Ignition Miss

[oldfaithful]

Well here is another ignition misfire problem. Its a 97 with 4.0 L and a stick. Over the last week or two its developed a miss under all conditions and its gotten worse. I checked out the ignition with an induction timing light just to verify that all cylinders were firing...and number 3 isnt firing at all. All of the other plug wires are showing random misses as they fire...which is seen in the timing light. I guessed coil and installed a coil pack but no change. Any ideas? Thanks a lot.

 

[BRUTUS_T_HOG]

A timing light is not the correct way to check for spark. If the spark plug is fouled and that cylinder requires very little voltage to fire a spark there won't be enough energy to trigger the timing light. The spark should be able to jump a 1/2" gap to ground from the wire. If the spark is forced to jump a gap your timing light would pick it up.

I use a timing light to check for spark during a no start diagnosis, but I force the coil to work a little bit so I know the timing light will trigger if a spark actually occurred

 

[oldfaithful]

Well problem solved. Thanks for the reply but just on a hunch i guessed that perhaps all of my "double platinum" plugs might be going bad...so i swapped the dead plug on no.3 for an old standard plug and guess what. The spark is steady on the induction timing light and the terrible miss is gone. Number 3 must have just been the 1st to go with the rest not far behind. Those double plat plugs were only 2k-3k miles old and still looked new...

Thanks, and good luck to you on your own wild goose hunt...

 

[TrexMex]

Well problem solved. Thanks for the reply but just on a hunch i guessed that perhaps all of my "double platinum" plugs might be going bad...so i swapped the dead plug on no.3 for an old standard plug and guess what. The spark is steady on the induction timing light and the terrible miss is gone. Number 3 must have just been the 1st to go with the rest not far behind. Those double plat plugs were only 2k-3k miles old and still looked new...

Thanks, and good luck to you on your own wild goose hunt...

Spec calls for Motorcraft AGSF22FM platinum spark plugs (SP-500). I have found the hard way to go by the book. BTW spec changed and the spark plug is longer on the 97 from prior years, dunno when it started, though. Just a heads up.