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Premium Member
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15,861 Posts
Discussion Starter #1
For those of you developing rough cold start idle, I think I've found the cause.

leaking injector(s)

to test:
start the car to verify rough idle
turn off the car
pull injector fuse on passenger side in the engine bay
start the car once

If it catches and dies, put the fuse back in and start it up.
The car should idle normally.

with as many miles as the first gens are getting now, it doesn't surprise me.

A good injector cleaning should fix it. The injector cleaning additives don't do much in this case, but worth trying once.

I tested the injectors on the bench with 2 AA batteries to see if there was some odd firing. 2 of mine had dull clicking.
I ran injector cleaning solvent through mine on the bench, then ultrasonic cleaned them and ran cleaner through them again.

Firing like a champ.
 

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209 Posts
For those of you developing rough cold start idle, I think I've found the cause.

leaking injector(s)

to test:
start the car to verify rough idle
turn off the car
pull injector fuse on passenger side in the engine bay
start the car once

If it catches and dies, put the fuse back in and start it up.
The car should idle normally.

with as many miles as the first gens are getting now, it doesn't surprise me.

A good injector cleaning should fix it. The injector cleaning additives don't do much in this case, but worth trying once.

I tested the injectors on the bench with 2 AA batteries to see if there was some odd firing. 2 of mine had dull clicking.
I ran injector cleaning solvent through mine on the bench, then ultrasonic cleaned them and ran cleaner through them again.

Firing like a champ.
i dunno if this could relate to my condition, my car idles rough when cold, It also idles rough if i turn on defroster or AC.
 

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Premium Member
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15,861 Posts
Discussion Starter #3
i dunno if this could relate to my condition, my car idles rough when cold, It also idles rough if i turn on defroster or AC.
AC and defrost is because it's running an extra accessory

defrost engages the AC so it can dehumidify the air entering into the cabin.

On cold start, it should jump above 1k rpms and then ease it's way down as the engine starts to warm up.

The kind of rough idle I'm talking about is where the needle jumps back and forth about 200 rpms.
 

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Registered
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23 Posts
For those of you developing rough cold start idle, I think I've found the cause.

leaking injector(s)

to test:
start the car to verify rough idle
turn off the car
pull injector fuse on passenger side in the engine bay
start the car once

If it catches and dies, put the fuse back in and start it up.
The car should idle normally.

with as many miles as the first gens are getting now, it doesn't surprise me.

A good injector cleaning should fix it. The injector cleaning additives don't do much in this case, but worth trying once.

I tested the injectors on the bench with 2 AA batteries to see if there was some odd firing. 2 of mine had dull clicking.
I ran injector cleaning solvent through mine on the bench, then ultrasonic cleaned them and ran cleaner through them again.

Firing like a champ.
Mine doesn't necessarily do it on a cold start, unless I'm cold starting it when it's hot outside. I will start the car and it will have a delay before going up to around 2,000 RPMs for the usual cold start. It will start normally and then boost up. In the mornings it works just fine though. But after I have driven the car and it's already warmed up, if it's before normal operating temperature and I started up again I will get a rough idle below 1,000 RPMs. I think it also might be a throttle body issue, going to swap it in the next day.
 

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Premium Member
Joined
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15,861 Posts
Discussion Starter #5
Mine doesn't necessarily do it on a cold start, unless I'm cold starting it when it's hot outside. I will start the car and it will have a delay before going up to around 2,000 RPMs for the usual cold start. It will start normally and then boost up. In the mornings it works just fine though. But after I have driven the car and it's already warmed up, if it's before normal operating temperature and I started up again I will get a rough idle below 1,000 RPMs. I think it also might be a throttle body issue, going to swap it in the next day.
You realize that the last post was from 6 years ago?
 

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Premium Member
2010 Hyundai Genesis Coupe 2.0T - Pirahna Racewerks Stage 1 - 285hp to crank - just getting started
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