Edmunds Answers



  • zaken1 04/07/12 3:32 am PST

    Sometimes when one section of a coil goes bad; it will cause the 2 spark plugs connected to it to become totally fouled with carbon. If that happens, and the coil is replaced; the motor will still not run on those 2 cylinders until the fouled spark plugs are replaced with new ones of the proper specification. If the spark plug wires have developed excessive resistance; this can also prevent two cylinders from firing. And bad plug wires can cause plugs to foul just like a bad coil can. So the plugs should be replaced whenever a coil or plug wires are suspected or found to be bad.

    Rather than assume there is no spark to those cylinders just because they don't fire; it is important to verify this assumption by physically testing for spark at both #1 and #4 cylinder plug wires: First connect a clean spare spark plug to the #1 cylinder plug wire, and clamp the threaded part of that plug to the metal of the engine; while leaving the #4 cylinder wire connected to the spark plug in the #4 cylinder. Check for spark at the plug that is clamped to the engine while the motor cranks or runs. If there is no spark at the #1 wire; move the external spark plug to the #4 cylinder wire, reconnect the #1 cylinder wire to its plug; and check for spark at the #4 cylinder wire. Only if there is no spark at either wire is it safe to conclude there is no spark getting to the plugs. A double outlet coil of this type CANNOT be tested for spark if one end is not conected to anything while testing for spark from the other end. Both ends must be connected to a spark plug that is touching the engine in order to have a complete circuit.

    Open circuited spark plug wires can sometimes destroy a coil or an ignition module. A spark plug wire in good condition should have less than 1,000 ohms resistance for each inch of wire length; so a 10 inch wire should have less than 10,000 ohms resistance. Of course; the ohmmeter must be connected to the metal terminal at each end of the wire in order to make proper contact.

    If there realy is no spark being produced by one section of the coil; the problem is most likely an open circuited connection in the 3 terminal low voltage wiring harness plug that attaches to the coil; or an open in the wiring harness or the plug at the computer end of those same wires.


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