Cold Start Rough Idle

Bryant Magana

New Member
Feb 25, 2018
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I have a really rough cold start which stalls the car out and runs rough and kills car have to try tapping the gas until it smooths out. Also long crank time on cold start as well. Once warm car will start no problem.
Tested Fuel pressure
KOEO : 0-5psi (if i prime the key about 3-4 time pressure will rise about 5/10 psi at a time till it reaches about 35 psi.
KOER : 37 psi (Fuel Regulator Vac off @ 41psi) no fuel coming out of regulator when vacuum line off it.

New Fuel Pump/Filter
New Coil
 
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Mustang5L5

i'm familiar with penetration
Mod Dude
Feb 18, 2001
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That prime behavior is not typical. When I first cycle my key, pressure rises from 0 to 39 psi on one key cycle.

Was the issue present before you changed the fuel pump? I’ve seen s split hose from the pump cause similar behavior
 

Bryant Magana

New Member
Feb 25, 2018
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That prime behavior is not typical. When I first cycle my key, pressure rises from 0 to 39 psi on one key cycle.

Was the issue present before you changed the fuel pump? I’ve seen s split hose from the pump cause similar behavior
Yeah that behavior seemed weird to me. It did that before i changed the pump which is why i changed it. its a new 155lb pump from LMR. also the fuel doesnt seem to bleed down super fast after i turn the key off. it does go down from about 35 to 29 right when i turn the key off then these two pictures are 22 minutes apart right after shutoff.
B83A4E94-0B69-45F7-AA25-759AD7055FB6.jpeg
61F70291-4CFF-40E1-ABBB-6336656DAFC9.jpeg
 

Steel1

Mustang Master
Aug 18, 2017
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Do you have an afr gauge on the car ?
Would be nice to know what it's doing during cold start, I would suspect your pretty lean.
 

Bryant Magana

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Feb 25, 2018
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Do you have an afr gauge on the car ?
Would be nice to know what it's doing during cold start, I would suspect your pretty lean.
I do not have a AFR gauge.
When I changed my ACT sensor there was fuel on the threads of it. Does that lead to a leaky injector? which is why i have cold start issues/slight misfire on cold start?
 

Steel1

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Aug 18, 2017
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Fuel anywhere except where it's supposed to be is never good, if external you are risking a fire.
I would take a good look at the area around the injectors/rail and check for leaks, make sure of that before proceeding.
Could that be the cause of rough cold start, probably not because a leak would be present hot or cold.

Reason I asked about afr is a richer fuel mixture is required when warming up.
 

Blown88GT

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Nov 13, 1999
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Yeah that behavior seemed weird to me. It did that before i changed the pump which is why i changed it. its a new 155lb pump from LMR. also the fuel doesnt seem to bleed down super fast after i turn the key off. it does go down from about 35 to 29 right when i turn the key off then these two pictures are 22 minutes apart right after shutoff.
B83A4E94-0B69-45F7-AA25-759AD7055FB6.jpeg
61F70291-4CFF-40E1-ABBB-6336656DAFC9.jpeg
Pressure not supposed to bleed down "super fast". Doing so will create other problems on hot start. Pressure look normal in those pics.

Are saying this: KOEO : 0-5psi (if i prime the key about 3-4 time pressure will rise about 5/10 psi at a time till it reaches about 35 psi.
is BEFORE or AFTER the new fuel pump?
It is not normal. What is fuel pressure when engine is started COLD?

You could be dumping excessive fuel into the intake. Stuck injector or bad ECU (never heard of injector driver failing ON, but it's electrically possible).
You will need a NOID light.
View: https://www.youtube.com/watch?v=5MmR-wXddVw
 
Last edited:

jrichker

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lowendmac.com
I have a really rough cold start which stalls the car out and runs rough and kills car have to try tapping the gas until it smooths out. Also long crank time on cold start as well. Once warm car will start no problem.
Tested Fuel pressure
KOEO : 0-5psi (if i prime the key about 3-4 time pressure will rise about 5/10 psi at a time till it reaches about 35 psi.
KOER : 37 psi (Fuel Regulator Vac off @ 41psi) no fuel coming out of regulator when vacuum line off it.

New Fuel Pump/Filter
New Coil
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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