I've been searching the forum for some time now reading threads about our spark plugs. My 8 has 33k miles on it, so I know that I'm nearing Mazda's recommended replacement interval for the plugs.

My question is WHY do they need to be replaced? I took mine out at 30k miles and they had a decent amount of carbon build-up, but carb cleaner, a toothbrush, and some elbow grease took most of it off. What is happening to the plugs (besides carbon deposites) that requires them to be replaced?

 

good question, if you were able to clean them that well why would they need to be replaced?

 

From a practical standpoint, if the car is running fine and the gas mileage has not dropped recently, you probably don't need to replace them. But, aside from the carbon buildup on the center insulator, one has to be concerned with the high temps the spark plugs are subjected to in this engine potentially causing internal breakdown and erosion of the electrode and cracking of the insulator.

 

I guess that's why I'm reluctant to change them...the car runs fine and I'm still averaging 20-21mpg. I guess I'm just trying to avoid the $120 raping for new plugs.

 

^ The white porclean stuff will break down and crack, I alctualy had one NGK plug fail in the miata, the center of it cracked and air was getting in/out around the plug. Also the metal electrode on the end will wear down, from heat of combustion as well as the spark itself (the spark can be very hot).

Also remeber than in a rotary engine, every pass of the rotor there is compression and combustion, the super-hot, super-high pressure, happens with ever pass of the rotor. Unlike a piston motor where every other pass of the piston is combustion, there is no cooling stroke, so it uses the plugs up twice as fast.

The plugs cost what 120 bucks for all 4, and wires (i always do plugs and wires every year) cost another 100 bucks max, so thats 220 a year, or every 2-3 years if you follow the mazda service. Its really not that much money/hastle every 30k miles.

 

It looks like its about 72 miles on I4 and about 1.5 miles on surface roads, so leaving at 7 we should be able to keep at least 80mph for a avg speed. making the trip about 1 hr.

 

It's also a good idea to remove them once in a while and apply anti-seize compound to the threads even if you're not replacing them. Sparkplugs that stay in an engine for too long can become very reluctant to ever leave.

 

^
Right. What I'm saying is why not just clean them every 10,000 miles and not have to replace them every 30k. This would also prevent seizing problems.

 

Here is a pic of a plug that had done 20000 miles....up against a new plug. The plug was still firing ok before removal.

Your call.

 

Gomez, why do you have to scare the poor guy, HUH?!

Yeah, I'd definitely replace them.

It's kind of how you see young healthy 'looking' people having heart attacks! No telling whats going on inside...