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Discussion Starter · #1 ·
I could use some advice on what is causing this misfire issue. It’s a 2018 ST3 with 62k. Never been tuned. Mods are fmic and bov. WMI was installed after issues started (see below). It’s a misfire on cyl 1 & 4 only at highway speeds cruising.

Issue started a couple months ago with a misfire on cylinder 1 and 4 a couple days after I got an oil change and replaced the BMS, due to windshield wipers going full psycho. It happened while cruising down highway in 6th at around 70mph. Flat road around 2700 rpm. Pulled over showed p0300, p0301, p0304. Immediately bought new motor craft plugs, and swapped coils around. Also reset code.

A few days later same code popped up. This time I figured it was carbon or injectors. Pulled intake manifold, cyl 1 and 4 were terrible so I cleaned them and installed a WMI kit to help. I also replaced all 4 injectors while I was in there. (This took me about 3 weeks between shipping and working)

I drove only backroads for a week or two and the car felt great. AFR’s are within range and I thought the problem was gone.
The first time I drove it on the highway after a I started cruising I felt hard hesitation again and got the same 3 codes.

Compression looks good, and also did a coolant leak test which looked fine. The valves also look spotless now. I am thinking it has to be a wiring issue but I am not sure where to start.
 

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How long do you cruise before this happens? Does it happen upon getting into boost some after driving steady speed for a while (like climbing a hill, passing someone, etc)?
 

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Discussion Starter · #4 ·
It’s usually about 8 min into cruising. It happens when I am just cruising at constant speed. I will start to feel a hesitation and vibrations then the scanner will show its misfiring. This is without any hard driving. If I downshift and accelerate it will typically go away. Typically lasts a max of 40 seconds at a time.
 

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It’s usually about 8 min into cruising. It happens when I am just cruising at constant speed. I will start to feel a hesitation and vibrations then the scanner will show its misfiring. This is without any hard driving. If I downshift and accelerate it will typically go away. Typically lasts a max of 40 seconds at a time.
Have you ever changed the purge valve assembly?
 
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How old is your purge valve?

Edit: beat me to it!
 

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I have not. I was reading the Purge thread, but wasn’t sure if that could cause the misfire on only 1 and 4
Considering this happens at steady cruise, when the intake is under vacuum. You would be drawing in raw fuel vapors. Once you lean on it and get into boost, you start forcing air back towards the tank and no longer are pulling the raw fuel vapors.

Have you bothered to watch fuel trims immediately prior to the misfires?
 

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Discussion Starter · #9 ·
Considering this happens at steady cruise, when the intake is under vacuum. You would be drawing in raw fuel vapors. Once you lean on it and get into boost, you start forcing air back towards the tank and no longer are pulling the raw fuel vapors.

Have you bothered to watch fuel trims immediately prior to the misfires?
looks like the moment the misfire starts my short term drops to around -8%. Then while misfiring stays between 0 and -3%. I also had a drop to -7 % five seconds before the event
 

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That doesn't seem super excessive, as long as long terms aren't also moving negative.

It's still definitely worth slapping a purge valve on for testing sake. It is a high failure rate item.
 
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How's the humidity level where you live? We've had some reports of guys having misfires related to moisture pooling in the intercooler and then it getting ingested when you get into the throttle a little bit.

Given that yours is doing it after only 8 to 10 minutes, though, unless the humidity is crazy high, that probably isn't it... More likely the purge valve instead of pooled moisture.
 

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How's the humidity level where you live? We've had some reports of guys having misfires related to moisture pooling in the intercooler and then it getting ingested when you get into the throttle a little bit.

Given that yours is doing it after only 8 to 10 minutes, though, unless the humidity is crazy high, that probably isn't it... More likely the purge valve instead of pooled moisture.
This is why I asked the conditions it happened under. Normally the condensation issue happens after steady cruise when you enter boost for the first time in a while. You get Into boost and all that water is blown out of the inside of the core and ingested.
 

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Discussion Starter · #14 ·
How's the humidity level where you live? We've had some reports of guys having misfires related to moisture pooling in the intercooler and then it getting ingested when you get into the throttle a little bit.

Given that yours is doing it after only 8 to 10 minutes, though, unless the humidity is crazy high, that probably isn't it... More likely the purge valve instead of pooled moisture.
I’m in Ohio so not the worst humidity. But the issue seems to happen even on mild days. On my way to grab a purge and see what happens.
 

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I’m in Ohio so not the worst humidity. But the issue seems to happen even on mild days. On my way to grab a purge and see what happens.
Man, I don't know what part of the state you're in, but in the northeast it's been humid as hell.
 

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Discussion Starter · #16 ·
That doesn't seem super excessive, as long as long terms aren't also moving negative.

It's still definitely worth slapping a purge valve on for testing sake. It is a high failure rate item.
Holy hell. I replaced the purge valve. I also sanded all grounds I could find and added an extra ground cable for fun. It feels like I am driving a whole new car. The purge valve plus possibly the fact that my valves are now squeaky clean transformed this thing. I have been disappointed the last couple months that my 22 year old Honda with 300k+ miles drives better than my focus. That all changed after replacing this. I have read the FUFF, but didn’t realize how much a difference this actually makes. No more highway vibrations, no more shutters after fill up, it’s incredible. I just did a 3 hr road trip and it felt fantastic. (1.5 hrs was glorious Ohio backroads). Thank you for making my do this. I am going to do a slightly modified version of FUFF with my old purge valve so I can replace this every time 10-15k with my spark plugs.
 

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Glad you got it sorted!

My only concern at this point is how badly your fuel tank might be deformed. Would be worth grabbing a bright flashlight and looking at the floor of your tank from underneath.
 
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