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

 

[Fless]

Those old plugs do look worn. What part number are they?

 

[tooleyondeck]

Those old plugs do look worn. What part number are they?

Old plugs are PN: 12567759

The summit plug wires I ordered were significantly shorter than OEM, despite their site saying that they fit. I test fitted one and it technically fit, but it was stretched tight and I didn’t like it.

I dropped them off for return today and I just picked up some Duralasts from the local parts house. If they end up being duds I’ll order some OEMs. New plugs are AC Delco 19307137 platinum tip. Gapping and installing them now.

 

[tooleyondeck]

Got the new plugs and wires in and took it for a test drive, fired right up and runs great. Data log looked good on about a 15min drive, did a couple of pulls onto the highway and no hiccups. P0300 can be a problematic code as we all know, thankfully this time it was the simplest scenario.

While removing the coils and dipstick on the passenger side, I found a small coolant leak from the rear A/C heater hose connector, so I went ahead and replaced that too.

 

[Gearz]

Yes there are 2 different spark plug wires depending on which coil packs you have and that's why you always go to the GM website and do a search and it will give you that information. It's not easy sometimes finding the correct parts. ( BEN THERE DONE THAT ) LOL

 

[mcorbitt52]

I wish I was so lucky to find a similar solution. My 2003 Tahoe (LT 5.3 V8) has had an intermittent misfire condition for nearly 5 years. I've been to multiple repair shops and I have replaced the following based on their "expert" diagnosis:
- spark plugs
- spark plug wires
- coils
- MAF
- TB
The misfires occur intermittently, but more often on a cold start. Sometimes I go weeks without an issue. On a few occasions I have had a misfire crop up while highway driving (however in these cases the misfire lasts only a few minutes).

What I have learned, is when the engine reaches normal operating temperature (approx. 208 degrees) and a misfire occurs, if I turn the engine off for about 30 seconds, it will always correct the misfire and run like a beauty.

I would appreciate any thoughts of wisdom on what some of you think could be the solution.

Mike

 

[tooleyondeck]

I wish I was so lucky to find a similar solution. My 2003 Tahoe (LT 5.3 V8) has had an intermittent misfire condition for nearly 5 years. I've been to multiple repair shops and I have replaced the following based on their "expert" diagnosis:
- spark plugs
- spark plug wires
- coils
- MAF
- TB
The misfires occur intermittently, but more often on a cold start. Sometimes I go weeks without an issue. On a few occasions I have had a misfire crop up while highway driving (however in these cases the misfire lasts only a few minutes).

What I have learned, is when the engine reaches normal operating temperature (approx. 208 degrees) and a misfire occurs, if I turn the engine off for about 30 seconds, it will always correct the misfire and run like a beauty.

I would appreciate any thoughts of wisdom on what some of you think could be the solution.

Mike

Well, If you've eliminated the parts mentioned as being culprits, I would check your intake manifold gasket next. If it happens most often on cold starts, the gasket may not be fully sealing until it warms up. I would also check your injectors and test your coil pack wiring harness for any shorts. There are a lot of things that can cause a random misfire code.