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1994 T Bird, 3.8 Auto.
I have been working on a rough idle, at all times. I was about to injector replacement. I decided to give diagnostics one more try. I did have a code 181 and 189. New O2 sensors have been installed.
I found the timing was at 15 degrees advanced, so I reset it to 10 degrees, with the spout removed.
Long story short, it idles smooth as silk. So why did 5 degrees advance make such a difference?
 

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Because the spark plug is firing at the wrong time which means the combustion cycle is off, putting alot pressure trying to force the piston down when it wants to come up. Thats why it idles rough because all of the pistons will experience this.
 
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