Engine Idles at 4000 RPM at start

red5.0fox

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Dec 1, 2019
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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.
 
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red5.0fox

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Dec 1, 2019
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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
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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
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0
1
15
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
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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

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Dec 1, 2019
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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

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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.
 
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red5.0fox

New Member
Dec 1, 2019
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0
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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
 

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jrichker

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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,
 
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General karthief

wonder how much it would cost to ship you a pair
Mod Dude
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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.
 
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red5.0fox

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Dec 1, 2019
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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
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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

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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

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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
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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.