Engine Idles at 4000 RPM at start

red5.0fox

New Member
Dec 1, 2019
12
0
1
16
Hope Mills, North Carolina
Just swapped in a 5.0 5 speed engine and tranny, but now everytime we start the car it revs right up to 4000 RPM and stays there. No idea what is causing this problem. We checked the TCP, MAF, salt and peppers, IAC, etc. but still happening. Can anyone help me with this? I have a video, but I can’t post it.
 
  • Sponsors(?)


red5.0fox

New Member
Dec 1, 2019
12
0
1
16
Hope Mills, North Carolina
Ok, what year car? What year harness? What computer? How much air is in the right front tire?
1992, the harness is the same year. Stock ECU out of the 5.0 donor car. Couldn’t tell you how much air lol.. underneath the intake manifold, there is a vacuum hole that we capped. Pretty sure it goes to smog. Don’t think that’s the problem, but it’s something we did.
 

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
17,287
5,586
193
polk county florida
Here is some help to see if you missed anything, of course you need to substitute 92 stuff when it comes to wiring because 92-3 wiring are exclusive. Also 02 wiring is different between auto and stick.
then you can go through this checklist for the idle stuff.
a vacuum leak will also cause a high idle, the list shows you how to properly set base idle.
 

red5.0fox

New Member
Dec 1, 2019
12
0
1
16
Hope Mills, North Carolina
Here is some help to see if you missed anything, of course you need to substitute 92 stuff when it comes to wiring because 92-3 wiring are exclusive. Also 02 wiring is different between auto and stick.
then you can go through this checklist for the idle stuff.
a vacuum leak will also cause a high idle, the list shows you how to properly set base idle.
Didn’t find anything in it. I’ve looked everywhere online, and NO ONE has had a problem of 4000 RPM at idle. Not sure wth is happening.
 

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
17,287
5,586
193
polk county florida
You have to do the checklist, I think nobody here can say exactly what your problem is without putting hands on your engine.
have you checked for codes?
stock swap or did you add modifications?
 

red5.0fox

New Member
Dec 1, 2019
12
0
1
16
Hope Mills, North Carolina
You have to do the checklist, I think nobody here can say exactly what your problem is without putting hands on your engine.
have you checked for codes?
stock swap or did you add modifications?
Bone stock, and haven’t quite checked for codes yet. I’m not a mechanic by any means, I’m 15 years old, so my dad has been helping me, but he’s one of those guys that just thinks if they cant figure it out, no one can, you know? So I’m trying to find out. We did do some of the checklist things though.
 

jrichker

StangNet's favorite TOOL
SN Certified Technician
Mar 10, 2000
27,436
2,869
234
74
Dublin GA
lowendmac.com
Tell your dad that at 73, I am probably older than he is,, so he can take advice from an old guy...


You guys with idle/stall problems could save a lot of time chasing your tails if you would go through the Surging Idle Checklist. Over 50 different people contributed information to it. The first two posts have all the fixes, and steps through the how to find and fix your idle problems without spending a lot of time and money. It includes how to dump the computer codes quickly and simply as one of the first steps. I continue to update it as more people post fixes or ask questions. You can post questions to that sticky and have your name and idle problem recognized. The guys with original problems and fixes get their posts added to the main fix. :D

It's free, I don't get anything for the use of it except knowing I helped a fellow Mustang enthusiast with his car. At last check, it had more than 250,000 hits, which indicates it does help fix idle problems quickly and inexpensively.
 
Last edited:

red5.0fox

New Member
Dec 1, 2019
12
0
1
16
Hope Mills, North Carolina
Tell your dad that at 73, I am probably older than he is,, so he can take advice from an old guy...


You guys with idle/stall problems could save a lot of time chasing your tails if you would go through the Surging Idle Checklist. Over 50 different people contributed information to it. The first two posts have all the fixes, and steps through the how to find and fix your idle problems without spending a lot of time and money. It includes how to dump the computer codes quickly and simply as one of the first steps. I continue to update it as more people post fixes or ask questions. You can post questions to that sticky and have your name and idle problem recognized. The guys with original problems and fixes get their posts added to the main fix. :D

It's free, I don't get anything for the use of it except knowing I helped a fellow Mustang enthusiast with his car. At last check, it had more than 250,000 hits, which indicates it does help fix idle problems quickly and inexpensively.
Just sent him the list. I’ll bug the hell out of him until we run through it, or I’ll do it myself. But now that that’s aside, I have 1 more problem. What the heck is this sensor? Cant find where it plugs in, but it was plugged in before we pulled the engine! Thanks for your help by the way!!
9358BDC0-191E-4586-B354-49B042F3B794.png
 

Attachments

jrichker

StangNet's favorite TOOL
SN Certified Technician
Mar 10, 2000
27,436
2,869
234
74
Dublin GA
lowendmac.com
That's the ECT (Engine Coolant Temperature) sensor. It tells the computer what temperature the engine is running at. The computer uses this information to set the spark advance and air/fuel mixture ratio,
 
Last edited:

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
17,287
5,586
193
polk county florida
There are two similar connectors in that area, one is for the sensor on the heater tube, the other is for the charcoal canister purge valve.
i think
i'll look in a few
Yup, only two there. The wiring for the purge valve has a pig tail from that connector to the valve down by canister along with the vacuum line.
 
Last edited:

red5.0fox

New Member
Dec 1, 2019
12
0
1
16
Hope Mills, North Carolina
There are two similar connectors in that area, one is for the sensor on the heater tube, the other is for the charcoal canister purge valve.
i think
i'll look in a few
Yup, only two there. The wiring for the purge valve has a pig tail from that connector to the valve down by canister along with the vacuum line.
Is there any chance you can take a picture/video showing where it goes? We have to find the other end.
 

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
17,287
5,586
193
polk county florida
I can describe it faster, below the air box is a plastic box with what looks like a mushroom on top, there is a plastic line that runs down the frame rail from the fuel tank that connects to the tank on one end and the purge valve is connected to another nipple with a vacuum line and the wiring pigtail up by the fuel lines on the block to connect the pigtail to that connector and the vacuum line to the front of the intake.
Confused yet? Google fox purge valve and you will see pics
 

jrichker

StangNet's favorite TOOL
SN Certified Technician
Mar 10, 2000
27,436
2,869
234
74
Dublin GA
lowendmac.com
I did some research, and that switch is way back in the engine near the firewall, this plug is right next to the distributor. I know you know wayyy more than me, but that doesn’t seem right?
You are indeed correct. I have edited my post to correct the error, thanks for the heads up.
 

Mstng93SSP

You have a nice rear end there Dave.
15 Year Member
Nov 29, 1999
1,979
2,307
174
Mililani, Hawaii
If that is the coolant temp sensor plug, the sensor is on the black metal coolant tubes that go to the back of the engine and into the firewall to the heater core. I have had a bad coolant temp sensor cause high idle on startup. Find the sensor and lug it in. It could be your problem.
 

red5.0fox

New Member
Dec 1, 2019
12
0
1
16
Hope Mills, North Carolina
I can describe it faster, below the air box is a plastic box with what looks like a mushroom on top, there is a plastic line that runs down the frame rail from the fuel tank that connects to the tank on one end and the purge valve is connected to another nipple with a vacuum line and the wiring pigtail up by the fuel lines on the block to connect the pigtail to that connector and the vacuum line to the front of the intake.
Confused yet? Google fox purge valve and you will see pics
You are indeed correct. I have edited my post to correct the error, thanks for the heads up.
Ya, it was the purge valve. We bought some vacuum to run it from the intake, to the sensor, and onto the charcoal canister. And the reason I couldn't find the other end is because the 5.0 canister was still on the donor car! Little errors lol. I dont think its 100% the idle problem, but I'm sure it'll solve the gas fumes, and maybe bring it down a little bit. We also messed with timing which helped a lot. Running through the checklist after we hook up the charcoal vacuum.
 
Thread starter Similar threads Forum Replies Date
africansnowowl Engine Another rough idle thread Fox 5.0 Mustang Tech 4
J Looking for 2005 GT OBD2 idle sensor reading 2005 - 2014 S-197 Mustang -General/Talk- 2
J Engine New Manifold nightmare idle and stall 2005 - 2009 Specific Tech 4
C 2013 GT low oil pressure at low RPMs or idle 2010 - 2014 Specific Tech 0
R Engine Remove the Idiot from the equation. Idle/hesitation issue. Fox 5.0 Mustang Tech 3
C 2014 Mustang Gt/Cs Idle and dying issue 2010 - 2014 Specific Tech 2
T MS3 AC Idle Up Settings Digital Self-tuning Forum 1
93silverlx50 Engine Car in limp mode, fuel spitting out of tailpipes, missing rough idle Fox 5.0 Mustang Tech 9
markinms Engine Should Auto Trans Be in Drive When Setting Base Idle Fox 5.0 Mustang Tech 7
R Fuel Throttle body idle screw 1994 - 1995 Specific Tech 2
darryl paarman A/C idle up control- Low input or High. ? Digital Self-tuning Forum 2
C Engine Gray smoke from exhaust. Smells like gas. Fox 5.0 Mustang Tech 15
lamrith New MSPnP2 in 94 Mustang, need help getting stable idle once warmed up. Digital Self-tuning Forum 1
J Idle issue still!! Replaced several parts.. looking for more ideas!! Fox 5.0 Mustang Tech 12
G 1998 3.8L V6 overheating at idle, gauge doesn’t go up. SN95 V6 Mustang Tech 3
bmgstang Need help with tune high idle Digital Self-tuning Forum 7
S Another Idle surge post with microsquirt Digital Self-tuning Forum 5
H Engine Tach goes to zero,engine goes to idle on 87 GT Fox 5.0 Mustang Tech 10
D 1994 Mustang Gt erratic idle 1994 - 1995 Specific Tech 1
A New fox owner, idle issues need some advice Fox 5.0 Mustang Tech 20
Mixveez Bad idle and dying in neutral. Fox 5.0 Mustang Tech 5
C Microsquirt tuning and surging idle problems Digital Self-tuning Forum 7
L High Idle and Hanging RPM. Tried everything, still no fix. SN95 4.6L Mustang Tech 0
J 2000 GT Idle/Running Issues SN95 4.6L Mustang Tech 6
9 Air Condition Idle up with 2-speed fan (MSPNP Gen 2 V1.2) Digital Self-tuning Forum 8
maisonmays22 Cannot figure out why my engine dies at idle Digital Self-tuning Forum 11
R Engine My 95 gt idling rough 1994 - 1995 Specific Tech 5
MSTNGGRL1995 Rough, loping idle and random attempts to die 1994 - 1995 Specific Tech 1
T Engine Foxbody idling surging and stalling Fox 5.0 Mustang Tech 10
B Slight surging idle issues Digital Self-tuning Forum 14
K Engine New Built Engine - Rough Start and Idle Surge Fox 5.0 Mustang Tech 3
G Engine Idle fussy only when 1st starting Fox 5.0 Mustang Tech 7
S Megasquirt PNP2 460SBF startup/idle issues Digital Self-tuning Forum 9
W 97 Mustang GT to 2011 Crown Vic Swap 95k Miles (Knock at idle) 1996 - 2004 SN95 Mustang -General/Talk- 1
Michael sommers 02 mustang gt auto knock only at idle in gear. SN95 4.6L Mustang Tech 2
Michael sommers 02 mustang gt auto knocking only when warm at idle in gear. 1996 - 2004 SN95 Mustang -General/Talk- 0
darryl paarman After revision - now hanging idle - Digital Self-tuning Forum 30
8 Plz Help! ‘88 Mustang will not idle and runs poorly Fox 5.0 Mustang Tech 4
F Idle Fluctuation Issue 98 GT SN95 4.6L Mustang Tech 0
D Engine Idle issues Fox 5.0 Mustang Tech 23
D Poor idle after full intake install SN95 4.6L Mustang Tech 3
wolfheads Irratic idle 2005 - 2009 Specific Tech 3
H Idle problem 2005 - 2009 Specific Tech 9
paddyrk Engine Hot idle oil pressure Fox 5.0 Mustang Tech 7
S 2000 Mustang GT surging idle/cruising SN95 4.6L Mustang Tech 3
E 2010 Mustang GT - randon, intermittent, fresh hard starts with rough idle. 2005 - 2014 S-197 Mustang -General/Talk- 0
M my '93 fox 2.3 idles @ 1200 2.3L (N/A & Turbo) Tech 4
H Car overheating when engine sits at idle SN95 V6 Mustang Tech 6
T 91 mustang idle surging Fox 5.0 Mustang Tech 5
Xenogenesis Timing issue at idle SN95 V6 Mustang Tech 1
Similar threads