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Q: Car shakes violently and 'check engine' flashes

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Earlier, I was driving my car and it started shaking violently - the whole car shook. The check engine warning light flashed on, so I went to an auto shop and plugged in the computer. It read: “Misfiring on #3.” After that I replaced all spark plugs, wires, and even the valve cover. No help at all to solve the problem! My car is still running, but it roughly idles and there’s no power when I accelerate. However... ‘everything gets okay’ when I drive past 25mph. What is your assessment of all this and how do I solve the problem?

A: Since you are only getting a misfire on cyl...

Since you are only getting a misfire on cylinder number 3 specifically, this informs me that the cylinder ignition is not the problem. Since the engine has a coil pack that powers two cylinders at the same time and if #3 was misfiring then most likely the cylinder number 1 would have a miss as well.

From what I can tell, you have a fuel injector that is clogged or not working. Another possibility is a leak in a vacuum for the cylinder.

Since you've mentioned that the problem goes away after 25 MPH then this tells me that the EGR is most likely stuck open. This would cause the cylinder to misfire at low speeds and would not effect the engine on higher engine speeds. Have the EGR checked to verify if it's stuck open.

I'd recommend getting some help from a certified mechanic for these checks. They will be able to diagnose what's causing your car to shake firsthand and fix it accordingly.

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