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2003 Mustang GT 5 spd.
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Hello all,
I own a 2003 mustang gt 5 speed. I recently have encountered a strange problem I have been unable to diagnose. The car runs fine at idle but at around 2.5-3k rpm it starts to stutter and struggle. The power seems to still be there. I currently am running the car on a 93 tune. My SCT tuner shows regular fuel pressure of 40 at the rail and a fuel pump duty cycle of 15% I replaced the fuel filter just in case with no change. I also disconnected each coil pack one by one to see if one was completely shot but each one, when disconnected caused the engine to run worse. There are no codes thrown and no other problems. I am at a loss considering it runs fine at idle and the fuel system seems to be ok. Thoughts and ideas are greatly appreciated!
 

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Welcome to the forums from Texas. What is your cars mileage and last time plugs were replaced.
 

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Spark plugs would be my starting point. Also look/inspect for vaccum leaks.

You said you already checked for codes and didn't see any because when you disconnected each cylinder at the coil and it ran even rougher. Yes, it will run rougher and not smoother when you disconnect or kill a COP for that cylinder unless the COP was totally dead in which you wouldn't notice any change. But I still wouldn't rule out the coil on plug if the problem persists after changing the plugs. I keep a new COP on hand for testing by swapping at each cylinder one at a time. Thats an option too because you could have a weak COP that isn't genersting enough output and it gets suppressed. And unless you have a hard misfire, it will not flash the CEL.

Without doing diagnostics on the car, anything at this point is best-guess but it is not expensive to change the plugs, and buy a test COP if that is less than spending $150 to pay someone to check it out. And not knowing if there was any scheduled maintenance completed at 100,000 miles it makes sense to do it now.
 
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2014 GT, 1967 Fairlane GTA
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Are you able to monitor or data log the car when you reach the problem RPM? I believe your tuner should allow that. Worn plugs as ONEZ ST suggested could cause it to break up at higher RPM. They should be replaced at 100k miles. I would check them to see if they look really old/worn. A bad tune could be the issue too. What other mods are on this car?
 

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Had this happen to the GT I had. I changed the spark plugs and it got worse. Found the COP boots were cracked. You can buy replacement boots so you don't have to replace the COPs just for cracked boots. Electrical tape around the boots work as a temporary fix.
 

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Hello all,
I own a 2003 mustang gt 5 speed. I recently have encountered a strange problem I have been unable to diagnose. The car runs fine at idle but at around 2.5-3k rpm it starts to stutter and struggle. The power seems to still be there. I currently am running the car on a 93 tune. My SCT tuner shows regular fuel pressure of 40 at the rail and a fuel pump duty cycle of 15% I replaced the fuel filter just in case with no change. I also disconnected each coil pack one by one to see if one was completely shot but each one, when disconnected caused the engine to run worse. There are no codes thrown and no other problems. I am at a loss considering it runs fine at idle and the fuel system seems to be ok. Thoughts and ideas are greatly appreciated!
Hi...it seems like you're having issue with the plugs, mostlikely just getting time to replace. The system youre describing is something a worn plug/plugs. Try a new set and see if this goes away. Good Luck
 
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