The Dreaded P0300 Code: Solved

[tooleyondeck]

2003 Yukon XL Denali LQ4 VIN "U", 209K, stock tune.

Took it to the store yesterday and noticed a misfire on the way home at around 1K RPM then got the flashing CEL. Eased it on home, hooked up VCM Scanner, and pulled the code: P0300 Random. Since I didn't have a specific cylinder code, I cranked it back up and started hitting the manifolds with an infrared thermometer. I couldn't get a discernable difference in temp, likely due to the engine being warm so I decided to wait until today to reassess as it will just be sitting in the driveway until then. It did not misfire on the next startup after I got back home.

Now, before I start blasting this thing with the parts cannon, I want to go through everything that I can to narrow down the issue. The plan for this evening is to:

- check spark plugs
- check injectors
- check grounds
- check MAF (clean if necessary)
- check TB (clean if necessary)
- check all associated connections and harnesses

What else am I missing? I want to be as thorough as possible and I know a lot of us have had this code and it ended up being from different causes. PFA. Thanks

 

[Fless]

I would recommend checking fuel pressure by the book procedure (at the rail, KOEO). Also, see if your scanner will read individual cylinder misfires, or beg or borrow one that can.

Has the 200k maintenance been done (spark plugs, etc.)?

 

[tooleyondeck]

I would recommend checking fuel pressure by the book procedure (at the rail, KOEO). Also, see if your scanner will read individual cylinder misfires, or beg or borrow one that can.

Has the 200k maintenance been done (spark plugs, etc.)?

Good call, if spark/air seem good I'll check fuel pressure to rule out the pump.

I'm pretty sure all HP Tuners can do is manually disable each cylinder to see if the misfire gets worse and help narrow it down that way (if it is a single cylinder). I have not replace the plugs/wires yet and I am hoping that's all it is.

 

[rockola1971]

Yes. First thing I would do it check to see which cylinder(s) are actually misfiring. Could be a bad knock sensor. Could be bad plug wire or ignition coil. Could be ALOT of things but you will need to know which cylinder(s) are the problem children, otherwise you are just guessing.

 

[justirv]

2003 Yukon XL Denali LQ4 VIN "U", 209K, stock tune.

Took it to the store yesterday and noticed a misfire on the way home at around 1K RPM then got the flashing CEL. Eased it on home, hooked up VCM Scanner, and pulled the code: P0300 Random. Since I didn't have a specific cylinder code, I cranked it back up and started hitting the manifolds with an infrared thermometer. I couldn't get a discernable difference in temp, likely due to the engine being warm so I decided to wait until today to reassess as it will just be sitting in the driveway until then. It did not misfire on the next startup after I got back home.

Now, before I start blasting this thing with the parts cannon, I want to go through everything that I can to narrow down the issue. The plan for this evening is to:

- check spark plugs
- check injectors
- check grounds
- check MAF (clean if necessary)
- check TB (clean if necessary)
- check all associated connections and harnesses

What else am I missing? I want to be as thorough as possible and I know a lot of us have had this code and it ended up being from different causes. PFA. Thanks

View attachment 433526

I have a 2002 Tahoe Z71. Same issue, but only when "cold". When engine warmed up, the problem went away temporarily. I found my issue was the Intake Manifold gaskets. They dry up/shrink and start leaking (drawing in unfiltered air) making your mixture lean. If you find this is your problem, might want to replace the knock sensors too since they'll be exposed.

 

[Scottydoggs]

if you go out and floor it wot and the cel starts flashing, with your miles id be checking the temp of the cats. clogged cats cause the misfires, (or you need a tune up super bad) only code you'll see is the p0300 and thats only if you scan it running still after the event. restarting the engine generally clears a pending code. those are the only two things that can make the cel flash.

so keep the parts cannon at bay. might just need a tune up, or worst case new cats. you could likely get away with cutting them off, deleting a few codes and be golden. iirc theres no inspection in FL?

 

[tooleyondeck]

so keep the parts cannon at bay. might just need a tune up, or worst case new cats. you could likely get away with cutting them off, deleting a few codes and be golden. iirc theres no inspection in FL?

No emissions inspections here (thank God). I would have put LTs on it long ago but my wife drives this mostly and it’s already pretty loud w/ the Jones Performance single chambers on it.

If you find this is your problem, might want to replace the knock sensors too since they'll be exposed.

I tuned them out after I dropped $225 on new AC Delco and the code immediately came back lol. Some say you need them, some don’t. I see it as I’m not going to cam or do any type of power adders to it.

 

[tooleyondeck]

Okay, so I got home from work and got to it. I tried an old trick I picked up and crunk the engine for about 15 sec then shut it off. I touched each each exhaust flange on the manifolds and found that cylinder 8 was close to ambient temp to the touch. I started there. I went to remove the plug wire and it immediately broke off and I did not pull it by the wire. I went ahead and removed them all.



I pulled the plugs and they weren’t the worst I’ve seen, but replacements were definitely in order. The plugs were AC Delco/NGK 12567759 Iridium.



I went ahead and ordered new AC Delco platinum plugs and some Summit 8mm wires, should be here Friday. Hopefully this fixes the problem but if the issue persists I’ll check the intake manifold gaskets as mentioned next. I did replace them with Fel-Pro when I first got the truck when I replaced the knock sensors.



I know this isn’t the cleanest engine bay as I’ve taken it down dirt roads on several occasions but it’s been such a reliable rig so I’ll probably take the time while it is down to clean it up a bit. I’ll report back after the new plugs and wires are in.

 

[blondie70]

Looks really nice and clean rite now !!! Good work.

 

[SilverSport]

sounds like your Yukon Denali will be running like it should soon...good luck

Bill