Help! Rough idle, hanging idle

zenboy99

Founding Member
May 12, 2002
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Madison, WI
After a failed TKO install I got sick of my Mustang sitting for 4 months, so I put in a King Cobra clutch. Beforehand the car was idling a little bit rough, and drove a little bit rough at low rpms, but no hanging idle. Now the car idles really rough (~1100rpm compared to 850rpm) and the idle hangs really bad off throttle and down shifting. I still have good power (compared to what I remember), and its not blowing any smoke.

I figured it was a vacuum leak, checked with carb cleaner but didn't find anything. I tore off the whole upper intake and found that one bolt had been stripped out. I found a longer bolt that managed to hold the rest of the threads and could be tightened down pretty good. I put everything back together, but the symptoms still remained.

Took out my JMS chip, same problems. Ran codes and found nothing.

The only thing that I can think of is that something is leaking around the mass-air meter. Thats in the fenderwell because of the AFM power pipe, and I don't have my lug nut key to get the wheel off and look in there. But I looked through the hole in the fenderwell and everything looked normal.

Anyone have any suggestions or ideas? Is there something I'm missing? This thing is running really rough.
 
It sounds like a vacuum leak to me. I would go over all the lines again. Maybe the pcv valve got pulled out of the lower intake? You could also clean the IAB located on the throttle body with carb cleaner.
 
All the lines that I see in the engine bay are fine. There is a line leading from the intake to the passenger side fenderwell that looks old, but seems to be OK.

I also cleaned the IAB, it wasn't really dirty at all.