Rough idle AFTER carbon cleaning?

[2015genesisNewbie]

This tells me it's happening only on cold start open loop mode, and the ECU can compensate when running closed loop with the O2 sensor inputs. So the telltale will be the ST and LT fuel trims.

Get a cheap OBD2 reader and torque app and watch your fuel trims. If they're way positive, you probably have an air leak or a bad MAP/MAF sensor. If they're way negative, maybe a sticky throttle plate or bad MAP/MAF sensor, totally clogged air filter (unlikely); or perhaps one or more sticky (open) injectors, but this is less likely with new replacements.
Very easy to screw up modern cars when pulling the intake off, with all the old plastic fiddly bits and gaskets. Could also be compression/plugs but you'd usually see a misfire code if really bad. Our cars seem to be moderately tolerant of cold start open loop "rough" idle short of complete misfire.

Do you know what the CEL code was before you took it in?

Update: 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?

 

[2015genesisNewbie]

This tells me it's happening only on cold start open loop mode, and the ECU can compensate when running closed loop with the O2 sensor inputs. So the telltale will be the ST and LT fuel trims.

Get a cheap OBD2 reader and torque app and watch your fuel trims. If they're way positive, you probably have an air leak or a bad MAP/MAF sensor. If they're way negative, maybe a sticky throttle plate or bad MAP/MAF sensor, totally clogged air filter (unlikely); or perhaps one or more sticky (open) injectors, but this is less likely with new replacements.
Very easy to screw up modern cars when pulling the intake off, with all the old plastic fiddly bits and gaskets. Could also be compression/plugs but you'd usually see a misfire code if really bad. Our cars seem to be moderately tolerant of cold start open loop "rough" idle short of complete misfire.

Do you know what the CEL code was before you took it in?

I have a BlueDriver OBD2 scanner. I will give you the exact short term and long term fuel trims within the next 24 hours

 

[2015genesisNewbie]

This tells me it's happening only on cold start open loop mode, and the ECU can compensate when running closed loop with the O2 sensor inputs. So the telltale will be the ST and LT fuel trims.

Get a cheap OBD2 reader and torque app and watch your fuel trims. If they're way positive, you probably have an air leak or a bad MAP/MAF sensor. If they're way negative, maybe a sticky throttle plate or bad MAP/MAF sensor, totally clogged air filter (unlikely); or perhaps one or more sticky (open) injectors, but this is less likely with new replacements.
Very easy to screw up modern cars when pulling the intake off, with all the old plastic fiddly bits and gaskets. Could also be compression/plugs but you'd usually see a misfire code if really bad. Our cars seem to be moderately tolerant of cold start open loop "rough" idle short of complete misfire.

Do you know what the CEL code was before you took it in?

You lost me in the beginning with the open loop and closed loop references. Can you please explain that to me.

 

[2015genesisNewbie]

This tells me it's happening only on cold start open loop mode, and the ECU can compensate when running closed loop with the O2 sensor inputs. So the telltale will be the ST and LT fuel trims.

Get a cheap OBD2 reader and torque app and watch your fuel trims. If they're way positive, you probably have an air leak or a bad MAP/MAF sensor. If they're way negative, maybe a sticky throttle plate or bad MAP/MAF sensor, totally clogged air filter (unlikely); or perhaps one or more sticky (open) injectors, but this is less likely with new replacements.
Very easy to screw up modern cars when pulling the intake off, with all the old plastic fiddly bits and gaskets. Could also be compression/plugs but you'd usually see a misfire code if really bad. Our cars seem to be moderately tolerant of cold start open loop "rough" idle short of complete misfire.

Do you know what the CEL code was before you took it in?

The part you mention about easy to screw up modern cars when pulling the intake off, that’s actually what I was thinking is possibly when they replaced the intake manifold gasket maybe they didn’t set it perfectly and it got twisted or pinched and that’s creating some sort of vacuum leak?

I was also thinking the same about the plugs because I guess apparently these cars are known to run rich and because they are GDI I thought maybe it was as simple as worn out or gunked up plugs so I pulled them about 6 months ago and I know looks can be deceiving but they looked fine to me. they are NGK iridium’s from the factory I even checked all the gaps and just ended up reinstalling them. After doing all that work should I have just went ahead and replaced them? Probably. did I? No. stupid? Maybe. But at the time my fuel trims seemed normal and I didn’t have any check engine lights or any codes so that’s what I chose to do.

I guess another possibility is it is the injectors and the dealership never replaced them and just told me they did?

I do not know what the check engine light code was before I took it in but I will try to dig up the paperwork. hopefully it is in my glove box. I’ll see if it’s on there and let you know. Thank you for your response, your help and your replies. I really appreciate it.

 

[Gunkk]

You lost me in the beginning with the open loop and closed loop references. Can you please explain that to me.

When a modern fuel-injected car first starts, the O2 sensors are not used by the ECU to control the air-fuel ratio. The ECU relies only its stored internal tables and other inputs like the MAP/MAF and temperature sensors. This is considered an "open loop" feedback control. Only pre-combustion inputs are used to determine how much fuel to inject into the cylinders. There is no feedback of exhaust gas composition (measured by O2 sensors) after combustion. So the ECU doesn't know if the end result from its stored tables and other inputs is good or not... and it cannot adjust for variations. So if there is a minor problem with too much/little air and/or fuel, the engine may misfire or run rough.

After the normal warmup cycle, the ECU will switch to closed-loop mode and will use the O2 sensors to feedback the oxygen content of the exhaust gasses so that the ECU can optimize the air-fuel ratio (aka "Lambda") for the best fuel efficiency, performance, and emissions. It does this by varying the fuel trims (shown as a positive or negative percentage variation from the standard amount of fuel the ECU should used based on its internal air-fuel mix tables). Once in closed loop, the ECU can see what is happening in the combustion process inside the cylinders (via the O2 sensors) and adjust the amount of fuel (typically) that is injected. This can completely compensate for small problems with the air and fuel system on a cylinder by cylinder basis.

This is why I mentioned fuel trims, as they are one of the first (and biggest) tell tale signs of air and fuel supply issues, whether too much or too little depending on which way the fuel trims are being biased by the ECU in response to off-target combustion.

Here is a reasonably good article that explains it in greater detail:

Open Loop vs Closed Loop: Understanding Fuel Injection

Understanding how open loop vs closed loop modes work in fuel injection — problems they can cause, how to identify them, and how to tune them out.

motofomo.com

 

[2015genesisNewbie]

When a modern fuel-injected car first starts, the O2 sensors are not used by the ECU to control the air-fuel ratio. The ECU relies only its stored internal tables and other inputs like the MAP/MAF and temperature sensors. This is considered an "open loop" feedback control. Only pre-combustion inputs are used to determine how much fuel to inject into the cylinders. There is no feedback of exhaust gas composition (measured by O2 sensors) after combustion. So the ECU doesn't know if the end result from its stored tables and other inputs is good or not... and it cannot adjust for variations. So if there is a minor problem with too much/little air and/or fuel, the engine may misfire or run rough.

After the normal warmup cycle, the ECU will switch to closed-loop mode and will use the O2 sensors to feedback the oxygen content of the exhaust gasses so that the ECU can optimize the air-fuel ratio (aka "Lambda") for the best fuel efficiency, performance, and emissions. It does this by varying the fuel trims (shown as a positive or negative percentage variation from the standard amount of fuel the ECU should used based on its internal air-fuel mix tables). Once in closed loop, the ECU can see what is happening in the combustion process inside the cylinders (via the O2 sensors) and adjust the amount of fuel (typically) that is injected. This can completely compensate for small problems with the air and fuel system on a cylinder by cylinder basis.

This is why I mentioned fuel trims, as they are one of the first (and biggest) tell tale signs of air and fuel supply issues, whether too much or too little depending on which way the fuel trims are being biased by the ECU in response to off-target combustion.

Here is a reasonably good article that explains it in greater detail:

Open Loop vs Closed Loop: Understanding Fuel Injection

Understanding how open loop vs closed loop modes work in fuel injection — problems they can cause, how to identify them, and how to tune them out.

motofomo.com

That makes perfect sense! Thank you for taking the time to give me such a great explanation!

Soooo does that mean if I were to have my scanner plugged in on cold start theoretically my short term fuel trim should be 0.0% (while in open loop mode)?

 

[2015genesisNewbie]

That makes perfect sense! Thank you for taking the time to give me such a great explanation!

Soooo does that mean if I were to have my scanner plugged in on cold start theoretically my short term fuel trim should be 0.0% (while in open loop mode)?

So I tried to look at live data as fast as possible immediately after starting the vehicle (it had been off since 6/29/21 6:50pm up until this point) but I couldn’t catch it in open loop mode…

 

[2015genesisNewbie]

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.

Dropbox

www.dropbox.com

 

[2015genesisNewbie]

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.

 

[Gunkk]

Thanks for the vid. I presume the video starts started shortly after engine startup. Fuel trims settle down to zero at 1 minute... which is very much normal. Anything within a few % of zero is normally just fine.

Yeah a cooked coil could be your rough idle. Coils can be funny with temperature. In the old school days a coil can could work fine when cold then fail when it heated up... just long enough to get you a few miles from home but not to where you needed to be.

I suggest checking the plug as well for any fouling, cracks, etc. as well.

If you were not getting spark intermittently when cold, you would definitely see misfires in the log. And unless the coil cut out completely the CEL probably would not flash. It's a goofy thing with our cars like I mentioned above in post #20. The ECU is doing its best impersonation of Kevin Bacon in Animal House while the city riots around him: "ALL IS WELL!"

If the problem follows the coil, then you have your culprit (or at least one of them). Last resort will be a compression test. A low cylinder might run rough when cold but relatively fine once heated up.

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[2015genesisNewbie]

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

Thanks for the vid. I presume the video starts started shortly after engine startup. Fuel trims settle down to zero at 1 minute... which is very much normal. Anything within a few % of zero is normally just fine.

Yeah a cooked coil could be your rough idle. Coils can be funny with temperature. In the old school days a coil can could work fine when cold then fail when it heated up... just long enough to get you a few miles from home but not to where you needed to be.

I suggest checking the plug as well for any fouling, cracks, etc. as well.

If you were not getting spark intermittently when cold, you would definitely see misfires in the log. And unless the coil cut out completely the CEL probably would not flash. It's a goofy thing with our cars like I mentioned above in post #20. The ECU is doing its best impersonation of Kevin Bacon in Animal House while the city riots around him: "ALL IS WELL!"

If the problem follows the coil, then you have your culprit (or at least one of them). Last resort will be a compression test. A low cylinder might run rough when cold but relatively fine once heated up.

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

 

[Gunkk]

A loose plug could absolutely have done exactly what you experienced: Low compression rough cold start + dirty coil jacket makes complete sense.

Fingers crossed and stay safe in the heat.

 

[2015genesisNewbie]

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

 

[Gunkk]

Excellent! Glad you got it sorted out.

 

[change741]

I am contemplating changing my spark plugs, DIY. 2011 3.8 Genesis. It looks like the intake manifold must be moved out to get at the passenger side plugs. Is that easy to do? I have some experience for small repairs. Should the gaskets between the manifold and engine be replaced at the same time?
Thanks