Persistent Cylinder #8 Misfire

[jophish]

Hey everyone,

Have had a persistent misfire on cylinder 8. Replaced and correctly gapped all the spark plugs, but did nothing to stop it from coming back. Any one have any advice on what’s next up the ladder? Coils? I’ve heard this may be an intake manifold issue as well.

Thanks in advance!

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[usgiorgi]

Before spending the money on a new coil swap the coil from a different cylinder to #8 and see if that one starts misfiring.
Does the car buck at all when you give it very mild throttle at around 2k rpm?

 

[jophish]

Before spending the money on a new coil swap the coil from a different cylinder to #8 and see if that one starts misfiring.
Does the car buck at all when you give it very mild throttle at around 2k rpm?

I will give that a try - haven’t noticed anything alarming regarding low-RPM engine response. The car is a 6-speed, so I’m not sure if the clutch might be allowing me to mask symptoms. I’ll keep an eye out though for anything at or around 2K RPM that might raise eyebrows.

 

[usgiorgi]

I will give that a try - haven’t noticed anything alarming regarding low-RPM engine response. The car is a 6-speed, so I’m not sure if the clutch might be allowing me to mask symptoms. I’ll keep an eye out though for anything at or around 2K RPM that might raise eyebrows.

I asked because 2015-17 GTs have an issue with the intake manifold runner controls cracking and causes the p0300 and p0308 codes. Mine started out with a p0308 and then a week later threw the p0300. The dealer replaced the whole intake manifold under warranty.

In either case, try the coil swap first.

 

[jophish]

I’ve had both codes, although I’m not sure I’m still under warranty. 2015 GT with 60,000 miles.

I’ll try the coils tomorrow and see how things hash out after the weekend of driving.

 

[usgiorgi]

I’ve had both codes, although I’m not sure I’m still under warranty. 2015 GT with 60,000 miles.

I’ll try the coils tomorrow and see how things hash out after the weekend of driving.

I'll put my money on it being the intake manifold. Try to give the car very little throttle between 1800-2200rpm and see if you feel stuttering/bucking.

 

[jophish]

I'll put my money on it being the intake manifold. Try to give the car very little throttle between 1800-2200rpm and see if you feel stuttering/bucking.

Anybody done this repair before? Not afraid of tackling it myself, but if it's serious amount of specialized tools and a high probability of screwing something up, I'd rather not.

 

[jophish]

One more thing - this code only seems to come up when I'm doing highway-speed cruising in 5th or 6th gear.

 

[usgiorgi]

Anybody done this repair before? Not afraid of tackling it myself, but if it's serious amount of specialized tools and a high probability of screwing something up, I'd rather not.

Oh to swap the manifold out is super easy. Maybe 30-40 mins. The cost is the annoying part.

 

[jophish]

Oh to swap the manifold out is super easy. Maybe 30-40 mins. The cost is the annoying part.

I'm seeing everything from ~$500 to well over $1K...will do more homework on this one for sure. Wouldn't mind upgrading a bit from OEM but don't need anything crazy. Main priority is avoiding poorly made stuff, learned that lesson with my old Camaro years ago - buy nice or buy twice.

 

[usgiorgi]

I'm seeing everything from ~$500 to well over $1K...will do more homework on this one for sure. Wouldn't mind upgrading a bit from OEM but don't need anything crazy. Main priority is avoiding poorly made stuff, learned that lesson with my old Camaro years ago - buy nice or buy twice.

Good time to get a gt350 one for sure if you plan on tunning.

 

[jophish]

Good time to get a gt350 one for sure if you plan on tunning.

To be honest, probably not - this is my DD, and while it's fun, I don't want to delve too deep into tuning it...I've got my Camaro for that.

 

[NYDP77]

I had the code on a 2017 GT turned out to be the intake manifold.

 

[wjfawb0]

I've had the P0308 and P0300 (#8 misfire and multiple random misfires) for about a year now on my 2015 GT with about 48,000 miles. I pulled my intake off and found the driver's side IMRC shaft was broken (5,6,7,8 cylinders). My mustang isn't a daily driver, and the codes were clearing up on their own until last week. The idle was rougher, and the highway fuel economy was down from 26 to about 22 mpg. A new intake is on order. I ordered online from a Ford dealer for around $320. I hear you can get it cheaper if you know the right people on forums.

Video showing the broken driver's side IMRC shaft (it's metal and not plastic as I say in the video):

 

[wjfawb0]

Here's a picture of the broken shaft:

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