[DrRobin]

I have had a slight rough running since August when I changed the CATs on a 2001 2.7L with 92,000 miles.

At first I got the odd misfire, but it was on a different cylinder and only happened at low revs after deceleration. My code reader said all four O2 sensors were slow to respond, so changed them but it didn’t make much difference.

I still got a misfire, about twice a week, but could avoid it if I kept the revs up or played the throttle a bit after deceleration and before feeding in the power. The misfire went when I fully charged the battery, but the car still didn’t run all that smooth, especially when warm.

After cleaning the throttle body, this improved it, so last to check was coils and plugs. After stripping out bank 1 last weekend the plugs all looked fine and I cleaned the coils, but noticed that two had a hairline crack. This weekend I obtained new coils and plugs and changed all six. The car is now running great, back to how it always was.

Looking at the coils all six had at least one hairline crack, they were Beru and changed at the main dealer at 55k miles (2009). The car now has 93k miles.

So my questions are

-Is 40-50k miles or 10 years about right for coils? It seems quite short, I would have expected 80-100k.

-What causes the coil to fail? Excessive heat?

The plugs were all fine, I can’t see on any of the receipts when they were changed, but it wasn’t in the last 6 years or 20-30k miles.

What is your experience?

Robin