Ignition issue? Paging Viperwolf

[needabronco]

OK here's an ongoing issue that I can't quite figure out. In the last 6 years I've been through 5 sets of spark plug wires, or approximately every 2000 miles or so.

It seems when I drive my bronco at highway speeds for 70+ miles I develop a misfire. When the torque converter isn't locked it's not as noticable as when I'm in overdrive then the shudder is very obvious. Every time I've traced the issue down to the spark plug wires. I change them out and all is good till the next highway trip. So far I've been through two sets of Bosch wires, Napa wires and 2 sets of Autozone wires (a cheap set and their premium set). I've never figured out exactly which cylinder is eating wires either so I don't honestly know if it's random or specific to one or more cylinders. I've changed exhaust manifolds from stock to WH shorties and I have plenty of header clearance.
I've got Mustang efi and an AOD, no codes and everything mechanically is in good to great condition.

What is causing this issue? My next step is going to be a new cap and rotor along with new wires, but I really want to figure out the cause so I can finally fix this issue. Thanks.

 

[Viperwolf1]

I'd say the wires or plug boots must be burning on something. It may not be obvious from the outside. You might be able to run a cylinder balance test as part of the KOER test. That might point to the cylinder(s). Might be a good idea to pull the plugs and see how they look too. If one has a huge gap the spark is more likely to jump to a ground through the wire insulation.

 

[broncnaz]

X2 on the plugs. Also if you dont already do it you may want to use a little dielectric grease on the plug boots as well. Might want to check your dist cap and rotor as well for any carbon tracking.

 

[Steve83]

Look at the inside back cover of any Haynes and compare yours to the photos. These may also help:

. . .

What brand & type of plugs are you using? Plain old Autolite copper resistors seem to work best in these old smallblocks.

 

[TwoDalesDad]

I just did a tune up on my ex wifez big bronco(351).....The only cylinder not firing was obvious....The header gasket had blown out and fried the wire in two. When I completed the task I pulled the paperwork to see when the last work was done. Ten years back...I used Ford Racing Wires, Autolight plugs, Cap, & rotor...no clue....The Bronco does run better......but she still won't let me drive it......

 

[ransil]

Open the hood at night and look for spark leakage.
I had misfires using platinum plugs, use cheap ones.
x2 on the dielectric grease on the boots

 

[707Bronk]

If you are fairly stock then I might start out with getting a set of OEM plugs. At least that way you are gauranteed to have the correct heat range, which is IMO the most important aspect of spark plugs. Aftermarket plugs don't always give you the correct heat range, and if you are using anything exotic then it becomes even less likely.

If you think your plugs and wires are getting burnt you may want to try those little protector boots.

Have you tried testing the wires to see which cylinder is burning up these wires? Determining if its one cylinder or if multiple different wires have failed might be a clue to something else.

Do you have any emission control equipment?

Definately check the cap and rotor and look for any carbon traces or cracks,

 

[needabronco]

I checked the plugs and number 3 and 8 were wet. I checked all of the spark plug wires over and ohm'd them and all check out. The cap and rotor were a bit suspect, and I noticed some discoloration on the ceramic of the spark plug where the metal meets the bodies, so I swapped out the plugs cap and rotor. I fired it up and verified the plug wires using my timing light. The only wire that showed any sort of inconsistency was the coil wire?

 

[Viperwolf1]

Coil wire can go bad too. Make sure it isn't rubbing on the bottom of the hood or anything.

 

[needabronco]

I checked it out too. I can't quite see how a coil wire would only affect 2 cylinders?

 

[Steve83]

If the coil wire was bad, its misfires would be present on all the others. The reason it appeared to have problems is that the timing light isn't designed to work at 8x the frequency of a cylinder wire - the strobe doesn't always have enough time to recharge & flash, so the light misses; not the engine.