Van has 135,000 miles on it. The CEL came on--solid not blinking. Scan showed a miss on #8. New OE platinum plugs have maybe 2,500 miles on them, so I'm thinking plug wire or injector (assuming the CEL returns). Coil packs and wires are original.

Is there any common issue to these engines or that van that might cause this outside the usual stuff?

 

It's not uncommon for coils to fail as they age, which would probably be the most likely culprit. I'd suggest swapping the ignition coil to an adjacent cylinder and see if the misfire follows the coil. If so, replace the coil...if not, look deeper.

 

I've ordered plug wires and coils. This is a van we plan to take on some longer trips and for the cost of replacing these old ones I'm willing to spend what it costs.

Thanks

 

I wouldn't do that. Ever heard of something called the bathtub curve?

https://en.wikipedia.org/wiki/Bathtub_curve

The highest likelihood of failure is at the beginning of a part's life, and as it nears the end. You could replace all 8 with Motorcraft coils and spend $$$ and most likely have good results, but others are substantially less reliable. Even with Motorcraft parts, I think you're even more likely to be in there again replacing a coil if you replace all of them.

 

This van has the two coil packs not coil on plug, so that cost isn't great. Given the original spark plugs had eroded to .70+ from .52, these coils would have been under significant stress as that plug gap widened.

Some years ago, I represented an ignition manufacturer and was taught with HEI ignitions, the demand on coils was much greater than the older, large, oil filled. Maybe they were trying to bump coil sales, but I often replaced these parts in my drivers. Cost of my samples was nil, but the engines ran stronger and smoother after.

 

Aaah, gotcha. Forgot that the early 4.6L engines used coil packs, which changes that dynamic a bit.

 

Repair shop says the initial diagnosis is a bad coil (pack). He's replacing, and we'll see what happens.
Thanks

 

save the good one for a spare

 

I feel like the TV guy, with a "Breaking News" flash . . .

Spoke to my mechanic later Tuesday--new coil didn't help. He says the spark on that cylinder, #8, is weak--yellow instead of blue/white and plans to replace the plug wire today to test. He also says it idles fine and runs well at low speed but put it under load and it misses on #8.

I had an Isuzu Rodeo V-6 a year ago that did almost exactly the same thing. Flashing CEL at 35+MPH. Replaced all the plugs and coil on #2, no help. Replaced the injector on #2 and it ran fine. Maybe was an intake gasket leak pulling some outside air near #2, but I replaced those in the process of replacing the injector. That fixed it.

 

Defective spark plug in #8. New Platinum Motorcraft plugs went in it no more than 2,000 miles ago, and #8 plug went bad.