Misfire at high rpm! Help

Ryanm2018gt · Jan 2, 2026

2018 mustang gt 10 speed I’ve replaced all vvt solenoids, new coils and plugs and I’ve been having a issue for a few months now where when I go WOT I get a flashing CEL and multi cylinder misfire code, purge valve seems good not stuck open or closed, brand new injectors DI AND port are both new, smoke tested and no vac leaks, new 02 sensors, anyone had this issue? I’m running out of ideas, I do have a P0451 code

Mcmahst · Jan 2, 2026

Have you checked the actual vacuum at idle to verify if a vacuum leak is likely? Smoke tests don’t always reveal vacuum leaks. Just for pease of mind, verify the idle vacuum to rule it out. Good luck.

Ryanm2018gt · Jan 2, 2026

Mcmahst said:
Have you checked the actual vacuum at idle to verify if a vacuum leak is likely? Smoke tests don’t always reveal vacuum leaks. Just for pease of mind, verify the idle vacuum to rule it out. Good luck.

I have, I checked the purge valve today at idle, and it was sucking air in like normal, un plugged it and vac went away like normal, this is really racking my head

Mcmahst · Jan 2, 2026

You know, if you eliminate all of the possible conditions, and the problem persists, then you have to consider if you have an ECU issue. It’s not something we usually consider, but when you run out of elements to test . . . Good luck.

Ryanm2018gt · Jan 2, 2026

Mcmahst said:
You know, if you eliminate all of the possible conditions, and the problem persists, then you have to consider if you have an ECU issue. It’s not something we usually consider, but when you run out of elements to test . . . Good luck.

I think I’ve figured it out and I hope this helps someone in the future, I think it’s the fuel tank pressure sensor, I’m going to replace that, the fuel rail pressure sensor, and the purge valve anyways because the car has 110k on it I’ll update in a few days or a week when I get everything replaced

Mcmahst · Jan 2, 2026

Did you run down all of the possible P0451 resolutions? One was a loose or damaged gas cap. Just wondering.

Bigred1971 · Jan 13, 2026

Mcmahst said:
Did you run down all of the possible P0451 resolutions? One was a loose or damaged gas cap. Just wondering.

Hey,im over here now,left VMF.

highplace11 · Jan 17, 2026

Ryanm2018gt said:
2018 mustang gt 10 speed I’ve replaced all vvt solenoids, new coils and plugs and I’ve been having a issue for a few months now where when I go WOT I get a flashing CEL and multi cylinder misfire code, purge valve seems good not stuck open or closed, brand new injectors DI AND port are both new, smoke tested and no vac leaks, new 02 sensors, anyone had this issue? I’m running out of ideas, I do have a P0451 code

Compression test all 8....

Mark

highplace11 · Jan 17, 2026

Ryanm2018gt said:
I think I’ve figured it out and I hope this helps someone in the future, I think it’s the fuel tank pressure sensor, I’m going to replace that, the fuel rail pressure sensor, and the purge valve anyways because the car has 110k on it I’ll update in a few days or a week when I get everything replaced

That car is too new and it breaks my heart you are spending so much on it... Needs a pro to look at it, and if you are correct a tune would have fixed that...

Mark

highplace11 · Jan 17, 2026

highplace11 said:
That car is too new and it breaks my heart you are spending so much on it... Needs a pro to look at it, and if you are correct a tune would have fixed that...

Mark

For the record vacuum leaks and misfires come through the emissions system, and makes me SICK in my stomach.... Unless you have to pass them, delete, cap and enjoy the car... First thing we do is eliminate that CRAP....

Mark

Misfire at high rpm! Help

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