2018 P0300:00

JamesZ · March 5

Hey y'all.

Searching for a reason and it lead me to this forum.

Cool to find, as maybe it'll help with my job.

I have a 2018 Chevy with a 5.3L and a misfire on #8 cylinder.

Coil is good, wire is good, injector is good, compression 170,

Swapped #2 plug for #8 (because I didn't have a new set of plugs), still #8 misfire.

Had another tech look at it, cleaned the plug and problem solved.

How the F?

If it was a bad plug, wouldn't it have followed to #2?

#8 plug works fine in #2.

I'm putting new plugs in today, all 8. Just bothers me that the problem didn't follow the plug swap.

rav3 · March 5

That is very strange. How many miles on ur truck. Any previous work done?

asilverblazer · March 5

1 hour ago, JamesZ said:

Hey y'all.

Searching for a reason and it lead me to this forum.

Cool to find, as maybe it'll help with my job.

I have a 2018 Chevy with a 5.3L and a misfire on #8 cylinder.

Coil is good, wire is good, injector is good, compression 170,

Swapped #2 plug for #8 (because I didn't have a new set of plugs), still #8 misfire.

Had another tech look at it, cleaned the plug and problem solved.

How the F?

If it was a bad plug, wouldn't it have followed to #2?

#8 plug works fine in #2.

I'm putting new plugs in today, all 8. Just bothers me that the problem didn't follow the plug swap.

Maybe the wire wasn't on good, good be the terminal in the wire is loose.

Maybe there was "enough" build up on the threads the plug wasn't grounding well.

Without more information how you came to the conclusion - I'd bet the wire wasn't as good as you say.

JamesZ · March 5

1 hour ago, asilverblazer said:

Maybe the wire wasn't on good, good be the terminal in the wire is loose.

Maybe there was "enough" build up on the threads the plug wasn't grounding well.

Without more information how you came to the conclusion - I'd bet the wire wasn't as good as you say.

Well, I'd imagine maybe. But two wires, two coils, swapped plugs, removed plug and reinstalled four or five times.

I think that would address those issues.

Once? Sure. But several times? Nah.

Idk, but I have eight new plugs going in. Truck runs smooth again. Just baffles me how after all that, the issue didn't change.

JamesZ · March 5

Wow. I'm a dumbass.

Working on a 351W, had that stuck in my mind. Firing order, number 8 is left rear. LS number 8 is right rear.

I'm tarded now.

mikeyk101 · March 6

5 hours ago, JamesZ said:

Wow. I'm a dumbass.

Working on a 351W, had that stuck in my mind. Firing order, number 8 is left rear. LS number 8 is right rear.

I'm tarded now.

No harm, no foul. A simple mistake that I'm sure a lot of us could make. At least you caught it. Keep the thread updated.

asilverblazer · March 6

23 hours ago, JamesZ said:

...But two wires, two coils, swapped plugs, removed plug and reinstalled four or five times...

On 3/5/2026 at 8:19 AM, JamesZ said:

Swapped #2 plug for #8... still #8 misfire... cleaned the plug and problem solved... #8 plug works fine in #2.

From this, the problem was located at cylinder 8, the only component that moved cylinders was the plug.

From the original post, the wire at cylinder 8 doesn't appear to have moved locations. The second post indicates maybe the wires were moved from cylinder to cylinder? I don't see any indication of how many times the wire at cylinder 8 was 'manipulated'. If there was some corrosion on the wire, it might have needed some cleaning on the terminal that was accomplished by scraping it off during the removal-reinstallation.

I'm not arguing, but like you, I suspect the 'real' culprit may not be the plug and the condition may return.

See post below, after reading the latest update.

Edited March 6 by asilverblazer
Read later post in thread.

asilverblazer · March 6

20 hours ago, JamesZ said:

Wow. I'm a dumbass.

Working on a 351W, had that stuck in my mind. Firing order, number 8 is left rear. LS number 8 is right rear.

I'm tarded now.

Am I understanding this correctly that you were working on the wrong cylinder and the second person worked on the correct one? Meaning, you never swapped the #8 components in the first place?

In which case, I retract my previous post, and it very well could be just the one bad plug at cylinder 8 (the right one).

Again, not trying to rub it in, just understand the process.

JamesZ · March 6

1 hour ago, asilverblazer said:

Am I understanding this correctly that you were working on the wrong cylinder and the second person worked on the correct one? Meaning, you never swapped the #8 components in the first place?

In which case, I retract my previous post, and it very well could be just the one bad plug at cylinder 8 (the right one).

Again, not trying to rub it in, just understand the process.

Exactly. I'm tired. #8 plug was worn out and foul. Eight new plugs and IT IS ALIVE!

JamesZ · March 6

These truck sit and idle a lot. Most run all day, whether driven or not. Guys don't pay fuel, so they run. Some get forgotten and are still idling the next day. Had an owners truck lose an engine with less than 80,000km on the truck. Dealership warranty the engine but said the idle time, if it was driving time, would have put the truck over 300,000km.

This truck we're speaking of has 136,000 km on it. New transmission last year. New injectors last year also. New starter last week.