What's Causing Periodic Engine Misfire?

Dear Car Talk

Dear Car Talk | Mar 24, 2015

Dear Car Talk:

I have a 2002 Mazda Protege, and it has been fairly reliable all these years. A little more than a year ago, I was driving home from work one night after a heavy rainstorm, and I hit a huge puddle at around 30 mph. A wave of water came over my hood and windshield. My engine light immediately came on, and my engine started to misfire. That continued until I got home. The next morning when I started the car, the engine light came on and the car misfired. After about 10 minutes of driving, the misfiring went away. That pattern continued for about three days. On the fourth day, the engine started normally and seemed fine. A couple of days later, the engine light went out.

Things were then fine for a few days, until it rained overnight. The next morning, the light came on again, and the engine started misfiring again. Now it happens whenever it has rained or when it's very humid. My feeling is that when I hit the water with a hot engine, something cooled and cracked, exposing something electrical, and the humidity is causing the problem. Any ideas?

-- Jim

This sounds like the most basic of water-related automotive issues, Jim. This car has an old-fashioned distributor cap and rotor. Most likely, when you forded the Nile that night, you got water inside the distributor cap, and it's causing the misfire by creating a short circuit. And I'd suspect that your problem is exacerbated by old spark plug wires that "leak" electricity when there's moisture or lots of humidity in the air.

This used to happen to cars all the time. Distributors and old wires would get wet on rainy days, and cars would die and strand people. AAA towing service still refers to that time as "The Golden Age"! But with distributorless ignition systems now, and fuel injection that prevents flooding, cars that don't start or run in the rain are really rare.

In your case, what's happening is that the moisture that's stuck inside the distributor cap is compromising your spark. The spark is sufficient when all the other conditions are perfect, but once rain or moist air steal additional energy via the old spark plug wires, the engine starts misfiring.

Eventually, as the engine heats up, the moisture in the distributor evaporates, the plug wires warm up and dry out a bit, and the cylinders all fire. But when the engine gets cold, the moisture re-condenses inside the distributor cap, and on the next rainy morning, you have the same problem.

While you might be able to fix it by simply removing the distributor cap and drying it out really well, I'd recommend replacing the cap, the rotor and the wires. That stuff's cheap, Jim. And it should solve all of your moisture-related problems. Except those bad hair days. Good luck.

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