Fiesta - Misfire codes and running rough

June 17, 2024
Fiesta - Misfire codes and running rough The reason for the low compression was easily visible after the head was removed
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The diagnosis on this 2011 1.6 litre Fiesta goes to prove that the basic mechanical tests are just as important as the more modern plug-in diagnostics methods. This car came in with a a misfire, and the codes P0302 and P0300 showing, both of which refer to a misfire issue.


Starting by checking the coil circuit, along with the HT leads and spark plugs, no problems were found. A compression check conclusively proved that the problem was a lack of compression on number two cylinder, so it was decided that the cylinder head needed to be removed.

Once the cylinder head was off and on the bench, the cause of the compression loss and the resulting misfire was easily seen. One of the exhaust valves had burnt away. The cylinder head was sent to be fully refurbished by our local machine shop before being refitted.

The misfire problem was rectified and the motor returned to running smoothly.

This is a good reminder to always check the basic mechanical points, ensuring there is a compression on the cylinders, before getting carried away with electronic diagnosis.

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