rpm problem

ryn62188 · December 19th, 2011, 06:49 PM

When ever I Rev up it takes long time for the rpm to lower. When I come to stop it idles at 1200rpm then drops to normal. I had a p1506 and replace the iac. Any ideas what could cause rpm to stay high?

blackout97 · December 19th, 2011, 08:00 PM

its normal.

97S281 · December 19th, 2011, 08:29 PM

Check the vacuum line coming out of the back of the plenum for cracks.

xploder17s · December 20th, 2011, 01:57 AM

on the back side of the plenum would cause p0171 & p0174. it would also cause a LEAN idle of 20.0 AFR... (found this out myself)

-remove iac electrical connector. the car should stall. only way to keep it running should be pedal acceleration. if it remains idling normal then the iac valve is stuck open. (self tested)
-cleaning it is damn near pointless. it fixes it for a time being before it is done permanently.(yup, done this too)

97S281 · December 20th, 2011, 05:35 PM

When I swapped out my plenum for a new one the vacuum tube on the back was a larger diameter on the new one. It created the same type of problem for me and I never got a code from it.

1996 GT · December 20th, 2011, 05:39 PM

^What type of plenum were you using? Just out of curiosity.

97S281 · December 20th, 2011, 05:40 PM

Ford.

BAD98GT · December 20th, 2011, 08:07 PM

the mustangs are notorious for the hanging idle.....it will hang for aa few seconds then drop to normal at idle....usually a vacume leak will cause it to hang a lil higher than 1200rpm...i had a vac leak and mine used to hang at 1800-2000rpm....but ever since i ported my plenum it does hang around 1200 rpm though.. what kind of mass air flow sensor do u have?

December 19th, 2011, 06:49 PM	#1
ryn62188 Newbie 97 mustang gt Join Date: Sep 2011 Location: San Pablo ca Posts: 16 iTrader: 0 reviews	rpm problem When ever I Rev up it takes long time for the rpm to lower. When I come to stop it idles at 1200rpm then drops to normal. I had a p1506 and replace the iac. Any ideas what could cause rpm to stay high?

December 19th, 2011, 08:29 PM	#3
97S281 Rollin' 18/ pushin' 2 1997 Saleen S281 Join Date: Feb 2007 Location: The lost highway Posts: 6,673 iTrader: 13 reviews	Check the vacuum line coming out of the back of the plenum for cracks. __________________ MODS IN GARAGE NCMM ORIGINAL BLOWN 96-98 CREW MEMBER # 1

December 20th, 2011, 05:35 PM	#5
97S281 Rollin' 18/ pushin' 2 1997 Saleen S281 Join Date: Feb 2007 Location: The lost highway Posts: 6,673 iTrader: 13 reviews	When I swapped out my plenum for a new one the vacuum tube on the back was a larger diameter on the new one. It created the same type of problem for me and I never got a code from it. __________________ MODS IN GARAGE NCMM ORIGINAL BLOWN 96-98 CREW MEMBER # 1

December 20th, 2011, 05:40 PM	#7
97S281 Rollin' 18/ pushin' 2 1997 Saleen S281 Join Date: Feb 2007 Location: The lost highway Posts: 6,673 iTrader: 13 reviews	Ford. __________________ MODS IN GARAGE NCMM ORIGINAL BLOWN 96-98 CREW MEMBER # 1

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December 19th, 2011, 08:00 PM	#2
blackout97 Regular 97 Mustang gt Join Date: Jun 2010 Location: Marion, Ohio Posts: 99 iTrader: 0 reviews	its normal.

December 20th, 2011, 01:57 AM	#4
xploder17s Regular Join Date: Apr 2008 Posts: 133 iTrader: 0 reviews	on the back side of the plenum would cause p0171 & p0174. it would also cause a LEAN idle of 20.0 AFR... (found this out myself) -remove iac electrical connector. the car should stall. only way to keep it running should be pedal acceleration. if it remains idling normal then the iac valve is stuck open. (self tested) -cleaning it is damn near pointless. it fixes it for a time being before it is done permanently.(yup, done this too)

December 20th, 2011, 05:39 PM	#6
1996 GT Traction...? 1996 GT/ 2008 GT Vert Join Date: Nov 2010 Location: Canada Posts: 2,207 iTrader: 0 reviews	^What type of plenum were you using? Just out of curiosity.

December 20th, 2011, 08:07 PM	#8
BAD98GT Enthusiast Join Date: Feb 2010 Posts: 953 iTrader: 0 reviews	the mustangs are notorious for the hanging idle.....it will hang for aa few seconds then drop to normal at idle....usually a vacume leak will cause it to hang a lil higher than 1200rpm...i had a vac leak and mine used to hang at 1800-2000rpm....but ever since i ported my plenum it does hang around 1200 rpm though.. what kind of mass air flow sensor do u have?

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