Misfire And Raw Fuel Smell

[bigdog225]

96 Cobra. Had a slight miss so decided I would change the plugs on the weekend as it wasn't anything that appeared major and was only noticeable upon quick acceleration. Before the weekend arrived the slight miss became more evident by a very noticeable shake at idle and an unburnt fuel smell in the car.

Replaced the plugs with motorcraft plugs. There was noticeable improvement but still a miss, a shake, and fuel smell. Checked the coil packs with an ohm reader and all checked out okay. Took it to a friends house that has a nice snap on scanner. No CEL codes and no evidence showing of a misfire from the scanner in live read mode. He also had some spare coil packs that were the same as mine and so I tried those since they were there but no change. It was late so I called it a day.

Today pulled the wires and checked resistance and all seemed ok. Varying very slightly from one wire to another. Started car and began pulling each wire off of the coil pack to check for a noteable change in idle. Found one that did not make a change in idle as all the other 7 did. Wire tests fine with meter though... 8.78 ... a little over a foot long wire. Seems that the wire is bad but test fine with the meter. Thoughts??? Trying not to just throw money at it without reason.

 

[trinity_gt]

Did you check the compression on the cylinder corresponding to that wire?

 

[bigdog225]

I have not. Ill see if I can borrow a manual tester and check that.

 

[Rusty67]

If you want to check it for spark you can leave the spark plug out of the cylinder (hooked to the COP unit of course) and have a friend crank the motor over to see if it throws spark or not.

Edit:
I just realized you have a 96 Cobra so no COP units right ? Just leave it hooked to the plug wire....

 

[bigdog225]

@ trinity_gt - found a compression tester to borrow but wont get until tomorrow. Please GOD let the compression be good!

@ Rusty67 - good idea which leads me to think I should have swapped the wire with another close in size and see the results and if there's any change. Maybe the wire performs well with a meter but not with the electrical load from the COP. I pray this is the case and will report what I find.

 

[bigdog225]

Had a chance to swap plug wires last night with one close by and the miss moved with it. Sounds like I have found the culprit. Picking up a new set of motorcraft wires today. My lesson learned so far, just because a wire checks out with an ohm reading doesn't mean anything... Ill report if the new set of wires resolves the problem.

 

[trinity_gt]

My lesson learned so far, just because a wire checks out with an ohm reading doesn't mean anything... Ill report if the new set of wires resolves the problem.

'Tis true. Plug wires can carry in excess of 30kV of spark voltage. The quality of the insulation is critical. If there are cracks, for example, in the insulation you can have the spark energy exit the crack to end up as corona discharge or even directly into the engine block (ground) if the plug wire is lying on a bracket or valve cover.

An ohmmeter won't catch this. What does the wire look like visually?

FWIW, to properly test a wire with an ohmmeter would should connect both ends and flex the wire. This might show any breaks in the internal conductor that may open up in certain wire orientations. But you're still not testing the HV insulation of the wire.

 

[bigdog225]

Visually the wire looks excellent. No cracks at all in the wire cover and the rubber ends still seem very nice and durable. Obviously in this case looks can be deceiving. Picked up the new set today and will install tonight. Will report the outcome.

 

[bigdog225]

Replaced the bad wire, fired it up and all was good again. After rejoicing I replaced the remaining wires.

 

[joshjwc9]

Replaced the bad wire, fired it up and all was good again. After rejoicing with a victory beer I replaced the remaining wires.

Fixed.

 

[bigdog225]

You know it