Intermittent flashing CEL at highway speed

[Bigred911s]

at about 35k miles on my 2016 S550 while on the highway cruising at 80mph, the CEL started flashing intermittently then would go out and then flashing agin. At that time I was planning on changing the sparkplugs and did so. After about another 4K same issue. Got off highway and no more flashing CEL and was fine even at full throttle.

Any ideas what it could be? If it were a bad ciol I would think I would have a CEL most of the time?

Here are what the plugs I replaced look,like.

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[ZX3ST]

Bad COP connection maybe? Possible the electrical connection opens up slightly when hot? Either that or a marginal coil. I'd guess connection, since you said it went away for a short time.
EDIT: I mean the connection from coil to plug, NOT the wiring on top of the coil.

Did you use any dielectric grease on reassembly? I've seen too many people grease the connection instead of the ceramic.

Plug colors look good, but did you check the gaps?

I know you said it didn't set a CEL, but did you scan for any pending codes?

 

[Sheenapple]

Check codes, likely misfire cyl 8 or 6 or an emissions “leak” code. culprit is either ignition coil, cracked intake manifold runner, or purge control valve. You already changed sparks and I assume you gapped them.

Troubleshooting steps should be plugs>coil>purge valve>intake mani replacement.

EDIT: to directly answer your question, a flashing CEL means misfire

 

[Bigred911s]

Yes it was random misfire in 4 & 8 cyl's. Plugs gapped right. Checked connectors on coils before swappin and were tight. Swapped coils with 1 & 2 cyl's.

Cleared codes and disconnecting batt cable for 30 mins.

Car runs great now no flashing CEL after 500 miles.

Weird couldn't find anything wrong. I'll monitor and see if it happens again.

 

[Sheenapple]

it took about 15k miles before my random CEL turned into the intake mani issue, so certainly your mileage may vary here. knowing what i know now, in your situation find someone who is either mechanically sound or someone who can slowly modulate the throttle from the driver seat. listen closely with the hood open to how the engine sounds at 1,850-2k RPM and 2,850-3k RPM. it shouldn't take but 30 seconds per RPM block. if you're able to replicate this at any point (whenever you feel like testing this), get a new intake

 

[Bigred911s]

I just ordered a new intake man for it. Verified nothing wrong with plugs, coils, fuel pressure, filter etc.

Random flashing cel comes from n at low rpm while cruising on hwy and it doesn't happen all the times me.

 

[cameronalford]

I just ordered a new intake man for it. Verified nothing wrong with plugs, coils, fuel pressure, filter etc.

Random flashing cel comes from n at low rpm while cruising on hwy and it doesn't happen all the times me.

Any luck? I have been chasing misfires everyday for over a week now. Same thing, it only misfires in 6th gear going down the freeway. SO WEIRD. I don't know what to do anymore. Did a new intake manifold fix it?

 

[Cobra Jet]

This doc may help to diagnose some random misfire issues:

https://www.mustang6g.com/forums/attachments/hd-misfire-detection-pdf.319596/

Have you tried swapping coils - meaning if your DTC was a misfire for cyl 2, have you swapped cyl 2 coil with say cyl 4?

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Also to note, the IMRC system has been known to prematurely fail and if it fails or is failing, it too will cause misfires. There’s a few threads on here about the IMRC system, search “IMRC” and you’ll find quite a few threads.

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