2005 4.0 v6 rough idle

mattmcd83

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I just bought my son a 2005 Mustang Premium Coupe 4.0 V6 with 90,500 Original miles and have a question about the idle while sitting still while in Drive its a little rough but when you get into the gas it runs fine. I don't believe the spark plugs have ever been changed is that the first thing I should check?
 

JimC

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At that mileage you are in the tail end of the recommended life span for the plugs so I would go that route first. On the 4.0 pay attention to the wiring. One side of the coil is in numerical order but the other side is not. Very easy to mix up the wires.
 

mattmcd83

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Thanks I pulled one out the other day it was a motorcraft platinum. Would platinum plugs been original equipment?
 

JimC

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Motorcraft would be original, but not sure about the "platinum" designation. It has been too many years since I owned a 4.0
 

TexasBlownV8

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Even if 'platinum' (and no, those wouldn't be original), check the driver side plugs. They tend to wear the fastest and cause idle issues. Very common.
Also, the throttle body can get dirty from the engine side, but that'll usaully cause the tb to stick a little.
 

mattmcd83

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Also I am not sure if the transmission fluid has ever been changed. It shifts good so I was wondering if I should change it or just leave it be since it’s not giving any trouble?
 

DieHarder

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Spark plugs and wires to start. Can't hurt.

Agree, my son's '06 benefited most by replacing the plug wires. (Buy OEM. Didn't cost too much. Pay attention to routing). Amazing difference. Idle smoothed right out. Ran like a new car. Wires coming up on 20 years; it's time to change them. I'd clean the throttle body as well. Can't hurt.
 

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