p0300 on highway only

[csk334]

I got a 2017 SIlverado 5.3L V8. Got it in January, and around April or so I had a misfire. Brought it to dealer and they switched coils around and stopped misfiring. Misfired again month or so later so got all my plugs replaced. Didnt work and also got CEL for EVAP so replaced all the EVAP stuff and it misfired again few weeks later. Threw Sea Foam through it and into the fuel as well. Misfired again few days later. Its popping up as a random misfire and it only happens on the highway (around 70 MPH or so). Not sure where to look to fix next. Bringing it back to dealer but also dont want them to say they cant recreate the misfire and dont know what to do. Was thinking maybe bad wire, coil, o2 sensor, maf, or crankshaft sensor. I guess it could be fuel injector too but not sure. Anyone else have this kind of misfire wild goose chase?

[rav3]

What was the new code? P0300 is a misfire but does say what cylinder. You really need to see the does code(s) otherwise your chasing a ghost, IMO. Just buy a cheap bluetooth OBDII dongle (see link below) & plug into the OBDII socket at the bottom of the dash. Download "TORQUE (lite) app to you phone, connect to OBDII & read your code(s) & log them. I'd clear codes & see what comes back. That's a start for now. Report back & someone will guide you to a possible fix. At this point you don't have enough information to know what & where to check.

How many miles on your truck? Do you have any maintenance records?

 

ELM327 OBD2 V2.1 Bluetooth Car Diagnostic Scanner Android Torque Auto Scan Tool

[It's Tim]

1 hour ago, csk334 said:

Brought it to dealer and they switched coils around and stopped misfiring. 

Probably time to find another dealer. 

[CrawlSlow]

Make sure your gas cap is tight. Try another gas cap? 

Edited September 3, 2021 by CrawlSlow

[JimCost2014]

17's do not have gas caps, they have the capless system.

[MikeBMW]

39 minutes ago, JimCost2014 said:

17's do not have gas caps, they have the capless system.

True, but when I first bought my truck, first thing they had to do was repair the capless tank flap.
Those can go bad, too.

[JimCost2014]

11 minutes ago, MikeBMW said:

True, but when I first bought my truck, first thing they had to do was repair the capless tank flap.
Those can go bad, too.

True, just part of the joys of buying used, not knowing the full history.

[MikeBMW]

19 hours ago, JimCost2014 said:

True, just part of the joys of buying used, not knowing the full history.

And, exactly why I bought the 5 year extended warranty.
Still deciding if that was a good deal as I haven't had to use it, yet.
But, I do get free oil changes for life! woohoo! (I think)

Edited September 3, 2021 by MikeBMW

[JimCost2014]

13 minutes ago, MikeBMW said:

Still deciding if that was a good deal as I haven't had to use it, yet.

That is never a bad thing, most warranties will buy out the balance (prorated) if you want out because you don't think you will ever use it. 

 

New or used, you just never know, this my 3rd new K2, with the first 2 having tons of problems.

 

100 has been a really great truck so far.

Edited September 3, 2021 by JimCost2014