Engine misfire

[Metro12]

I have a 2019 NBS 5.3 with 20,700 miles.  Yesterday I took it out after it’s sat on a trickle charger inside a garage since October.  I got a shudder at slight acceleration and a check engine light flashed.  Turned it off, noticed reduced power and the shudder/engine light came on again.  Checked under the hood and found a mouse nest under the engine cover.  Towed to dealer.  Dealer called that evening and said it had a code for a single cylinder misfire, they moved the spark plug and then got a misfire on that cylinder, so said the plug need replaced.  They said no wires were chewed, so the mouse didn’t cause it.  Also said the oil cooler line had a leak that would be a warranty repair.  I agreed to replace the plug since I was already on the hook for shop time since it’s not under the power train warranty.  Dealer calls this afternoon and said another cylinder is misfiring and they moved that plug and the misfire followed and now they’re pushing for a $699 “tune up” to replace plugs and wires.  I told them no.  Plugs, wires, brakes, rotors, oil and filters are within my capabilities and work I’ve done in the past, it’s $100 for me to cuss and scream for an hour in my garage and do it myself.  What I’ve never done in the past is replace plugs at 20k miles, nor have I had a plug fail and 12hrs later have another one fail.  I told the dealer I’m concerned something else is wrong and they’re just replacing plugs.

 

Any ideas on something else that might legit be wrong, or are they just trying for a $699 spark plug job?

[asilverblazer]

17 hours ago, Metro12 said:

Dealer called that evening and said it had a code for a single cylinder misfire, they moved the spark plug and then got a misfire on that cylinder, so said the plug need replaced... Dealer calls this afternoon and said another cylinder is misfiring and they moved that plug and the misfire followed...  What I’ve never done in the past is replace plugs at 20k miles, nor have I had a plug fail and 12hrs later have another one fail.  I told the dealer I’m concerned something else is wrong and they’re just replacing plugs.

I don't disagree with their diagnosis but I agree with your concerns too. 

 

All you can do is check the codes and inspect the plugs yourself to confirm. 

[Metro12]

Called GM CC, and expressed my concern that two plugs went bad at 20,700 miles, 70,000 miles ahead of the service schedule.  
 

wait and see

[newdude]

Truck sit a lot?  Do you start it when its sitting or do you just leave it on the trickle and not start it?

[Metro12]

Sat since late October but I started probably every other week, pulled it out and ran it for 15-20min 

[newdude]

1 hour ago, Metro12 said:

Sat since late October but I started probably every other week, pulled it out and ran it for 15-20min 

 

 

That's likely the problem and why you've got one or more plug bad.  If it sits for long periods of time, don't start it until you are planning to drive it.  Floods the cylinders on these DI engines and fouled the plugs.  The Corvettes are notorious for this.  We've had 3 different C8s in for it.  Owners started them in storage.  Come off the tow truck running on 3 or 4 cylinders.

 

If you are thinking of engine lubrication purposes, if you really must, put your foot to the floor with the gas pedal to enter clear flood mode.  This will crank the engine without firing the fuel injectors.  

 

 

 

 

Edited February 7, 2024 by newdude

[asilverblazer]

34 minutes ago, newdude said:

Floods the cylinders on these DI engines and fouled the plugs.

 

If you are thinking of engine lubrication purposes, if you really must, put your foot to the floor with the gas pedal to enter clear flood mode.  This will crank the engine without firing the fuel injectors.

I wonder why this is happening?

 

Why the need to enter "clear flood mode"? Once the engine has fired and ran any amount of time the extra fuel from a flooded engine should have been cleared.

 

I would think if it is so rich to cause these significant of miss fires it would kill the catalytic converter too.

[newdude]

1 hour ago, asilverblazer said:

I wonder why this is happening?

 

Why the need to enter "clear flood mode"? Once the engine has fired and ran any amount of time the extra fuel from a flooded engine should have been cleared.

 

I would think if it is so rich to cause these significant of miss fires it would kill the catalytic converter too.

 

 

In storage sense, clear flood would allow the engine to crank and circulate oil without actually firing the engine.  

[asilverblazer]

I see what you're getting at...

 

...I thought you were indicating the engines were flooding.

[Metro12]

So, if not driving it once every week, either let it sit on the trickle charge and don’t start, or start and rev the engine WOT? 

[Bikerjon]

1 hour ago, Metro12 said:

So, if not driving it once every week, either let it sit on the trickle charge and don’t start, or start and rev the engine WOT? 

No. newdude is advising leave it sit on trickle or do not start it but crank the engine over at WOT or gas pedal pinned to the floor. Gas pedal pressed all the way down turns off the fuel injection, (engine will not start). This will allow the oil to circulate without starting and flooding the cylinders.

[Metro12]

9 minutes ago, Bikerjon said:

No. newdude is advising leave it sit on trickle or do not start it but crank the engine over at WOT or gas pedal pinned to the floor. Gas pedal pressed all the way down turns off the fuel injection, (engine will not start). This will allow the oil to circulate without starting and flooding the cylinders.

Ahhh, ok.  I was unaware such a feature existed!

Truck ran fine for the ride home from the stealership.  They replaced the second spark plug for no charge

 

Anything available to try and clean out the gunk that’s built up?

[asilverblazer]

I think the best solution is to drive it once a week. Long enough to get up to operating temperature and burn off any condensation that builds up (prevent sludging the oil). This would prevent any build of "gunk" too.

[Gangly]

Fuel stabilizer might help as well.  I put Stabil in darn near anything that is gonna sit dormant for a month or more.

[ember1205]

30 minutes ago, Gangly said:

Fuel stabilizer might help as well.  I put Stabil in darn near anything that is gonna sit dormant for a month or more.

 

Stabilizer may help to prevent the issue from getting worse, but I don't believe it will help to clear any buildup that's already there. A fuel system cleaner would be more adept at that. One could even add a heavy dose of SeaFoam to the gas. I've use SeaFoam in small engines to clear buildup in the carbs by putting a few oz in a half tank of gas, running it for five minutes, then shutting it down to sit overnight. After that, I run the rest of the tank dry and continue to add 1oz per tank going forward for a few more tanks of fuel. Seems to help substantially.