Hanging idle / start up rev - Mustang Evolution

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Old 08-31-2019, 09:59 PM   #1
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Join Date: Aug 2013
Location: Victoria
Region: Canada
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Hanging idle / start up rev

Hey guys! I've been doing some restoration work on my 99 mustang with 2002 gt drive train. It was in a sorry state when I bought it back after a few years of owning new mustangs. Since I got it back, I replaced a coil pack, seafoamed it, put new oil, replace air filter, replaced all the plugs, and now ovr been trying to figure out why my idle hangs. I understand that some of this is normal but it seems to take a second after coming to a stop even with the brake off. I looked into my IAC. Tried unplugging it and improved my idle, but did not kill the engine as it should have. Inspecting my vacuum lines, I don't see or hear anything that sounds like it could be leaking. So I decided to replace the IAC. Since then, the car behaves the same but with one new added symptom. The car spikes up to 3k or so on start up, almost as if I started it with my foot on the pedal. Anyone have any ideas where to look next? Of note, I did unplug the battery and try to drain the remaining charge before starting the vehicle after the IAC swap. Could this be a vacuum issue, exhaust leak (haven't ruled it out), or tps issue?

I appreciate any advice. I've done some research but as always, I seem to be encountering some bizarre mix of symptoms that make it hard to assume what the issue is. My mechanical skills are moderate at this point. I can do most of these smalls things.

Il post more story about the car later. It was my college car and I've owned the car on and off since 08!

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Old 09-05-2019, 04:38 PM   #2
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Location: Around Orlando FL
Region: Florida
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Just had the same issue. Car kept dying at stop lights. Replaced iac, then had hanging/high idle but didnt die.

In the interum, we replaced a ps hose. I decided to bleed the brakes, and that helped a little (i guess hydroboost and vacuum lines are related).

I also replace a couple vacuum hoses and used clamps to seal them shut. I found the leaks by using an unlit proane torch around the lines, replaced those lines and added clamps. Now the car runs like new

Hope this helps
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