Not sure. Primary method to test is install/swap and again if you remove the two-wire connector and the engine stumbles it was working. References: https://haynes.com/en-us/tips-tutorials/simple-guide-your-cars-ignition-coil-pack More in-depth explanation on the circuit:
Perform a resistance test with a multimeter. pin1 to pin2 = 0.50 to 0.60 ohms. each pin to the secondary plug connection spring should read 5730 ohms. this is for 05 to 10 oem cops only. I do not believe these numbers will work for the older 2 valve COP's but I could be wrong. Compare readings from cylinder 8 COP to the others that are working. If they all check exactly the same, then swap the boots only between cylinder 7 and 8 to see if the misfire changes from P0308 to P0307. Boots have been known to develop pin holes and cause arcing to the cylinder head casting.
I'm having the exact same issue in my 2007 GT. P0308. New plugs installed. And new coils coming today. Another place to look could be a leaking/cracked intake manifold creating problems in Cyl 8. Let me know when you fix your issue and what was the cause.
I swapped out the plug and coil in cylinder 8, going to take it for a drive tomorrow and see how it is but based on initial running it feels like it's still doing it so I'm suspecting the injector.
Well- it's looking like the coil was most likely the culprit. I installed one of my old coils in place of the newer one on Cylinder 8, and swapped the plug for one of the other autolite's I pulled out (that I did not think was bad). Took it out for a drive this afternoon and it ran perfect the entire time even with 3 or so times of shutting it off and restarting.
The fuel filter is often over-looked, even by the dealership. My son's Saleen Gurney started running rough and losing fuel mileage (16mpg dropped to 11mpg) and the dealership looked at the fuel pump, injectors and everything else -- it ended up being the fuel filter. Mileage & performance were restored, including 21mpg on the freeway (the highest he ever experienced).
In my case the fuel filter shouldn't be an issue...I replaced it in winter 2020 but since then have only put maybe 3000 miles on the car. I'll probably replace it this winter though.
^ Why? Unless you are running E-85 or came into a bad tank of gas its fine...unless you just enjoy piddling and spending money.
So, a little more to add to this saga- After being perhaps a bit over confident that the #8 coil was the only issue, I swapped back in two of my original HT1's for the newer ones I had installed, and put the boot from the new coil onto the older coil. Well, it was unseasonably warm yesterday so I got the car out to run to the store and whaddaya know- it was misfiring again. So apparently there is either a problem with the boot from the newer coil, or one of my original HT1's is bad. Wish this was a little easier to diagnose. Guess I'm swapping everything back to how it was.
