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Discussion Starter · #1 ·
Car: 2002 V6 Mustang
Miles: 117,000
Mods: Cold Air Intake
Recent work: (3 months ago) New Alternator as the last OEM one failed. Alt was tested before we installed it. No problems what so ever.

Problem :

Alrighty, so about a month ago, my car began stuttering upon acceleration and acting rough on idle after a cold start. The issue 100% goes away once the engine has reached operating temp. Rarely does it stutter or lag while warm but it always has a lagged acceleration. Past 3k rpms the problem goes away completely regardless if it's at temp or not. I religiously allow my car to warm up for a couple minutes before taking her out to the road as well. So, rough idle SOMETIMES (idle is typically very smooth), car runs rough and has lagged acceleration when below operating temp, issues go away when warm, above 3k rpms there's no issues.

What I've done to combat the problem:
  • I religiously clean my engine bay and keep the throttle body and IAC valve spotless
  • I checked for vacuum leaks, non to be found (soapy water method)
  • Got new double platinum plugs, no difference
  • Spark plus wires are less than 2 years old
  • Spark plugs are gapped to spec and to torque spec
  • Old plugs had no residue, only a little bit of oil on the threads from a previous valve cover oil leak, threads were thoroughly cleaned with a microfiber cloth
  • Been going to different gas stations
  • Used Techron complete fuel system cleaner twice back to back, no difference
  • No codes are found
What I haven't done but will do:
  • Check and clean the MAF sensor
  • Use the most powerful fuel injector cleaner my Autozone has
  • Change fuel filter
What I think is the problem:
* Coilpacks maybe? I'm going to buy the screamin demon coilpacks and will re-gap my plugs. My thought process is that a more powerful spark would fix this issue and perhaps give me better mpg.

So, am I going in the right direction or does it sound like the fuel system? I'm really at a loss and I miss not worrying about the engine running smooth. So I'm all ears.

13 Posts
I can think of two potential issues. 1 - inside the IAC. They carbon up and don't work properly. Cleaning the outside doesn't fix this. Has to be removed and inside cleaned off the car. Or it could be done altogether. 2 - MAP/BAP sensor may be bad. Try replacing with a known good one.
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