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Sooty Tailpipe and (Very) Mild On-Idle Stumble


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So, let me preface this with... I'm probably worrying about nothing here, but that's what I do.

 

My 15 5.3 has developed a mild stumble at idle in the last 15-20k miles.  It kind of feels like a misfire, but not that intense.  The best way I can describe it would be 'bumpy'.  I've only started to care about since it's been getting worse.  In one of my last visits, the dealer did an induction service (supposedly), but my throttle body wasn't particularly clean when I removed the tube a few weeks later.  It wasn't dirty, but not spotless.  Also, in the last couple weeks it's gotten bad enough to happen at every stoplight and the intensity is increasingly perceptible in the tach.

 

Also, my the inside of my exhaust pipe is covered in soot.  I've never noticed any smoke, but I haven't monitored my oil levels enough to know if I'm actively burning it.  

 

  • 2015 5.3L Gas (not FF)
  • 65k miles or so
  • I've run 93 octane exclusively for over a year due to persistent load-induced knocking on 87 and occasional knocking on 91
  • I have an AFE dry-flow filter in the stock box
  • I have the Airraid MIT
  • I have some damage to the woven exhaust coupler after the catalytic converter.  It's not shown any signs of significant leakage, but something o/r banged it up a bit.  
  • Stock Tune (afaik), but the pinging on 87 has made me wonder.  Honestly, if this is the engine after a tuner they should have gotten their money back.  

 

At lunch, I fired up TorquePro and look a look at my fuel trims as I drove back to the office.  The LTFT peaked at -7ish (both banks)  but it didn't stay there long.  The STFT moved around a lot but rarely deviated from -8, +8.  I can do some logging if that might help.

 

My theories that I hope to work through are:

  1. Dirty Air Filter
  2. Excessive PCV flow/buildup.  I have a can of CRC intake valve cleaner and I might break down and buy a catch can... I dunno.
  3. Oil Burning Unlikely based on never seeing smoke, but I may pull a few plugs and see what they look like.
  4. EGR sticking open a bit
  5. O2 Sensors

 

Can you think of anything else I should look at or consider?

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Also, my the inside of my exhaust pipe is covered in soot.

 

Just a FYI this is normal on most direct injection motors from all manufacturers. 

 

This article from Car & Driver explains it better than I can.

https://www.caranddriver.com/features/a22974104/black-exhaust-pipes-car-direct-injection/

 

Some guys report pinging even with bone stock trucks running 87. Most engine tuners say 87 is to low with the compression ratio and amount of spark advance/retard and recommend running at least 91. With that said pinging with 91 would be bothersome to me.

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I can save you time a post your next question/issue. X banks running rich intake system cleaned. Now I need 4 new o2 sensors and all new y pipe with cats. Or as I call it second side of issues cylinder x misfire. After buying all thatbiff you still had a issue injector and lines. 

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2 hours ago, 1SLOW1500 said:

I can save you time a post your next question/issue. X banks running rich intake system cleaned. Now I need 4 new o2 sensors and all new y pipe with cats. Or as I call it second side of issues cylinder x misfire. After buying all thatbiff you still had a issue injector and lines. 

Um... Thanks... What happened?  Did the intake cleaning mess up the exhaust system?

 

 

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Um... Thanks... What happened?  Did the intake cleaning mess up the exhaust system?
 
 
I'm guessing debris and oil build as per the gm bulletin. And that is what they performed the "clean" however that clean is know to be super hard on the cats and even burn them out. Also a gm doc on that. But as most people get it around the 60-70k mile mark gm is just getting you out of the warranty. Once the o2 command more fuel be it the down stream do to the cats performance or ups do to bad read. You run richer. I say richer because the di engines run rich already. Common other issues are missing and for the dollar we just swap out the coil wires and plugs but also it can be injector. I'm seeing lots of injector issues as of late.

Sent from my SM-N960U using Tapatalk

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6 minutes ago, 1SLOW1500 said:

I'm guessing debris and oil build as per the gm bulletin. And that is what they performed the "clean" however that clean is know to be super hard on the cats and even burn them out. Also a gm doc on that. But as most people get it around the 60-70k mile mark gm is just getting you out of the warranty. Once the o2 command more fuel be it the down stream do to the cats performance or ups do to bad read. You run richer. I say richer because the di engines run rich already. Common other issues are missing and for the dollar we just swap out the coil wires and plugs but also it can be injector. I'm seeing lots of injector issues as of late.

Sent from my SM-N960U using Tapatalk
 

The number one reason I am running the dual catch cans and running Techron Injector cleaner (only at oil change time) on mine. Hopefully can avoid ever using the intake cleaning chemicals. Will bore scope at 20K to take a look.

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24 minutes ago, 1SLOW1500 said:

I'm guessing debris and oil build as per the gm bulletin. And that is what they performed the "clean" however that clean is know to be super hard on the cats and even burn them out. Also a gm doc on that. But as most people get it around the 60-70k mile mark gm is just getting you out of the warranty. Once the o2 command more fuel be it the down stream do to the cats performance or ups do to bad read. You run richer. I say richer because the di engines run rich already. Common other issues are missing and for the dollar we just swap out the coil wires and plugs but also it can be injector. I'm seeing lots of injector issues as of late.

Sent from my SM-N960U using Tapatalk
 

So, then what should I do here?  Should I try to run the CRC cleaner through the intake or what?  I don't want to destroy my cats and O2 sensors.  

 

Geoff 

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This thread is why I run E85 in the spring through fall.

Yes slightly lower MPGs, but absolutely Clean Tailpipe, and my butt dyno says thumbs up! :thumbs:

I know you non-flex fuel guys cannot run it without a conversion.

These LXX engines tend to run rich like 1Slow mentioned.

You can see evidence of this on cold startup with a puff of black sootty matter exiting the tail pipe.

 

 

Edited by Sierra Dan
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2 minutes ago, Sierra Dan said:

This thread is why I run E85 in the spring through fall.

Yes slightly lower MPGs, but absolutely Clean Tailpipe, and my butt dyno says thumbs up! :thumbs:

I know you non-flex fuel guys cannot run it without a conversion.

These LXX engines tend to run rich like 1Slow mentioned.

You can see evidence of this on cold startup with a puff of black sootty matter exiting the tail pipe.

 

 

I wish I could run E85...I'd do the conversion but the closest E85 is about 160 miles away.

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Fortunately, we have about 5 stations around here.

I will be switching over probably in the next few weeks once temps reach the 50's

E85 and winter do not mix well.

One other note in my experience with E85.....   the truck also shifts like butter.

 

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So, then what should I do here?  Should I try to run the CRC cleaner through the intake or what?  I don't want to destroy my cats and O2 sensors.  
 
Geoff 
Not a ton you can do but be aware of the known issues and replace parts and service accordingly. I have run seafoam SLOWLY in the running engine, in gas tank,and even in oil before a change. If you can scan and see fuel trims or o2 sensors then you would know to replace them. I do believe intake and valve build up is bad in there at this point.
Catch cans are a must and gm as well as the Gov know this. But on the cost, and up keep it just doesnt happen. But Google search and high end race shop and look at there builds catch cans and breather tanks one every car and truck.
If you get to the point of needing a injector you have to buy lines also and the intake comes off. Clean it off truck but please take the time then to look inside the intake ports. It will make you sick to your stomach the aluminum is trashed. It will never just clean up.

Sent from my SM-N960U using Tapatalk

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26 minutes ago, diyer2 said:

but I haven't monitored my oil levels enough to know if I'm actively burning it.  

You don't check your oil regularly?

:)

Honestly, no... I took for granted that a vehicle this new wouldn't have a problem.

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5 minutes ago, 1SLOW1500 said:

Not a ton you can do but be aware of the known issues and replace parts and service accordingly. I have run seafoam SLOWLY in the running engine, in gas tank,and even in oil before a change. If you can scan and see fuel trims or o2 sensors then you would know to replace them. I do believe intake and valve build up is bad in there at this point.
Catch cans are a must and gm as well as the Gov know this. But on the cost, and up keep it just doesnt happen. But Google search and high end race shop and look at there builds catch cans and breather tanks one every car and truck.
If you get to the point of needing a injector you have to buy lines also and the intake comes off. Clean it off truck but please take the time then to look inside the intake ports. It will make you sick to your stomach the aluminum is trashed. It will never just clean up.

Sent from my SM-N960U using Tapatalk
 

Okay, well, I guess I can start with the chemical and see what that does.  I hope it doesn't destroy the truck though... Geeze.  I never bought into the hype about catch cans, but I get the logic.  I'll look into one of those too.

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1 minute ago, papageoff said:

Okay, well, I guess I can start with the chemical and see what that does.  I hope it doesn't destroy the truck though... Geeze.  I never bought into the hype about catch cans, but I get the logic.  I'll look into one of those too.

NOT saying they (catch cans) are the end all cure all, but they are being used for a reason...here are some dirty valves and the picture of the oil in the glass is what my catch can caught after just 100 miles of easy driving...multiply that by 50 times and that would be what gets sucked down your intake and through your valves every 5000 miles which is the average oil change interval...nasty stuff.

Carbon-Buildup.jpg

Carbon-Buildup1.jpg

100.jpg

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