Dear Car Talk:
I got gas from Sam's and my 2001 Honda Civic started misfiring.
I changed the spark plugs and air filter. I put HEET, gas treatment and injector cleaner in the gas tank, but the engine is still misfiring. -- AJ
I got gas at Sam's once, too, and I haven't eaten there since.
There's something law students learn called the "post hoc fallacy," AJ. I learned it watching Perry Mason. In Latin, it says "post hoc ergo propter hoc." That's Latin for "Nuh uh, wasn't Sam's fault!"
The translation is actually "after this, because of this." And it's called a fallacy since lots of people mistakenly assume that because Event Y happened right after Event X, then Event X must have caused Event Y. But that's not always true.
If you start dating your secretary and your gutters start to leak, the first event did not necessarily cause the second event. But if you start dating your secretary and then you find yourself divorced, living in a washing machine box and grilling squirrels under a bridge, then X caused Y.
In your case, it's less likely that the misfiring was caused by getting gas at Sam's, and more likely that it's related to the fact that your car is 18 years old.
The first thing I'd hope for is a bad ignition coil. If the car is misfiring continuously (for instance, if it's misfiring at idle) it's easy to test for that. While it's running, try unplugging one ignition coil at time. Each time you remove a coil, the engine should run worse. So, if you disconnect one coil and nothing changes, that's likely the bad one.
You can test this further by replacing that particular coil. If the car then runs perfectly, you're all set. If it's still misfiring, then you could have a valve that's too tight in that cylinder, or if it's been too tight for a while, you could have a valve that's burned out. Another possibility is that you have a timing belt that jumped.
Check the coils first, AJ. And if you end up solving the problem, we'll conclude that's a direct result of writing to us. Post Car Talk ergo propter Car Talk.