CEL code P0303?

motopilot

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CX-7 Sport Platinum Silver, 2002 Protege
Guy at the autoparts store said is was a #3 cyclinder misfiring. I am going to look at the plugs once it cools off but any other ideas? The car started running a little rough at idle (going to try that EGR thread) and now when driving unless you are doing 65+ you can feel the roughness. Does anyone have any other ideas? Car is a 2002 Protege 2.0L with 105K on it. I have off this week so trying to get this done while I am off.
 
If it's just that one cylinder misfiring, first check your spark plugs (I would replace them @ 105K miles) and your spark plug wires (ditto for replacement). Also check your fuel injectors by swapping two of the same color injectors (if the problem moves to another cylinder, your injector is bad). Only swap purple for purple, and green for green, don't mix the colors.

If none of those work, you should check your compression to make sure it is in spec, and also check for air leaks between intake manifold and cylinder head, and also between the two parts of the intake manifold (spray some unlit propane around these seams and if your engine noise/speed changes, you've found a leak).

If none of that stuff works, then you've got a wiring issue, which will be a PITA to track down (usually). The nice thing about a misfire in only one cylinder is that it really narrows down your troubleshooting.
 
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