Diagnosing spark plugs

[alvocado]

I'm trying to diagnose a rough idle and would like some input from those who've chased down similar issues. I ran an STP Pro Series intake cleaner and two bottles of B12 in the fuel tank to clean up valves and fuel injectors. The idle is still a little rough with some consistent missing. Drive RPM seems fine with no noticeable issues and no codes. I pulled cylinders 1 and 3 plugs which have 55k on them, OEM AC Delco and the electrodes appear fine. There's a little carbon on the bases so any input on whether or not this is an indicator of minor carbon fouling and potentially a leaky injector would be appreciated. The truck has 155k miles, no issues in 10 years and no oil consumption. 

[Ian Mills]

Why did you show 1 and 3, were they different than the others?

Those plugs don't look too shockingly bad to me. Carbon fouling that would have a noticeable effect on your idle is usually up on the strap or the electrode. The color of the strap looks ok to me too. Have you inspected or replaced your spark plug wires recently?

Also, LS motors are known to have a light misfire at idle. It might just be the way it is.

[alvocado]

1 and 7 would show oil fouling if the AFM solenoid was passing oil so I figured I'd start with one of those and check a nearby for comparison. The wires are original. 

[Ian Mills]

I see your thinking. I really don't think the condition of those two plugs would cause or result in a misfire. At 55k you could replace them for maintenance purposes. I would consider doing the wires too, they could have bad internal connections.

 

Have you checked other stuff like a vacuum leak or a dirty throttle body?

With no codes though, it's possible that it's just how the engine wants to idle. 

[diyer2]

Simple test is to raise the hood when its dark and look for spark jumping. Otherwise coils or wires?

Edited April 30, 2021 by diyer2