P0305 Misfire cylinder 5 continuing issue

[NASCAR Mike]

2017 Expy with 5 Star 93 octane tune and NGK 6510 / LTR7IX-11, one heat range colder than stock because of the tune. SUV has 27k miles and has had the tune since 4k miles with zero issues until now.

I keep getting a P0305 code on the #5 cylinder. I swapped the plug (no arcing on existing) with a new NGK 6510 / LTR7IX-11 and gapped at .032 per 5 Star. All the other plugs are the same (NGK 6510) at .032 gap. I also install a new Ford DG549 Motorcraft Ignition Coil.

I cleared the DTC codes and start her up. Rough running at idle and CEL comes on - same P0305 code. The roughness disappears when you drive it.

What else should I be looking for? Defective injector? I have read some people gap at .028 but I can't believe I am getting spark blowout at startup and idle.

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[99WhiteC5Coupe]

Although you use 93 octane - do you use Top Tier fuel? You may have a clogged fuel injector.

You may want to try Techron Fuel System cleaner according to directions on the bottle label.

 

[NASCAR Mike]

I always use 93 octane. I did add fuel injector cleaner at last fillup but only have less than 20 miles on it since.

Oddly enough, the misfire is worse with the new Ford coil and the new NGK sparkplug.

I am going to put the original coil back in and see what happens.

 

[99WhiteC5Coupe]

I always use 93 octane. I did add fuel injector cleaner at last fillup but only have less than 20 miles on it since.

Oddly enough, the misfire is worse with the new Ford coil and the new NGK sparkplug.

I am going to put the original coil back in and see what happens.

I have owned and worked on cars / trucks for more than 50 years. I have always used OEM parts, especially tune-up parts such as plugs. My experience is that they always fit and function well.

I’m curious - why did you switch to a different brand of spark plug? Have you considered returning to the OEM plug (I know you have a tune)?

 

[NASCAR Mike]

NGK makes about the best plugs out there. Besides I needed one heat range cooler because of the tune and I could not find one heat range colder in a Ford Motorcraft plug.

 

[LokiWolf]

That is the wrong plug!

Have you checked the gap since install. The plugs you installed come from NGK with a .043 gap. You are gapping them down A LOT to get to .032, and even more to get to .028 that most tuners recommend on these motors. They will slowly expand.

The correct one range colder plug to run is this:

https://www.ngk.com/ngk-95605-ltr7bhx-ruthenium-hx-plug

I am sure there is an Iridium version of that heat range/gap, but would need to look it up.

 

[LokiWolf]

I have owned and worked on cars / trucks for more than 50 years. I have always used OEM parts, especially tune-up parts such as plugs. My experience is that they always fit and function well.

I’m curious - why did you switch to a different brand of spark plug? Have you considered returning to the OEM plug (I know you have a tune)?

PLEASE stop recommending Ford plugs for those that are tuned. Heck, they aren't that great for non-tuned.

There was a time when the plugs were good. I even used to recommend the Ford GT(Ford Performance) plugs for the 3.5 several years ago. They were one heat range cooler, and GREAT. That is not the case anymore. Ford's most recent plugs are CRAP. They are made by NGK, but are the lowest common bidder version, made to barely meet the service interval under light duty use.

It has been WELL documented here, that even those that are stock, have seen MANY improvements, including idle, power, and MPG, switching to the plugs I linked with no other change, even at factory gap, WELL before Fords recommended replacement interval.

Plugs should not be where you save on these motors.

 

[NASCAR Mike]

That is the wrong plug!

Have you checked the gap since install. The plugs you installed come from NGK with a .043 gap. You are gapping them down A LOT to get to .032, and even more to get to .028 that most tuners recommend on these motors. They will slowly expand.

The correct one range colder plug to run is this:

https://www.ngk.com/ngk-95605-ltr7bhx-ruthenium-hx-plug

I am sure there is an Iridium version of that heat range/gap, but would need to look it up.

I have been using this plug for years without an issue. When I checked plugs 5 (misfire cylinder), 2, 3, 6, all were still at .032 from 4k miles ago.

I can change them to the 95605 but if the plugs were the issue, wouldn't all of the cylinders be missing?

 

[LokiWolf]

I have been using this plug for years without an issue. When I checked plugs 5 (misfire cylinder), 2, 3, 6, all were still at .032 from 4k miles ago.

I can change them to the 95605 but if the plugs were the issue, wouldn't all of the cylinders be missing?

100!

They will work fine as long as they hold their gap. The issue with any plug when you are varying the gap, if there is A LOT of deviation from the factory gap, under heat they are more likely to distort back to that gap. The 95605's are recommended, because they start closer. I have seen those 6510's expand SO many times.

You have another issue, especially if you verified the gap, and switched plugs.

Compression test on that cylinder? Any difference on the Misfire with stock tune? Have a way of logging while doing a pull?

 

[NASCAR Mike]

I returned the computer back to stock and it is still missing. Pending P0305 code on the scanner. I am thinking it's a bad fuel injector at this point.

How many times do I need to start/run the car before I bring to the dealer so they don't see reflash? (In case it's worse than an injector!)