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Random cylinder misfire, dealer is stumped.

Ablaze

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Hey guys. I've got a bit of a situation.

Clue 1 - Brand new car arrives at the dealer. I asked them to check for some known issues during the PDI and I'll pick it up a week or so later. They said everything looks great. Drove the car back home 1000 miles. After the 100 mile mark I got into the gas a bit. Exhaust mode selection unavailable. I *think* I had a misfire somewhere one time but I didn't have my tuner on me and I don't remember if I got a CEL or not. Wife and I had at least a dozen plus starts/stops on the way back though. I didn't think anymore of it. Car ran absolutely fantastic the entire time, all smiles.

Clue 2 - Dropped the car off for for detail work. Two weeks later I pickup the car. Looks perfect. Starts up no issues. I'm driving it back and more than half way home I goose it (just rolling into the gas) and I get a sharp cut in spark, felt like someone tapped the brakes lightly. CEL. I remember this for sure. Car still runs fine but I don't play with it anymore and cruise on home since something is obviously wrong.

Clue 3 - I get home and notice some things on the list weren't done by the original dealer. I wouldn't have taken it to them to get fixed anyway, always intended to check everything once I got home. I plug in my tuner and get the following codes: P0300, P26C6, P2BF5, P2BF9, and P2BFD. I schedule an appointment with my local dealer.

Clue 4 - Local dealer fixes everything on the list except the misfire on Day 1. Day 2, they check a few things to try and fix the misfire and when they start up the car they find it misfiring right at idle on random cylinders. I asked the service manager which cylinders but he never got back to me. They pull fuel to test as they can't think of anything else. Day 3, fuel is fine so they call the Ford hotline which tells them to check spark plugs and coils. They don't work on the car the whole day. I know where this is going.

Day 4 will be tomorrow. This isn't my daily. I'm not upset and I haven't yelled at the dealer yet for sitting on my snack pack car the entire day without doing any work. Tomorrow they say their master tech has 2 jobs ahead of me (although they'll have had the car for 4 days now). I don't think they want to work on it anymore when they could be banging out no brainer oil changes. The service manager made comments about no one really wants the car there (I don't think he wasn't being condescending, he sounded more concerned) and no one feels comfortable driving it.

I'll be picking it up tomorrow either way, I don't want them sitting on it. I'm thinking about asking for a Ford specialist to fly in and work on it. I heard this is possible for specialty cars like the GT500. If they say no, I'll end up taking it to another dealership. I'm also thinking about asking them to drop the fuel tank, drain it, then put in fresh 93 and see if it clears up regardless of their test results. I remember doing chemistry working as a plant operator and half of the guys did it wrong then just copied the previous shifts' chemistry numbers and changed them slightly, so I'm not sure I trust their results. It's getting cold here fast so my window to work on the car is year is shrinking rapidly.

You guys know a lot more about this stuff than I do. Any advice on the best path forward?
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Snoopy49

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This is from the 2020 Mustang Service Manual.
Unfortunately this manual doesn't describe these pinpoint tests.

P0300 - Random Misfire Detected
Description:
This DTC sets when the misfire detection monitor detects an engine misfire in multiple cylinders or the PCM cannot identify which cylinder is misfiring.

Possible Causes:
  • Damaged camshaft position (CMP) sensor
  • Low fuel (less than 1/8 tank)
  • Stuck open exhaust gas recirculation (EGR) valve
  • Restricted EGR passages
  • Misfire monitor neutral profile correction has not been relearned since the last mechanical repair
Diagnostic Aids:
One or more EGR passages may be restricted.

P26C6 - Exhaust Flow Control Valve "A" Control Circuit Low
PINPOINT TEST H: RH (RIGHT-HAND) TAILPIPE ACTUATOR FAULTS

P2BF5
- Exhaust Flow Control Valve "A" Position Circuit Range/Performance
PINPOINT TEST H: RH (RIGHT-HAND) TAILPIPE ACTUATOR FAULTS

P2BF9
- Exhaust Flow Control Valve "B" Control Circuit Low
PINPOINT TEST I: LH (LEFT-HAND) TAILPIPE ACTUATOR FAULTS

P2BFD
Exhaust Flow Control Valve "B" Position Circuit Range/Performance
PINPOINT TEST I: LH (LEFT-HAND) TAILPIPE ACTUATOR FAULTS
 

Epiphany

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Everything you wrote is what would have gone through my mind too. Sucks to have to go through this with a new car. Good luck and I hope you end up with an experienced mechanic that can put this one to bed in short order.
 

Classic Lover

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Sounds like a wire or computer issue, just a guess but it doesn’t sound like a fuel problem and if it’s missing on random cylinders probably not a bad spark plug. I’d try swapping in a new pcm and see what happens.
 

AMBJR

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Sounds like a wire or computer issue, just a guess but it doesn’t sound like a fuel problem and if it’s missing on random cylinders probably not a bad spark plug. I’d try swapping in a new pcm and see what happens.
I had a similar problem, like going into "LIMP MODE" in the first week I had it!!! I was able to drive it home!!! The next morning it was "OK" and never came back again!!!
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