Strange return to idle issue

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Strange return to idle issue

Postby UrSobsessed » Mon Dec 18, 2017 2:21 pm

Hey guys,

Long time no post for me. My car has been running great but has recently developed a strange issue when returning to idle. When the engine is at 2000 + rpm's and I let off the throttle, the rpm's drop normally but when hitting ~1000 rpm's the engine revs on it's own and bounces around 1300-1500 rpm's for up to 30 seconds before calming down to normal idle of 850. If, when this is happening, I hit the throttle quickly to ~3000 rpm's it will usually drop the idle to normal quicker. It happens mainly when ambient temps are ~55°F and colder. Usually worse when colder. If I unplug the IAC and use spark based idle control only, the issue goes away.

The system has been pressure tested and no leaks were found. I have cleaned out the IAC, which is only 5 years/30k miles old anyway. I have messed with the throttle valve when it happens and that is not sticking. I asked Marc about the diode required for VEMS failing but it doesn't sound like that's an issue either. I have looked at the settings in VEMS and nothing seems out of sort. I've made no idle changes to the tune to cause this anyway. It might just be the IAC itself but I'm trying to not just throw parts at it until last resort.

Do any of you guys have any ideas on where to look? I'm drawing blanks.

I'll attach a .config when I remember how to do that. lol. I'll upload a vid later as well.

Thanks in advance for any assistance!

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Re: Strange return to idle issue

Postby ChrisAudi80 » Tue Dec 19, 2017 1:12 am

Remind me, is this your AEB or 5 cylinder?

If unplugging the IAC makes it go away, it looks like you have your answer.

Attaching a log is easy. Just scroll down and press upload attachment, browse for the right log, then attach.
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Re: Strange return to idle issue

Postby PRY4SNO » Tue Dec 19, 2017 9:48 pm

Only thing I can think of is the coolant temp sensor at the back of the head... my UrS4 did some funny things with idle (and slight misfires up top) when that was on it's way out.
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