2015 Hyundai Genesis Sedan 5.0L V8, 8-speed automatic transmission 60k miles. Rough idle on coldstart only. Once the car has been driven the idle is so smooth you can’t even tell the car is running. No loss of power car runs great. No check engine light or any dash light and no codes found using BlueDriver scanner. I’ve had the car 3 years and it’s been doing this for about a year now. Always used 87 octane with no problems but recently switched to 93 to see if that would solve the issue but it has not. GDI engine all 8 fuel injectors were replaced under warranty 1 year ago at 54k miles. I use full synthetic 5-20 every 5k miles. Please help.
Throttle body is clean
So, as embarrassed as I am to admit this, I guess the problem was the loose spark plug in cylinder 6 the whole time? 🤦🏻♂️
But the whole reason I pulled the spark plugs back in late September 2020 to begin with was because of the rough idle 🤦🏻♂️🤦🏻♂️😅😅
soooooo 🤷🏻♂️
regardless car seems fine again
When was the last time you had a basic tune up? Air filter, spark plug change, etc? Are all your ignition coils working normally? Are any damaged?
Both air filters are clean. I actually suspected and was kind of hoping it was the spark plugs so I pulled all 8 and inspected them and made sure they were all gapped correctly and I know looks can be deceiving but they look normal. No obvious signs of excessive wear or damage. They are NGK iridium which is the factory spec and about as good as spark plugs get in my unprofessional opinion. As for the ignition coils I don’t have any check engine light or codes pulled using BlueDriver scan tool and besides the Cold start idle, runs drives operates idles fine otherwise so I don’t think it’s any of the ignition coils. Been scratching my head so long I’ve made it through the skin and now I’m scratching my skull 😩
Keep us updated in the morning. The reason I mentioned coils, I’ve had an ignition coil do exactly that when it was just starting to go out. Over time it kept getting worse until 1 day it went completely & the engine was misfiring continuously.
Will do! Good to know! Thanks 🙂
So, as embarrassed as I am to admit this, I guess the problem was the loose spark plug in cylinder 6 the whole time? 🤦🏻♂️
But the whole reason I pulled the spark plugs back in late September 2020 to begin with was because of the rough idle 🤦🏻♂️🤦🏻♂️😅😅
soooooo 🤷🏻♂️
regardless car seems fine again
Thank you for reporting back. Happy motoring.
Clean your air sensor. It can cause this behavior when dirty. And just to be thorough, swing your throttle plate open and check backside of it. Front can be clean, back can be dirty. Harder to clean the backside on some cars.
After doing a little research in some other forums I did see some others complaining about issues with the Throttle Position Sensor failing under warranty. Could cause similar issues. Something to ask your local dealer about. Didn’t see any recalls on that but you may have one for an ABS system fire hazard. Check into that.
No MAF sensor. MAP sensor. Nothing I can do with it besides replace it but I don’t think that would make sense being it’s only happens on cold start and goes away within about 5-10 min. Front and back of throttle plate clean. I suppose it’s possible Throttle position sensor could be the culprit but why only on cold start and not all the time? As for the ABS recall, I still need to get that done. Thank you for your response! 🙌🏼
Sometimes I wish I had a MAF sensor I could clean 😅
You do, should look something like this
That’s for a 2014 3.8L V6. I have a 2015 5.0L V8 with a MAP (manifold absolute pressure) sensor. To my knowledge vehicles have one or the other, not both.
I know what year it’s from. I posted it as a reference. You gotta have a sensor monitoring inbound air flow, even if it’s a MAP. Still can be cleaned.
Ok I was unaware you could clean a MAP sensor. I thought it only applied to MAF sensors 😅🤷🏻♂️
I will give it a shot. Thanks
If that doesn’t help your gonna have to start looking at your throttle position sensor or throttle body. They pull double duty when engines cold. Supposed to not open as far and basically be your choke.
Removing the MAP sensor was a huge pain but I got it done and unfortunately, there wasn’t even any buildup therefore it felt like a waste of time but I sprayed it down really good with CRC MAF cleaner and reinstalled. I also removed the throttlebody and sprayed and cleaned the front and back of the throttle plate (again) but there wasn’t any buildup on that either. Anyways put everything back together and the car still does the same thing. As for the position of the throttle plate at idle, it is almost completely shut but not completely. I’d say maybe at like a 1° angle 0° being completely shut. I understand it could be 1000 different things but the fact that it’s only on cold start and after five minutes of driving it idles butter smooth after that; does that not narrow down the possibilities? 🤷🏻♂️
Only other possibility would be the tenp sensor that controls the choke. It doesn’t choke when cold and runs rough until warm. You habe eliminated everything elee.
So, as embarrassed as I am to admit this, I guess the problem was the loose spark plug in cylinder 6 the whole time? 🤦🏻♂️
But the whole reason I pulled the spark plugs back in late September 2020 to begin with was because of the rough idle 🤦🏻♂️🤦🏻♂️😅😅
soooooo 🤷🏻♂️
regardless car seems fine again
Weird that would only affect it when cold. Glad you found it
Agreed! Thanks again
Since you've addressed the usual causes, maybe your temp sensor is telling the ECU the engine is warmer than it actually is, and it's not delivering the correct air/fuel ratio for the actual temp. I would imagine a high level scan tool would show if this is the case.
Idk what “mode 6” is but here are some screen shots of scan and a Dropbox link to a screen recording of the live data.
https://www.dropbox.com/s/wttv0cd3iljp650/Video%20Jun%2030%2C%2013%2027%2011.mov?dl=0
So, as embarrassed as I am to admit this, I guess the problem was the loose spark plug in cylinder 6 the whole time? 🤦🏻♂️
But the whole reason I pulled the spark plugs back in late September 2020 to begin with was because of the rough idle 🤦🏻♂️🤦🏻♂️😅😅
soooooo 🤷🏻♂️
regardless car seems fine again
Is there anything, such as white smoke or steam, visibly coming out of the Exhaust before it warms up and smooths out?
Yes. Not excessive but light white smoke on start up but goes away once warmed up.
So, as embarrassed as I am to admit this, I guess the problem was the loose spark plug in cylinder 6 the whole time? 🤦🏻♂️
But the whole reason I pulled the spark plugs back in late September 2020 to begin with was because of the rough idle 🤦🏻♂️🤦🏻♂️😅😅
soooooo 🤷🏻♂️
regardless car seems fine again
so like I said no check engine light or any codes pulled from the OBD2 however I was browsing through the mode 6 data and noticed there were 10 misfire counts for cylinder 6. Just pulled cylinder 6 ignition coil and it is brown and has build up. pulled cylinder 2 ignition coil and it still looks brand new. going to swap them and see what happens.
Good news: found loose plug, tightened it, and switched cylinder 6 coil with cylinder 1 coil. No more mode 6 misfire data on any of the cylinders.
Bad news: I don’t think that was the culprit for the rough idle on cold start only but i won’t know until morning. I’m in central Texas and it’s 90°F + pretty much daily now so it takes my car sitting overnight to cool down 😅🤷🏻♂️
So, as embarrassed as I am to admit this, I guess the problem was the loose spark plug in cylinder 6 the whole time? 🤦🏻♂️
But the whole reason I pulled the spark plugs back in late September 2020 to begin with was because of the rough idle 🤦🏻♂️🤦🏻♂️😅😅
soooooo 🤷🏻♂️
regardless car seems fine again