View Full Version : Coils / Intermittent "missing" on my 99 STS

08-06-04, 09:52 AM
Hi there,

I have an intermittent problem on my '99 STS. Seems also to happen only when warm. Problems is symptoms of severe misfire, whole car shaking... definetly not running on all cylinders!!

I do get the flashing yellow light - indicating misfire - in *some* of these intermittent conditions, others I get like misfire but the SES light isn't on.

No codes stored either.

I've replaced the plugs and wires, but I've come to the conclusion that perhaps it's the coils... reason is I noticed (on my 'vette) that if I pulled a spark plug wire out (so it was running on 7 cyls out of 8), the engine was not that rough... it'd take 2 cyls to really get a rough, bucking engine like I'm having.

So, I figured - what with new plugs and wires, and it's an intermittent problem - perhaps 1 of the coil packs is bad... this would give misfires in 2 cylinders, hence a real shaky engine.

Trouble is, as this intermittent, I can't reproduce it on demand, ready for me to diagnose!!

So, my question really is: is trouble like this (bad coil packs --> intermittent misfires) "common" [or rather is it uncommon?]?

If they are intermittenly bad, there's nothing I can do to figure out which of the 4 coil packs to replace, right? or is there some sort of test?

At worst I can replace all 4 for a $130 set; but $20 or so each is cheaper still!

08-06-04, 11:19 AM
Just replace 1 coil....and retest. If you have the same problem....reinstall the original coil and try replacing the next one. And so forth... Its a $20 test.

Logan Diagnostic

08-06-04, 01:38 PM
Aha - lateral thinking, didn't think of that!! I guess it must be Friday... or the heat ... shoulda thought of that...

Yes, a $20 test is what I'll do!

Many thanks, and have a great weekend. Greetings from England.

08-06-04, 10:48 PM
I had a similar problem. What I did was, pull one wire from the coil and start thr car and watch for a spark. Keep doing this until you find the bad coil, if that is the problem.