Q: van has a constant miss on 1999 Mercury Villager

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one code reader said cylender 4 misfire, so i changed plugs. still missed. my mechanics reader said random misfire, he said change wires, i did. still misses. is there a coil pack? should i look there?
(2) Answers
First you need to find out why #4 cylinder was missing, was it a plug, a wire etc? It could be a lean condition, so I would check your Long Term Fuel Trim, that can cause a random misfire, or a leaking intake manifold. Make sure that your Long Term Fuel Trim is not above 9% for either Bank 1 or Bank 2, if it is, you may have a vacuum leak or a defective Mass Air Flow Sensor or other things. I would also do a compression test just to be sure that the engine is still tight, you want to see at least 150 psi in all cylinders.
Try the distributer cap and rotor. Could also be a bad injector (of somthing else in the fuel system) or something mechanical. Run a compression check on the cylinder.