Q: Cylinder 3 misfiring during rain intermittently. Got worse, now CEL is flashing. Changed plugs, #3 had carbon fouling. Next steps?

asked by on September 12, 2017

Hi! I have a 2009 Chevy Cobalt with 106000 miles. The transmission went out earlier this year, so I had it replaced with a new transmission (honestly should've just called it then, but whatever). A few months later I started to have an engine misfire on cylinder 3. It was infrequent and usually after rain or when the humidity was high. The light went off so I didn't worry about it. Now its back and the misfiring is consistent. Just in the last few days I've started detecting the odor of unburnt fuel and the check engine light has started flashing. I changed out the spark plugs, the plug on cylinder 3 was dark and ashy (guessing carbon fouling). Thinking about changing out the coils, but the fouling makes me think it's a sensor or other problem. The car only has a low value, is it worth taking any additional steps? If so, what steps should that be? Can I drive it at all or should I stop altogether?

Thank you, David

My car has 106000 miles.
My car has an automatic transmission.

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If you drive the car while the engine is actively misfiring, that can damage the catalytic converter. The problem you are writing in about is likely readily repairable and thus I suggest you request a misfiring diagnostic and the responding certified mechanic will get this diagnosed and resolved for you. "Global" engine sensors are not a likely cause of the problem because you seem to have a problem in only 1 cylinder. The coil itself should be tested including the power to the coil. There is also the possibility of a malfunctioning fuel injector and, worst of all, low cylinder compression due to a leaking valve but the mechanic will get you an authoritative answer and quickly. Finally, regarding your question about the economics of "repair versus replace", although there are some complexities in the calculation, generally speaking if the cost to repair and restore your present vehicle and make it FULLY functional is less than the cost of acquiring another vehicle, you should repair what you have. If you have further questions or concerns, do not hesitate to re-contact YourMechanic.

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