Question about 2003 Mercury Grand Marquis

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A coil on plug is breaking down causeing misfire, how to locate which coil is defective?

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These coils usually fail because the oil sel around the plug lets oil wick up the plug and shorts the coil. If you find an oily plug they you most likely have found the bad coil.
You can have the dealer hook up their diagnostic machine and they can tell which cyl it is.

Good Luck

Posted on Oct 14, 2010

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Posted on Jan 02, 2017

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Could be a coil breaking down , you did replace them did you ? Take a spray bottle of water an spray the coils , mite find cause of the misfire . Also look at coils for a discoloration , a rainbow affect . Or you can power brake the vehicle an take a test light an put neg . clamp end on battery neg . an then move the point end around the coils .

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2007 jeep grand cherokee is skipping and misfire codes


There are several different things that could make it misfire ! not just plugs ,does it have plug wires or is it coil on plug coils. Could have a coil breaking down. Does it do it in all driving conditions ? Idle , cruising speed ,under a light load and what codes .po300 random misfire or a specific cylinder misfire code ? P0306 cylinder #6 etc.

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Car keeps missing when driving , engine light came on auto zone ran a test on it i was told it could be about one of four different problems concering ignition or circuit prblems.


A misfire can be caused by a number of different defects
  • A spark plug may be breaking down under certain conditions.
  • The cables which carry power to the plugs may be defective.
  • If the "Check Engine" light came on the fault would be most likely found in the ignition system.
  • The ignition coils
  • The crankshaft or camshaft pickup sensors.
  • The computer output circuit may be defective
  • Any one of the above problems may be caused by the electrical circuits or a defective component
Auto Zone gave the correct information.
Regards Jonhgee10

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Hello I have a1997 xk8 runs great but my check eng. light came on and the code that it read was a misfire code were do I start troubleshooting.


If it was a specific misfire code go to that cylinder and check the following: plug, wire, and coil, if it is a random misfire code, start by checking your spark plugs(including the gap), plug wires, and coil packs.

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cylinder #6 is misfiring check plug(if gap incorrect can cause misfire, fouled etc) , coil on plug (or Wires) poss spark plug coil pack (check for cracks and heat damage in insulation of pack)

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Engine light on


They are still looking for a cause because it isnt misfiring right now or because they cant find the cause of the stored misfire codes ?
If it is misfiring right now and they cant figure it out then perhaps you should find new mechanics !
I have seen a lot of bad coils on the PT.
Misfire is usually caused by spark plug wires first, spark plugs 2nd, and coil third.
If it isnt misfiring now, spray down the wires with water. If it misfires after that replace the spark plug wires and spark plugs and try again.
If it still misfires then the problem will likely be the coil.



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#2 cylinder misfiring....changed plug and plug wire....still misfiring....what other situations could be causing the misfire??


Other causes could be: faulty coil; faulty fuel injector; poor compression; defective computer

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this could be caused by a couple of things. Defective plug wires, coil packs cause this as does the Ignition control module. Also being overful on oil will also cause a multiple misfire on your particular vehicle and others. Hope I narrowed it down some for you.

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Check spark plugs and wires. Could also be defective Coil.
I recommend replacing all plugs and wires at the same time.

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