Misfire

[Russell Lloyd]

Hi, folks. So, my Caddy is misfiring. No SES light -- yet. Had this once before and just needed one coil pack replaced. No SES so no codes yet, so was thinking of just replacing all the coil packs. I see them for as cheap as $9 online. But what is a "good" pack at a good price? Thanks!

[Cadillac Jim]

The SES light won't come on until you have a miss that is consistent enough to cause possible CAT damage if left too long. If you run the codes, you will probably have a P0300, and if you use a code reader, there will be a P030n where n is the cylinder number that tells you which cylinder is missing. If you have two cylinders that are fired by the same coil, that will tell you which coil is bad. If you have a different pattern, it's probably plug wires, plugs, plug boots, or another cause.

A bad plug wire is a whole lot more common than a bad coil. Take off the beauty cover and see if you can hear arcing, or if you can see it in the dark. Also, make sure that the plug wires are all parallel and in the notches provided in the beauty cover to keep them apart. If they touch or cross, they will eventually cross-arc to each other.

Then, there are spark plugs, plug boots, and the possibility that a tiny oil leak has filled up a plug well with oil or some such. One thing to look for is a bad or missing ground from the ignition module, the assembly that the coils bolt to. There are two, one through the wiring harness and one from the module, and if both aren't there you can have intermittent spark problems or drivability issues.

But, to answer your question, Rock Auto sells economy line coils for as little as $15 (not in my car!!!), OEM quality coils from Standard Motor Products and others for about $32, and AC/Delco coils for $44 or $49, depeding on which coil you have. AD/Delco lists two GM part numbers, D555 and E530C. I have no idea what the difference is. Most aftermarket coils cross over to the D555.