misfire sounds like an ignition problem, at least to me, meaning the fuel and compression were there, but for whatever reason, the required spark was missing or weak. This might lead an aspiring mechanic to troubleshoot the daylights out of a perfectly good ignition system. I do not care to discuss how I know this.

In OBD-speak, misfire simply means one or more cylinders is putting out low power compared to the others. Pretty much anything affecting the output of a cylinder can set a misfire code. It can be low compression. It can be a valve not opening fully. It could be a clogged fuel injector. And yes, it could be an ignition problem, too, but don't overlook the other possibilities, too. [/soapbox mode off]

 

Yes I get p0308. Once I replace the coil I can drive it without misfire for 5 or 10 or 20 miles the first time, but upon restart up it misfires and has restricted performance message. Everything was good before I replaced plugs and coils, so why would injector or something else suddenly go bad. It idles rough, but runs fine at any other speed.

 

next time it happens, swap the coil to another cyl and see if the problem follows it

 

 

Check the connector for the fuel injector on cylinder #8 - make sure the connector is fully seated and locking tabs are present.

 

Bad wire?

 

After much more trial with error, I closed the gap on the plug an non measurable amount. I reinstalled the plug with one of the new coils which previously failed. It all is working so far. I have driven only 25 miles, but have restarted it 4 or 5 times without a misfire. I am hopeful the plug gap was the problem. Lesson learned is to always check the gap.