Q: Dealer says replace spark plugs and decarb, but missed oil leak on right side of engine. Throwing p0303.

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Had water pump and timing belt replaced last month. Now throwing a P0303 code for a cylinder 3 misfire. Took to dealer for diagnosis, they said new spark plugs and fuel system service (decarb is what they called it). They couldn’t get to it that day, so I came home.

I had noticed prior to taking to dealer some oil drops on the front passenger side, almost at the radiator. This morning there is fresh oil in the same place, and on the plastic cover at the bottom right below the timing belt area. To the left of one of the larger pulley wheels there appears to be some oil. Dealer did not mention any of this which was two days ago.

I had 102k service two years ago at a diff dealer, which was replacing the timing belt, spark plugs and coolant flush. They said I shouldn’t have to do plugs unless there is a problem. Looking for a 2nd opine as to whether I should expect this and should them not seeing the oil be a red flag.

My car has 142000 miles.
My car has an automatic transmission.

As you know code P0303 relates to a misfire, specifically in cylinder #3. This is likely unrelated to the oil drops you are seeing as those are more likely related to a gasket or a seal leaking. Engine misfires can be caused by many different problems and are generally caused by either a spark or fuel issue or in some 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. Due to the number of different things that may cause an engine misfire, the quickest and easiest way to diagnose this type of problem is to hook the vehicle up to a scanning tool which will download any potential fault codes that may have registered in the vehicle’s computer as a result of the poor running condition. I would recommend having an expert from YourMechanic come to your location to diagnose and inspect your vehicle.

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