My car has 25k miles on it and after my last track session, I decided to check the condition of the spark plugs. (easy to do since the car was already on jack stands for brake bleeding).

I found the plugs to be heavily covered in deposits and some of them were pretty black. Also one of the leading plug wires did not fit properly on the plug. It was very loose and was held on mostly by the boot. Internally it was very coroded and green, as was the plug it was connected to. I'm guessing that plug wire was loose from the factory, and arcing.

The other leading plug wire wasn't assembled correctly. While it did fit tightly, the internal connector was angled off center which made getting it back on a chore.

I ended up buying new spark plugs and have new wires on order (Magnecor). The maintenance schedule for plugs is every 35K miles (schedule 2) but I think that is too long to wait. I'm going to start checking them every ~15K and replace if necessary.

After changing the plugs, the car ran better. It now has a smoother idle and appears more responsive. Subtle difference but detectable. Still waiting on the wires though.

 

Agreed. I pulled mine last month and noticed the same heavy dark brown and black deposits. Mine had 29000 miles on it at the time.

 

Mine were thrashed at 40k - warranty work

 

just changed my plugs, all was fine

the leading plugs will always look dark and the trailing plugs will always look reddish brown, just the nature of the beast, it's not necessarily indicative of them not working well

 

What is the average life of original spark plugs for a new model conventional piston engine car these days, in comparison? I think my other car ('05 Nissan)'s plugs are good for over 100,000 miles.

 

These plugs are rated to last that long too. I think with all the issues with PCM flashes they were fouled earlier than expected

 

I think that cgrx is probably correct, but even piston engines fail to meet their "good for" lifetime. My old '92 Nissan engine promised 100k out of the spark plugs, but I never found that to be the case. In fact, I found substantial improvement out of changing them every 20k (Of course, those plugs were much cheaper...). Besides the 100k rule is probably if the engine runs perfect, with perfect maintenance, and perfect gas... you get the point.

 

The Mazda maintenance schedule I am using is for plug changes every 35,000 miles.