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High idle while rolling, drops to normal when stopping

5K views 35 replies 20 participants last post by  dave6666 
#1 ·
Hi guys,

After letting off the gas and putting the car in neutral to coast to a stop the idle will only drop to around 1500 RPM. It does this until I get close to actually stopping, at which point it drops to a normal idle. The car is a 2001 GTS and it does this around 75% of the time.

Any ideas on what could cause this?
 
#4 ·
Normal. Should do it all the time. Or better yet, it better do it all the time. I haven't checked if there is any difference with or without the AC on when approaching a stop. I know in my SCT software, I can set idle decell to where ever I want. No harm in checking the IAC pintel for sticking regardless. Oh and mine is a 2001 GTS...at one time at least.
 
#5 ·
Normal for mine. When I get below around 10 MPH with the clutch in, the idle drops to the normal 750 RPM.
 
#7 ·
This is normal for practically all fuel injected manual shift cars as well as some autos. The main purpose is to provide additional air for a slightly fuel rich condition following a gear change. It also improves driveabilty on cars with aggressive cam timing, as EllowViper explained it should do that at all times at normal operating temperature.
 
#9 ·
my 2001 tuned with an SCT does this but more often than not it keeps fluctuating, like i'm repeatedly pressing the gas pedal until i reach less than 5mph or so. i don't remember the exact range but maybe 600 to 1500rpm? it's somewhat annoying because considering how loud the exhaust is, everyone around me probably thinks i'm trying to rev the engine at them. i cleaned out the IAC but it still does it.
 
#12 ·
Pretty sure the first mark on the tach is 750.
 
#14 ·
I stand corrected. Just went out to the car and looked. Hell, I never even noticed that first line before.
 
#16 ·
On my '01 the first line on the tach is 500 rpm. There is no line at 250. There is also a line at 750, and my car idles slightly below that.

Also, your idle hang is not normal. Mine used to do that - somewhat annoying - but I was able to make it stop. I much prefer driving the car without it doing that.
 
#18 ·
My idle hangs now that I'm on an SCT tune for the Paxton.

It irritates the crap out of me. I know it's in the tune because it never did it before I put on the supercharger and flashed with the SCT.
 
#24 ·
I can rule out almost all of the above theories.

When I did my build, I cleaned the throttle bodies, IAC, and made them like new. Even replaced the throttle body and IAC gaskets with new OEM.

Also, during my build, I installed a brand new battery. And for whoever mentioned vacuum leak.... no, because when the idle falls to normal it idles perfectly and doesn't miss a beat which wouldn't happen with a vacuum leak.

It's definitely tune related. Tori at Unleashed sent me a new tune because I told him it was hanging so I'll report back and see if it fixed it.
 
#28 ·
I will get a video of the symptoms within the next few days. The car is currently on jack stands because I'm having problems with my back-up lamps not coming on (suspect it is the back-up lamp switch on the transmission).

For the idle issue, I have already disconnected the battery, and then also disconnected all 3 plugs from the ECU and let that sit for awhile before connecting it all back up. I believe the IAC is held on by a Torx bolt which I don't have laying around, but will try to clean that next.

With any luck that'll "fix" it, but if not I will get the behavior on video. I should have done that in the first place :(

Thanks to all the helpful replies in this thread! It's really interesting that some people say this is normal and others not...
 
#30 ·
Try scanning the PCM IAC counts, make note of the counts while stationary as well as the RPM at those counts. Use RPM from the scanner. Drive the car, clutch in coasting, monitor counts make note of RPM. If counts are higher and RPM also the PCM commanded the action. If the counts while coasting are the same as stationary but the RPM is higher then another air source is bypassing the throttle valve(s). Cars w/SCT can be tailored to suite the tuner/owner, Chrysler had to tune to suite the EPA, CARB, and a gaggle of other legal concerns. Also try monitoring counts w/AC high blower then quickly tuning off while monitoring idle stability. Counts should increase with load, decrease without while idle remains fairly constant.
 
#32 ·
This+++ The SCT lets you do allsorts of silly things with IAC settings/counts. I'm on the road right now and can't access where I set mine at for various conditions. Different for many conditions to include cold start, idle, warm idle, coasting, closed loop, open loop. In fact (if I recall) at WOT the IAC is wide open allowing every bit of available air to get into the induction system. Easy to tune out but I like having a high idle (1500) approaching a stop...and then seeing it drop to a commanded 780 rpm. But that is just me.
 
#31 ·
It's the IAC valve right behind the passenger side Throttle body. Clean it and your good. Or buy another one. Easy change. Guy i bought the car off of told me about it. It clogs up about every 3000 miles. Just stab the throttle real quick also and that clears it sometimes.
 
#34 ·
AEM has a rolling idle speed parameter that can be tuned to above the base idle speed, easily changed if you don't like it. I would encourage you to leave it as is, there is a reason for that. I have not seen data from a stock Viper PCM but have seen data from countless other makes (many Mopars) and most if not all are programed to do this. I applaud your tuner for including this factory like idle tune.
 
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