Missfire below 4k that clears up the second you go wot and comes and goes

[coyote_nik]

Bought my 2013 mustang gt like 2 months ago from a dealer, was all fine for the 1000mile road trip i did to college, now it has randomly developed a missfire at part throttle below 3k rpm but the second you go over 3k rpms with more than half throttle it dissapears, i have replaced plugs, coils, and harness wires, i have gotten crackes manifold and injector suggestions from both my instructors with 1200hp drag coyotes so that was going to be next check, any ideas on what could cause this

 

[86GT351]

Did you gap the spark plugs or take them out of the box and install? Your symptom is of a bad spark plug.

 

[JC SSP]

Any codes? If not, check plugs and coils. I hope you used factory OEM quality or better. The cheap aftermarket stuff can causes problems.

Before you pull the intake do a smoke test or simple soapy water spray bottle to check for vacuum leaks.

 

[coyote_nik]

It just throws an evap code and the cylinder 2 and 8 missfire, no fuel pressure codes when it happens, just put new motocraft plugs and C&L coil since the old owner used an ugga dugga to put them in before i bought it and cracked all the coils, so i thought new coils and plugs set to the right gap would fix it, but it didnt, so i hopped on all data and the evap system runs off manifold pressure and it is throwing the small evap code leak which is why i am now suspecting manifold, just wonder if anyone else ran into this problem before and if it wad manifold becuase everyone i have asked and all the all data flow charts say 20 different things.

 

[coyote_nik]

Any codes? If not, check plugs and coils. I hope you used factory OEM quality or better. The cheap aftermarket stuff can causes problems.

Before you pull the intake do a smoke test or simple soapy water spray bottle to check for vacuum leaks.

Only codes are the missfires and small evap and ik the evap runs off manifold vaccum so thats why i was thinking manifold,replace plugs with motocraft ones, and C&L coils and that didnt do anything

 

[86GT351]

Only codes are the missfires and small evap and ik the evap runs off manifold vaccum so thats why i was thinking manifold,replace plugs with motocraft ones, and C&L coils and that didnt do anything

Ditch the coils for Motorcraft

 

[JC SSP]

Swap coils from cylinders 2 & 8 to other cylinders. If it stops and happens in other cylinders then coils are bad and replace with motorcraft.

 

[skwerl]

It just throws an evap code and the cylinder 2 and 8 missfire, no fuel pressure codes when it happens, just put new motocraft plugs and C&L coil since the old owner used an ugga dugga to put them in before i bought it and cracked all the coils, so i thought new coils and plugs set to the right gap would fix it, but it didnt, so i hopped on all data and the evap system runs off manifold pressure and it is throwing the small evap code leak which is why i am now suspecting manifold, just wonder if anyone else ran into this problem before and if it wad manifold becuase everyone i have asked and all the all data flow charts say 20 different things.

As others have stated, start by swapping coils with the affected cylinders and known good cylinders. If the misfires stay on cylinders 2 and 8 after doing that, then you may want to closely inspect (or pressure test) your intake manifold. If the previous owner did the 'ugga dugga' torque method on the coils and plugs then he may have overtorqued the manifold and cracked it.

 

[RedCar]

Since you have an Evap code, I'd start there. The evaporative emissions recirculation valve at the front of the intake manifold could be stuck partially open causing a vacuum leak. If you unplug the electrical connector from it (it should then close), then remove the vacuum line from it, there should be no vacuum felt there with the engine running. However sometimes the leak is very small though and you won't be able to feel it. You could also cap off the vacuum port on the valve and go for a drive and see if the misfire is gone. You can also build a smoke machine out of a jar, a pen and a bicycle pump (instructions are on YouTube) and use it to test the evap recirc valve separately, and then the intake manifold for leaks. I made one and it works great. It proved that my evap valve and intake system had no leaks at all. My misfire was from a bad upstream O2 sensor, but you'd likely have a code for that.

 

[eighty6gt]

Sounds like sunken exhaust valve(s,) a friend's 5.0 pickup went down to this issue.