Need help please! Flowmaster FlowFX

[Gestanley]

I put a set of Flowmaster FlowFX straight through mufflers on my 2017 Mustang GT and immediately after my check engine light came on, code P0300 misfire. Could this be that my O2 sensors are reading incorrectly now? And if so, will wideband sensors fix the problem? Would it be the upstream or downstream sensors? Thanks in advance for any info!!

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[K4fxd]

Adding a muffler should have no effect on a misfire condition.

Did you mess with the front of the system?

Try a crank re-learn

 

[MiamiGT350]

Crank relearn should fix it, but I agree with @K4fxd that your exhaust shouldn't have anything to do with a misfire.

You may also want to try disconnecting the battery for a few minutes and then start the car and idle for 3 mins. Turn it off for a min and then idle another 3 mins. Then drive around and do a 2-3 times over 45mph, then let off the gas and let the car descelerate to a stop.

Reply back to the thread if the issue persists.

 

[Gestanley]

Crank relearn should fix it, but I agree with @K4fxd that your exhaust shouldn't have anything to do with a misfire.

You may also want to try disconnecting the battery for a few minutes and then start the car and idle for 3 mins. Turn it off for a min and then idle another 3 mins. Then drive around and do a 2-3 times over 45mph, then let off the gas and let the car descelerate to a stop.

Reply back to the thread if the issue persists.

Thanks guys, I haven't done any mods on the exhaust up stream, only mufflers the car already had the resonator delete. So is this process that is explained the crank reset?

 

[MiamiGT350]

Thanks guys, I haven't done any mods on the exhaust up stream, only mufflers the car already had the resonator delete. So is this process that is explained the crank reset?

You need Forscan and a laptop for the crank relearn which can help to clear occasional misfires.

The method I described is different but should help it re-profile any mild changes in air / fuel ratios, if that is the cause (unlikely)

How frequent is the misfire code and does it happen at specific rpms or conditions? Warning: do not rev your engine high if it keeps happening.

Did you touch anything other than the resonator recently?

 

[Gestanley]

You need Forscan and a laptop for the crank relearn which can help to clear occasional misfires.

The method I described is different but should help it re-profile any mild changes in air / fuel ratios, if that is the cause (unlikely)

How frequent is the misfire code and does it happen at specific rpms or conditions? Warning: do not rev your engine high if it keeps happening.

Did you touch anything other than the resonator recently?

I have forscan and port reader with a laptop. As I was leaving work yesterday I decided to bring up my voltage gage on my display and noticed that it was reading low, about 12 while driving engine light still on. I stopped at a store shut the car off went in and when I got back out on the road I looked down and notice the engine light had turned off and my voltage was reading around 13.8. The rest of the way home it would go from that to around 12 and back. Can a low battery or bad alternator cause these issues? Nothing around the cats or O2 sensors have been disturbed. Thanks!

 

[KingKona]

I have forscan and port reader with a laptop. As I was leaving work yesterday I decided to bring up my voltage gage on my display and noticed that it was reading low, about 12 while driving engine light still on. I stopped at a store shut the car off went in and when I got back out on the road I looked down and notice the engine light had turned off and my voltage was reading around 13.8. The rest of the way home it would go from that to around 12 and back. Can a low battery or bad alternator cause these issues? Nothing around the cats or O2 sensors have been disturbed. Thanks!

1. Your voltage gauge is showing what volts are being outputted by the alternator, NOT what voltage state the battery is in. That usually means that the battery isn't needing the juice from the alternator.

2. Well....how old is your batter? How often do you drive your car?

 

[Gestanley]

I'll have to see how old it is. I drive it everyday. I think I'll get the battery and alt checked and go from there.

 

[Gestanley]

Battery and alternator checked out fine so I done a crank sensor relearn with forsan. Guess I'll see if it worked. Scanner is not reading any codes yet. Battery is about 16 months old.

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