In the past month I've had two Coil-on-Plug failures with the 2005 Scion tc I bought 4 months ago for my son. It started running really rough and the check engine light came on. I have an OBD II scanner so I hooked it up and got a P0303 (#3 cylinder misfire) fail. So I went to Advanced Auto Parts and asked for a set of plug wires and plugs. Thats when I learned of Coil-on-Plug and was told each coil was $140. So I bought the spark plugs and swapped 'em. No help. Then I took the Coil-on-Plug off the #3 cylinder and swapped it with the #4 cylinder. Now my OBD II sensor told me the problem shifted to a #4 cylinder misfire. So, knowing that at AAP they are $140 each, I went to AutoZone and was told they are $120 but they had to order it. So I went across the street to OReilly's and bought one for $100, but it too had to be ordered. The next day I picked it up, installed it onto the #4 spark plug and the problem was gone. Three weeks later, yesterday, the problem came back, only this time I had a #2 cylinder misfire. I swapped the #1 and #2 Coil-on-Plugs and sure enough, I now had a #1 cylinder misfire. Called Oreilly's, ordered another one, picked it up and installed it on the #1 plug. Once again the engine runs smoothly and I no longer have a check engine light.

I've searched the forums here and on google and nowhere have I found any evidence that Coil-on-Plugs are high failure items. As a matter of fact, every employee I asked at the auto parts stores told me they don't see many failures. Does anyone know whether there is something that can be causing these things to fail on this car? When I replaced the spark plugs I noticed that the wire harnesses to each of the 4 Coil-on-Plugs had been tampered with to reveal the wires (the protective sleeving was cut and pulled back about 4 inches). My gut feeling is that these two Coils had been causing intermittant problems for the previous owner and the mechanic was never able to correctly isolate the problem (at least that is what I'm hoping for). If anyone has any ideas or insight I'd love to hear from you. I sure hope this Scion doesn't turn out to be a lemon for me. The Consumer Reports ratings and appraisals of Scions convinced me to get one for my son.

tia,
Enrique

 

my gut feeling is the previous owner was an idiot and probably abused the car.

tried to mod the vehicle.. and maybe even tried installing even a worse performing spark plug system by adding wires (i.e. nology) because he didn't know better.

i can't imagine why a professional mechanic would tamper with the system by troubleshooting the wires like that. however, i have not ever seen the coil-on-plug system cause issues like you have described before.

at that point, i would hope that the previous owner hasn't caused any further lasting and/or underlying damage. that's the thing about buying a used vehicle.. you don't really know.

wish you the best of luck, and i'm glad you got it running smoothly again.
you have iridium spark plugs installed right? when you replaced them? if you did?

 

Yep. I bought and installed iridium plugs.

 

I have had VERY minimal problems with coil on plug coil packs going out. We have made TONS of power with them (up to 520whp) without the OEM coilpacks going bad.

Now it may be a different story if the previous owner did something like a grounding kit or hacked up some the coil pack wires to use something like an ignition amplifier and did not repair it correctly.

 

i've owned my 2005 tC for 5 years now since it was new. 80k miles, was supercharged and now turbocharged, and has had 3 seasons of autocross, plus thousand upon thousands of miles of cross country road trips. not a single problem.

it just sounds like you bought a mutilated car from an idiot. take the time one weekend and check out the whole car and see if there are any other problems that might come up from what that hack may have tried to do.

 

I think you guys a probably right on this one. I never considered that someone would alter critical componets of a perfectly good engine, and to do so would be way over my head. The only work I do on my cars is making an effort to diagnose and repair on my own. If the problem isn't obvious, I gotta take it to the shop. I hate working on cars but because I do maintenance for a living, I can figure alot a stuff out myself and therefore save myself a few bucks. If I lose another coil I'm gonna take it to the shop and let them figure out what I got myself into.