2006 yukon xl 1500, 4th cylinder misfire

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rockola1971

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P0300 is just a generic misfire code. It will update soon with which cylinder(s) are misfiring. Id swap #4 and #2 spark plug wire and spark plug just to rule them out too.
 
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AlaskaEd

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Depending on specific codes, it can take X number of drive cycles and/or x number of miles before enough errors occur for the CEL to light up on the dash.

If you have a Tech 2, great! If not, you may still be able to dive in deeper into the computer to see what pending codes might be collecting in there.
Tech 2 will certainly get into the pending data that might be stored and can also show the misfire counts in the live data.
If you plan on keeping your Yukon a while, do yourself a favor now and invest in a Tech 2 scan tool. Clones are about $300, but the information they can give you is off the charts!

Another option, if you need something for other vehicles, a good bi-directional scan tool like an Autel tablet would be the way to go. Better ones run $800+, but can do a lot of the stuff the Tech 2 can.

However, there are specific directions with detailed steps written into the field service manuals. Those instructions in the FSM are specific guides on using the Tech 2 to do all the mechanical, electrical and functional diagnostics for the GM mechanics. (Not to mention... all the module testing and programming that the Tech 2 can do, that others can't...)

Not an official plug for Tech 2... Just wanted to mention that you might want to dive in deeper to the stored data and live data to see if there are things happening that exist, but not enough to trip the CEL on the dash just yet. You may have misfires still on 4 and just haven't been told about it.

Keep us posted. Good luck!
Not always, but sometimes... A camshaft position sensor can cause a misfire that stays with the same cylinder.
P0300 is just a generic misfire code. It will update soon with which cylinder(s) are misfiring. Id swap #4 and #2 spark plug wire and spark plug just to rule them out too
P0300 is just a generic misfire code. It will update soon with which cylinder(s) are misfiring. Id swap #4 and #2 spark plug wire and spark plug just to rule them out too.
So after I swapped the coil and wire from cylinder four to two and then that cylinder started misfiring too, I decided to replace the coil and wire that were originally on 4, but I left the new one on 2 because of the swap from four. The P0300 code is gone, but the P0304 code is back. Not sure what’s happening with the fourth cylinder still. It has the coil and wire from 2 which I had no problem with, and I put a new spark plug in (even though I had just recently replaced that plug.) When I pulled the plug from four I also pulled the plug from 2. They were installed at the same time, but the four looks fouled. Picture below. Maybe the coil on four isn’t even getting power to send a spark? 7BAE98CC-4C42-4E59-812B-5BCF8415C19C.jpeg
 

MassHoe04

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Where is all that grease from?
Maybe the grease is preventing proper grounding of the plug in that location?

Maybe those Bosch plugs aren't playing nice with the electricicals...
It is possible the Bosch is giving too much resistance for what the coils and wires are feeding them.

Cheap enough to try AC Delco plugs (properly gapped, of course) in those two cylinders to see if that makes any difference.
 

S33k3r

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I've heard nothing but bad things from running anything but AC Delco, with maybe the exception of NGK. Bosch, specifically, has been hated in several of the GMT800 groups of which I am a member. It is not that Bosch sucks, it is that GMT800s don't seem to like them.
 
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AlaskaEd

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Where is all that grease from?
Maybe the grease is preventing proper grounding of the plug in that location?

Maybe those Bosch plugs aren't playing nice with the electricicals...
It is possible the Bosch is giving too much resistance for what the coils and wires are feeding them.

Cheap enough to try AC Delco plugs (properly gapped, of course) in those two cylinders to see if that makes any difference.
I have anti-seize on the sparkplug threads and dielectric on the tip. I could try an ac delco for four, the original plug I pulled out of it when the code started presenting had been firing but the gap was ridiculously large. I put in a bosch put it looks different than its neighboring cylinder in two. I’ll show a picture comparing cylinder two and four bosch plugs, four is brown.
 

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MassHoe04

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I have anti-seize on the sparkplug threads and dielectric on the tip. I could try an ac delco for four, the original plug I pulled out of it when the code started presenting had been firing but the gap was ridiculously large. I put in a bosch put it looks different than its neighboring cylinder in two. I’ll show a picture comparing cylinder two and four bosch plugs, four is brown.
Try getting all the Dielectric out of the boot with a spray solvent (electric contact cleaner or Brake Kleen). You want direct contact in the boot between the wire connector and the plug.

Dielectric grease used to be the old-school solution to a lot of things back in the days of my father or grandfather. But that was before cars had computers and all kinds of sensors.

I had thought Dielectric grease would help when I had my 05 Wrangler that was running rough. The dielectric grease actually interfering with the current from the wire to the plug. I cleaned it out and that made a huge difference.

You mentioned new wires, so the boots should be able to keep water out just fine without the dielectric grease mucking up the connection.
 
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AlaskaEd

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I've heard nothing but bad things from running anything but AC Delco, with maybe the exception of NGK. Bosch, specifically, has been hated in several of the GMT800 groups of which I am a member. It is not that Bosch sucks, it is that GMT800s don't seem to like
Try getting all the Dielectric out of the boot with a spray solvent (electric contact cleaner or Brake Kleen). You want direct contact in the boot between the wire connector and the plug.

Dielectric grease used to be the old-school solution to a lot of things back in the days of my father or grandfather. But that was before cars had computers and all kinds of sensors.

I had thought Dielectric grease would help when I had my 05 Wrangler that was running rough. The dielectric grease actually interfering with the current from the wire to the plug. I cleaned it out and that made a huge difference.

You mentioned new wires, so the boots should be able to keep water out just fine without the dielectric grease mucking up the connection.
Dang man, I did not know that. I’ll clean the boot up and see how that goes. Thanks.
 

MassHoe04

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Heat actually dried out the dielectric grease on the Jeep. I was like paraffin wax and cakey. Took a bit of effort to get it out of the rubber boot on the spark plug wire.
 

rockola1971

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I would suspect the Spark Plug Wire(s) are the culprit and/or the dielectric grease in them as MassHoe04 mentioned. That #4 has a problem. I would also be looking down in that cylinder to make sure there is no obvious funny business going on down in there. Exhaust smell "sweet"? See any green or orange/pink on top of piston crown? See any marks are the crown that arent suppose to be there? The spark plug on #4 doesnt appear to be covered by oil ring blow by.
 

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