Cold start rough idle.

[Denalibro76]

Afternoon all. My L9H has a cold start issue. I’ll start it up, it fires then it seems like it stalls for a half second then fires up again. When it does it idles rough accompanied by rich smelling exhaust and a bit of smoke out the tail pipe. This lasts for around a minute or two, then smooths out. Codes for misfire and O2 sensor bank two sensor two stuck rich. I thought it could be a leaking injector(s) so I pulled them, bench tested and cleaned. No leakers were found. Wondering what I should check next? My plan was to check the plugs and replace them.

 

[opfor2]

Are you using E85 fuel?

 

[Fless]

Just a thought... the evap purge valve on the engine can stick open -- intermittently -- and cause a rich start.

Lots of tests that could be done, maybe an injector balance test. Any misfire counts for individual cylinders?

What do the fuel trims look like?

Have you monitored fuel pressure when it acts up?

 

[Foggy]

Don't know what year you have, etc
But try a fuel trim re learn/reset procedure esp if it is flex fuel
and maybe try a higher grade of gasoline as well

 

[Denalibro76]

Are you using E85 fuel?

Premium (91)

 

[Denalibro76]

Don't know what year you have, etc
But try a fuel trim re learn/reset procedure esp if it is flex fuel
and maybe try a higher grade of gasoline as well

I’m running 91. It’s an 09’ Yukon Denali. How do you do a reset/relearn? Apologies if this is a dumb question…

 

[Fless]

Some of these NNBS fuel systems that have the virtual alcohol sensor for E85 can mis-calculate. Sometimes it's a function of weak or worn O2 sensors, but you can check what the PCM thinks the alcohol is in the tank. If you're running E0 or E10, then the alcohol reading should be low. But if the reading is high, the PCM is going to command too much fuel into the combustion chamber and make everything rich. If that happens, generally the engine will run like poo all the time, but YMMV.

 

[Foggy]

I’m running 91. It’s an 09’ Yukon Denali. How do you do a reset/relearn? Apologies if this is a dumb question…

Bi Directional Scanner. You will be able to look at fuel trims as well as alcohol fuel %
These flex fuel engines use "virtual" info calculated by the O2 sensors... and it sucks !!
They get out of whack sometimes and cause all sorts of weird issues.
I disable the flex fuel on the tunes I do... But Can't run E85 anymore.. But E85
isn't really common around me, so I never run it anyway

 

[Denalibro76]

Bi Directional Scanner. You will be able to look at fuel trims as well as alcohol fuel %
These flex fuel engines use "virtual" info calculated by the O2 sensors... and it sucks !!
They get out of whack sometimes and cause all sorts of weird issues.
I disable the flex fuel on the tunes I do... But Can't run E85 anymore.. But E85
isn't really common around me, so I never run it anyway

Just an update, I used a good Mac Tools scanner and my Alcohol % was sitting at 30%!! Reset it and all is good now. Started up just fine this morning. Will scan it again later and see where it’s sitting. Looks like that was the issue.

 

[Foggy]

Just an update, I used a good Mac Tools scanner and my Alcohol % was sitting at 30%!! Reset it and all is good now. Started up just fine this morning. Will scan it again later and see where it’s sitting. Looks like that was the issue.

If it gets out of whack again , it's time for new upstream oxygen sensors.
Get the GM ones.. The aftermarket on our gen iv engines are ******