Engine misfire after header install

[Ntuomi]

I recently installed stainless power 1 7/8 catted headers on my 2017 gt. After install I went for drive to test out and everything ran fine until I got on the highway. After shifting into 6th gear the car started running rough and the check engine light came on. I ran codes and it said cylinder 4 and 7 are misfiring. At lower speed and gears the car seems to run fine but once I get into 5th and 6th on the highway with high sapped the car starts running rough. Any suggestions?

Sponsored

 

[WildHorse]

could be a header leak. Hammer it in 2nd or 3rd briefly and see if the codes come back.

 

[Ntuomi]

the code stays on and doesn’t go away. Even when getting off the highway reducing speed. The check engine light is constantly on but it only blinks while on highways. I just don’t notice it in lower gears like I do in 5th or 6th

 

[WildHorse]

Did you raise the engine for the install ?

 

[Ntuomi]

Yes I took the motor mounts off in order to get headers into position.

 

[Jsolman]

There are sensors in the original headers o2.
The first set nearest the engine check for rich/lean mixture. It does this on a delay in the ECU. Each pulse takes X amount of time to reach the sensor. Aftermarket headers usually move the sensor away from the engine. So you need to amend the time setting, to match the new sensor difference.
Otherwise the engine thinks it’s running rich/lean on the wrong pulse, and try’s to correct, causing misfires etc.

 

[robvas]

There are sensors in the original headers o2.
The first set nearest the engine check for rich/lean mixture. It does this on a delay in the ECU. Each pulse takes X amount of time to reach the sensor. Aftermarket headers usually move the sensor away from the engine. So you need to amend the time setting, to match the new sensor difference.
Otherwise the engine thinks it’s running rich/lean on the wrong pulse, and try’s to correct, causing misfires etc.

Hasn't this been debunked by all the people that run headers without a tune just fine and all the tuners who tell people they don't need an update after adding longtubes?

 

[Jsolman]

Not a clue tbh, but it’s the only thing I was told to amend in the map, post header swap from stock to Kooks.
Explained as above, by the tuner.
More to keep the throttle response more accurate, as opposed to constantly adjusting.

 

[TonyT930]

Yes I took the motor mounts off in order to get headers into position.

Did you tune it?

Sponsored