Question about 1991 Ford F150

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Why does a spark plug fire at idle but not on acceleration ?

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You may need to check the ignition wires and the distributor cap/rotor.
Older ignition wires will "break down" and the spark will actually short out on nearby metal/engine block etc.
Try this test. With the car running, parking brake set, and foot firmly holding the brakes, put the car in Drive and accelerate. If the engine misses under a load, it's most likely the ignition wires..


Posted on Jul 09, 2010

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Posted on Jan 02, 2017

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If the ignition system can't provide sufficient voltage the spark won't jump and the cylinder(s) won't fire or will only do so sporadically.

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