• Mustang Forums
  • 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk-
  • Fox 5.0 Mustang Tech

Car idles up randomly

  • Thread starter Thread starter Mustang5L5
  • Start date Start date Jul 2, 2010

Mustang5L5

That is…until I whipped out my Bissell
Mod Dude
Feb 18, 2001
43,192
17,889
224
Massachusetts
Jul 2, 2010
#1
  • Jul 2, 2010
  • #1
This prob just started this summer.

Car will either idle normal at around 800 or idle high at 1500 rpm. It does not surge. It does it more when the car is first started and stops once it gets hot. Revving doesn't lower it and like I said...it's not a surge.

Weird part is I was at a light in neutral idling at 800 and the rpms just randomly rose up to 1500 rpm. So the iac opened up on it own by some signal

I've cleaned the iac. Biggest issue is if I stall or shut off when the idle is up....the car will not restart unless I push the gas pedal to the floor and crank it more than usual.

No codes stored....all 11s.

After car is hot it doesn't do it...so after 30 mins it is fine. But if I shut the car and restart...it sometimes goes back to the high idle. It's totally random, and like I said...not a surging idle at all.

It almost revs up like whe you do a koer test for codes.

Any ideas?
 

Mustang5L5

That is…until I whipped out my Bissell
Mod Dude
Feb 18, 2001
43,192
17,889
224
Massachusetts
Jul 2, 2010
#2
  • Jul 2, 2010
  • #2
Btw...car is an aod that was converted to 5 spd. I added the Vss but kept all the aod wiring and EEC
 

281pony

Active Member
Aug 31, 2003
2,681
2
46
Oly, WA
Jul 2, 2010
#3
  • Jul 2, 2010
  • #3
i recall my car doing similar things to this with the sticky idle before my dyno tune. he went over a whole checklist of stuff real quick he was "turning off" for me to cure these issues.

have you ever considered a dyno tune? do you have anyone local? if it comes to me what he turned off with my tune, ill report back. i thought it was along the lines of smog iirc. it made a huge difference though when he did it even idling on the dyno.
 

Celeb

20+ Year Stangneter
Sep 8, 2002
736
13
59
Here and there
Jul 2, 2010
#4
  • Jul 2, 2010
  • #4
Possible failing TPS?

When mine was on it's way out idle would rise and hold but would quit when it was hot.
 

2000xp8

SN Certified Technician
Aug 8, 2003
8,016
1,613
194
NJ
Jul 2, 2010
#5
  • Jul 2, 2010
  • #5
A few years back i had this issue.
It was the TPS.
 

HISSIN50

"How long does it take to get help in here?
15 Year Member
Nov 29, 1999
31,179
33
129
Jul 3, 2010
#6
  • Jul 3, 2010
  • #6
Another vote for it possibly being a TPS issue.
 

Mustang5L5

That is…until I whipped out my Bissell
Mod Dude
Feb 18, 2001
43,192
17,889
224
Massachusetts
Jul 3, 2010
#7
  • Jul 3, 2010
  • #7
It could very well be the tps....I have suspected it of being on it's way out.


Good thing I have several spares.

Thanks guys
 

jrichker

StangNet's favorite TOOL
In Remembrance. Thank you for your contributions
Mar 10, 2000
27,512
2,811
234
Dublin GA
Jul 3, 2010
#8
  • Jul 3, 2010
  • #8
The TPS is the #1 suspect for an idle that rises and then goes back down when you turn the ignition off.

See the "Surging Idle Checklist for help with all your idle/stall problems. Everything you need to find and fix idle problems is in there.

The first two posts contain all the updates to the fixes. At last count there were 24 possible causes and fixes for surging idle/stall problems. I continue to update it as more people post fixes or ask questions.
 

Mustang5L5

That is…until I whipped out my Bissell
Mod Dude
Feb 18, 2001
43,192
17,889
224
Massachusetts
Jul 31, 2010
#9
  • Jul 31, 2010
  • #9
Well, tps replaced and still does it.

But...

New code appeared. Now I get a consistent code 21 (ect sensor) when I run the koer test after the car has been warmed up for a while. Hoewver, when I run a koeo test just before, I get code 11 (pass)

Is it normal for ect code to not appear in koeo but always in koer?

Anyway, got a new sensor...just need to install.
 
H

Haus

Member
Jul 20, 2005
408
0
16
Jul 31, 2010
#10
  • Jul 31, 2010
  • #10
That ECT will fix you hot/warm start problem most likely
 

Mustang5L5

That is…until I whipped out my Bissell
Mod Dude
Feb 18, 2001
43,192
17,889
224
Massachusetts
Aug 1, 2010
#11
  • Aug 1, 2010
  • #11
Replaced the ect and problem solved.

No more codes, car runs nice.


Got to do a cylinder balance test too...different story there.
 
You must log in or register to reply here.

Similar threads

C
Engine Idle cuts out
  • claypoolxx
  • Mar 7, 2026
  • Fox 5.0 Mustang Tech
  • 2
Replies
22
Views
565
Fox 5.0 Mustang Tech Jun 5, 2026
KRUISR
7
Engine Random hanging idle
  • 706 Saleen
  • Feb 11, 2026
  • Fox 5.0 Mustang Tech
Replies
9
Views
453
Fox 5.0 Mustang Tech Feb 19, 2026
86HO5.0
8
K
Resolved Need some assistance. Holley Terminator X alongside H/C/I. Very rough/inconsistent idle
  • karhoot
  • Jun 6, 2026
  • Fox 5.0 Mustang Tech
Replies
0
Views
73
Fox 5.0 Mustang Tech Jun 6, 2026
karhoot
K
G
Idle surging after warm up
  • GALLY3309
  • Jun 10, 2026
  • 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk-
Replies
1
Views
70
1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk- Jun 10, 2026
Noobz347
Engine Surging idle on start-up
  • keel
  • Aug 11, 2025
  • Fox 5.0 Mustang Tech
Replies
16
Views
794
Fox 5.0 Mustang Tech Oct 31, 2025
keel
Share:
Bluesky Email Share Link
  • Mustang Forums
  • 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk-
  • Fox 5.0 Mustang Tech
Menu
Log in

Register

  • Forums
  • What's new
  • Media
  • Resources
  • Contact
  • Sponsor
X

Privacy & Transparency

We use cookies and similar technologies for the following purposes:

  • Personalized ads and content
  • Content measurement and audience insights

Do you accept cookies and these technologies?

X

Privacy & Transparency

We use cookies and similar technologies for the following purposes:

  • Personalized ads and content
  • Content measurement and audience insights

Do you accept cookies and these technologies?