What a worn spark plug looks like
This is the sparkplug that I pulled out of the BMW E30. I'm not sure of the mileage, but it's about a worn as it gets. As a kid, I always wondered why spark plugs need to be replaced since it was just electricity passing from one point to another. The reality is that there's really not much going on aside from that so a worn plug is a very uncommon sight.
Heres a short lesson on spark plugs.
Spark plugs have 2 electrodes that are spaced apart by some dimensiom called a gap. An electric charge is applied to one side which then jumps from one electrode to another, creating a spark.
Over millions of cycles, the electrode, which is made of metal, is eroded by the electric current. While this won't prevent the spark from being generated, it does reduce the efficiency of the spark. For cars that run significant advance or retarded timing (such as cars with turbo or superchargers) or with high revving engines, you'll have a noticeable effect on your engine that could cause damage if left unchecked.
So how did this sparkplug get this bad?
Lots and lots of miles is the only answer. The BMW service manual calls for spark plug replacement at 30K miles as a preventative maintenance item, but this car must have went over the maintenance cycle by a considerable amount to show this much wear!
It would have been great to get a back to back comparison of how the car performs with new spark plugs, but the new plugs were being delivered a week later and I had to tackle the fuel injectors.
With the new injectors and plugs installed, the motor is butter smooth at startup and idle. I can't tell you if the smooth operation was contributed mostly by the plugs or injectors, but I can say that the engine is considerably smoother than before!
Steepshot
haha that plug truly doesnt look very useable anymore. :D
I just posted some pics about my Yamaha XT restauration, check it out :)
I drove one of my first cars for a long time without changing spark plugs until is started running really rough and had no power. This was a 4 cylinder Chevy Cavalier, so it was never very powerful to begin with, but I had to shift down to second gear to go up a decent hill.
When I pulled the plugs they were so worn that the curved electrode was burned all the way back to the base. The effective gap was about 0.200". One of them was even cracked in half right through the ceramic on the outside of the engine.
It's a wonder that engine ran at all.
Just saw this! I think you further validate that swapping out your sparkplug result in little if any performance gains unless your plugs are visibly worn out. Ie they haven't been touched in over 100k miles.