fluctuating rpms 02 GT


New Member
Sep 29, 2021
daytona beach
new here, my name is nicole and i currently own a 2002 5 speed mustang gt.
she was an auction find, had 134k miles when i bought her now has 145k.

my issue is, when i cold start the engine, it’ll idle high then drop, idle high then drop. the rpms get up to a grand, then drop to a little under 500 almost seeming like it’s about to stall, this cycle continues until the engine is warmed up.
once the engine warms up, the rpms rest at 600/700 (as it should).

it still boggles my mind why every time the rpms fluctuate so drastically from a cold start, it’s a little frustrating.

i experienced this same issue when i first got the car, so i replaced the plugs and coils and it fixed the problem.
i just really hope it’s not a misfire or anything along those lines.

given the circumstances, i’m leaning towards either a vacuum leak or IAC valve.

any suggestions would be helpful and i would really appreciate it!
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Mustang Master
Sep 26, 2017
It probably has an intake air leak
Any unmetered air will cause your symptom
When the O2 sensor sees oxygen (air) that the MAF does not know about
The processor sends more fuel (increases the injector pulse width) and the engine speeds up
When the closed throttle idle speed reaches around 1000 the processor shuts the fuel down because at ,8v the TP says it should be at idle
This causes the bad surge as it repeats over end over
A thorough visual inspection is needed and listen close for hissing noise
You can take some non flammable brakleen and spray it around the intake gasket area while watching for a stumble
You can use carb foam (upper intake cleaner) or flammable brakleen if you got marshmallows
If you are lucky it just has a cracked vacuum hose
Even if you are unlucky intake gaskets are not that big a deal
Check the intake air tube for a split
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Founding Member
Aug 25, 2001
Rogue River, Oregon
My wife's Roush does the exact same thing, but only when the A/C is on. My Bullitt doesn't but no codes on either car. Weird but no codes and both cars run fine at all driving speeds so I don't lose any sleep over it.