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Q: Bad 5th cylinder - misfiring?

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Does it need valve job or piston work

My car has 175000 miles.

It is unlikely that your motor needs a valve job or piston work judging by the amount of miles, absent some kind of catastrophic failure inside your motor. Often times, engines will have misfires for a wide variety of reasons that can cause a "dead cylinder" problem.

Engine misfires can be caused by a list of problems, but there are a few suspects that occur more than others. Generally, misfires are caused by either a spark or fuel issue or in cases, a combination of both. Spark related problems generally will result from things like ignition coils, crankshaft position sensor, spark plugs, spark plug wires or ignition modules not working properly. When the misfire results from a fuel related issue, this is commonly related to a lean fuel condition (lack of sufficient fuel supply to the motor).

Fuel related misfires can be caused by many different things such as low fuel pressure, faulty or dirty fuel injectors, a faulty O2 sensor, a dirty or failing mass air flow sensor, a faulty or dirty idle air control valve or a vacuum or intake leak. When the fuel supplied to the combustion chamber is insufficient, this results in an ignition (spark) that is igniting a less than balanced load of fuel and air. This results in a misfire or an explosion in the cylinder that is much less powerful than the other cylinders. This creates a loss of power that resonates throughout the motor additionally causing other problems with ignition and fuel timing. I would recommend having an expert from YourMechanic come to your location to diagnose your misfiring problem.

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