Q: I got gas from a discount station, 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. Is discount gas really that bad for my car?

A: I've gotten gas at discount stations, too, and then I vow to never eat there again.

There's something law students learn called the "post hoc fallacy" (I learned it by watching "Perry Mason"). In Latin, it's "post hoc ergo propter hoc," which can be roughly translated as, "after this, because of this." It's called a fallacy because lots of people mistakenly assume that because Event Y happened right after Event X, the first caused the second.

But that's not always true. If you start dating your secretary and then your gutters leak, the first event likely did not cause the second. But if you start dating your secretary and then you find yourself divorced, then X did cause Y.

In your case, the first example is more fitting. It's less likely that the misfiring was caused by where you bought gas and more likely that it's related to the fact that your car is 18 years old.

The first thing I'd look 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 a time. Each time you remove a coil, the engine should run worse. So, if you disconnect one coil and nothing changes, that one probably isn't doing anything.

If that's not the problem, then you could have a valve that's too tight, 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, and if you end up solving the problem, we'll conclude that that's a direct result of writing to us.

Commuter concerns

Q: My husband and I have a 2014 Fiat 500 with 75,000 miles, but we need a second vehicle because we're both on call for our jobs. I commute about 45 minutes to an hour each way; my husband is just a mile from his job.

He wants me to take the new vehicle and he'll take the Fiat. He'd really rather have a truck, but I'm worried that it would cost more in gas. What do you suggest?

A: I agree with your husband that with you spending two hours a day commuting, it makes sense for you to have the more reliable vehicle. And the Fiat doesn't fit that bill. It's fun to drive, but its reliability record is among the worst of all cars rated by that major consumer magazine we all read.

It is perfect for your husband's commute, however. What's the worst that can happen if it breaks down? He'll never have to walk more than a mile. Buy the new car for yourself and an oversized umbrella for him in case he has to walk in the rain.

When the Fiat dies — or requires its next major repair — let your husband get his truck. Your focus on the cost of gas is laudable, but it's much more of an issue with your two-hour-a-day commuting habit than his quick buzz through the neighborhood. He's driving a total of 10 miles a week to work. Even if the truck gets only 15 miles per gallon, it'll still take him five months to go through a tank of gas.