The importance of Spark Plugs

Regardless of how old or new your vehicle may be, check your spark plugs every now and then.  One of my cars is a Mazda RX-8.  Last summer, just after I bought it, the engine failed.  It happened just after I made some changes and experienced a massive oil leak, so it was probably my fault.  I wound up replacing the engine with one from a donor car.  That donor car had just 30,000 miles on it.  That's 70,000 miles less than the spark plugs are supposed to last.  Well, I had some terrible mis-firing under 50-75% throttle in the upper revs.  I changed out the coils and wires, knowing those to be week points on the car.  Most of the issue was taken care of, but it was still not running right.  I pulled the plugs and and was shocked to see them caked in crud.  With that much build-up, it's a wonder the car ran at all.  I changed in some fresh plugs and now she runs like a dream and the fuel mileage has gone up considerably.

Today I ambled out to the garage to start working on my Explorer.  The transmission is shot and needs to be replaced, but I really didn't feel like wriggling around on the ground today.  So instead I popped the hood.  I'd noticed the plug wires looked pretty damned old when I first bought the truck, so I figured I should at least check the plugs.  The gap is supposed to be .044 of an inch.  The first plug I pulled was fouled (nice and gray, though, so at least the fuel mix is good, and there's no evidence of coolant or oil leakage).  The surprise, though, is that the gap was .084 of an inch.  Almost double what it was supposed to be.  Once she's warm the truck seemed to drive just fine, so my fuel mileage on the highway should have been around 19mpg.  But she lacked "oomph" and my highway mileage was closer to 16.  Given my rather heavy right foot, you may not be surprised.  However, I've been pretty disciplined with this truck, so I should have been getting better mileage.  Now I know why I wasn't.  Given the state of the drive train I haven't bothered getting new plugs yet.  That'll probably happen this weekend.

The reason spark plugs are so important is obvious when you think about it.  In gasoline engines, the fuel-air mixture is ignited by a spark of electricity jumping the gap between the cathode and anode of the spark plugs.  Every time the spark jumps the gap, a little bit of spark plug material goes with it.  Over time this causes the gap to grow.  Eventually the gap grows wide enough to make the spark weaker, which in turn leads to less complete ignition of the fuel-air mixture.  With less complete ignition comes more pollution and lower gas mileage.

I'll let you know, when my Explorer is back on the road, what the results were of resolving the plug gap issue.