# Darn P0300 Error code



## 2004chevy2500hd (Dec 19, 2010)

Guys - Wondering if you can offer a bit of advice. Have a 2004 2500HD - Had plugs and wires changed (Napa parts) - general tune-up. O2 sensor replaced...all of the "normal" tune-up specifics. The multiple random misfire code disappeared - and then came back.

Sequence of events:
1) Purchased truck used
2) Truck had check engine light on - had the following errors
- O2 sensor
- P0300
3) Replaced plugs & wires / O2 sensor
4) P0300 disappeared 
5) Two months passed - hauled 4200lbs of dog food (w/trailer) - just the other day...
6) check engine light came back on
7) had error code checked and saw that it was another P0300
8) was plowing and all of a sudden noticed that check engine light was off (yay!!)
9) confused - I searched this forum (chevy) and found this:
http://www.plowsite.com/showthread.php?t=62238&highlight=p300
..from some time ago.....

I went to the local autoparts store and picked up some SeaFoam to give it a whirl and wondered what the best plan of attack would be. Should I dump the whole thing into the gas? Or do the thirds thing that is recommended here - on their web site:

http://www.seafoamsales.com/how-to-use-sea-foam-motor-treatment.html (check out video on right)

Thank you


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## sweetk30 (Sep 3, 2006)

also what brand of spark plug was installed ? 

todays newer stuff dosnt like diffrent brands/model plugs. 

i always install exact style/model factory plug. or you can get small skip/miss.


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## leepotter (Nov 25, 2009)

I've had nothing but problems with Bosch Platinums if that is what you used.


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## mrv8outboard (Dec 29, 2009)

I use Seafoam in this way. I let the engine ingest it through a vacuum port or a small vacuum line into the intake. I use the whole can and it is like fogging out and engine, let it sit an hour or so, then run it off. Others bring up a great point about the plugs. Use what OEM replacements not a cross. Also you did get the premium wires from NAPA not the cheaper ones. You get what you pay for. You could also get some MAF sensor cleaner and clean your MAF sensor, be very careful when you do it as you can damage it.


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## sweetk30 (Sep 3, 2006)

leepotter;1193506 said:


> I've had nothing but problems with Bosch Platinums if that is what you used.


x1000 here as a mechanic for over 10+ years.

unless its a german car motor.


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## 2004chevy2500hd (Dec 19, 2010)

Funny you guys say that - I picked up and installed the Bosch Platinums - and then was told to switch them back to AC Delcos....Lost out on the money I spent on the Bosch had them in there all of one day - (call it - "paying for the experience" laughing ) - Plug wires are the OEM as well. - 

I'll try the vacuum line suggestion here in the next day or so 

Drove the truck around today - did some minor plowing and no check engine light...Oh well, I'm hoping that this is good news.

I'll keep you guys posted and thanks again!!


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## sweetk30 (Sep 3, 2006)

take the plugs back and tell them thay didnt work right. made the motor skip didnt work as advertised . 

but now YOU know what the correct way to do the tune up is . :salute:


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## 2004chevy2500hd (Dec 19, 2010)

Excellent suggestion - I'll do that - 

Why didn't I think of that?


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## sweetk30 (Sep 3, 2006)

daaaa  :laughing:

not in that mind set. you just want it to run right.


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## 2004chevy2500hd (Dec 19, 2010)

lol - very true sir - very true - thanks again


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## hummertech (Jan 14, 2007)

what engine is in the truck?

i would use ac delco plugs. most ac delco replacement plug are iridium now. 

the plastic intakes are known to leak.


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## 2004chevy2500hd (Dec 19, 2010)

hummertech - I have the 6.0L in there. Great engine - not so great MPG



Yeap - AC Delco's are in there now 

I still have to try the SeaForm - laughing - just need to find the time to do this.....


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## sweetk30 (Sep 3, 2006)

04 2500hd. i be bettin since plugs its gas  but if i recall o3 was last for 8.1L so he has 6.0 if i was a bettin man.


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## 2004chevy2500hd (Dec 19, 2010)

smiling - SweetK30 - Want a challenge? Wanna help me replace this 6.0L with the 4cyl from my Accord? 

totally kidding.....

80,000 miles and running strong


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## stacks04 (Jan 22, 2008)

I would do a throttle body clean, clean the maf sensor, get some injector cleaner and watch were you buy fuel. It could be as liittle as some ****** fuel and causing intermittent skip.


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## sweetk30 (Sep 3, 2006)

2004chevy2500hd;1196139 said:


> smiling - SweetK30 - Want a challenge? Wanna help me replace this 6.0L with the 4cyl from my Accord?
> 
> totally kidding.....
> 
> 80,000 miles and running strong


cuttin tourch and welder . not a problem. might not look good when done tho. :laughing:


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## 2004chevy2500hd (Dec 19, 2010)

stacks04 - I have to try these after the SeaForm treatment - it looks OK for me - Keeping my fingers crossed


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## herbert02 (Jan 18, 2011)

Did your check engine light come back on? Have the same exact truck and the same problem. Mine comes on when im on the highway. Did plugs, wires, upper intake gasket, mass air flow, sea foam, injector cleaner. Still no luck. Spent $500 at the dealer for them to just guess what the problem was. Ive given up on it.


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## hummertech (Jan 14, 2007)

6.0l should be coil near plug. do you have access to data? misfire history will tell you which cylinder is missing. kinda helpful to know which cylinder.

they are know for cats clogging up also. will cause a misfire and burn exhaust valves if clogged for long periods of time.


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## 2004chevy2500hd (Dec 19, 2010)

Guys - Thank you for the feedback. Good news is that my check engine light has not come back on (yet). Smiling - I bought the truck used (from a landscaper) and spent darn near $3000 on repairs - nothing major - but still not what I wanted to dump money on. 

Tires
brakes (front and rear)
rotos (rear)
Parking break
All fluids
Tune-up (plugs and wires)
fan clutch
wheel bearings

As for the data on the check-engine light - my next door neighbor in a SnapOn dealer who has access to those really nice computers that you can hook up to the truck - I'll have to get him to do that....

I guess that it's not just me who has the check-engine light mystery


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## herbert02 (Jan 18, 2011)

This multiple cylinder misfire is messed up. I have done everyting. As anyone heard of the upper intake in the 6.0 warping? I know they are plastic intakes but I have a hard time believe this rumor.


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## Woodenshoe (Oct 30, 2010)

We have a similar truck, with a lot of electrical issues...
Go to someone with a GOOD scanner, it will tell which cylinder was misfiring, and will give a misfire count for each cylinder. We had a few bad coils, and all of the ground points were rusted. We took off all the coils, wire wheeled the ground contact points, added direct ground wire from the coils to the battery, and the misfires are gone!!


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## The PuSher MaN (Jan 14, 2009)

Don't run that seafoam though your truck unless you want to replace that o2 sensor again. I ran seafoam in a 160k mile truck that ran fine previous and it caused me alot of headaches screwed the o2's up and it hasn't been right since i think it can clog cat's too.


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## mrv8outboard (Dec 29, 2009)

The catalytic converter remanufactures here recommend using Seafoam to clean the catalytic converters out when you get the dreaded "P0420 catalyst below threshold". But if your engine is old and has years of build up you might find that you treat it once to clean the engine the a second time for the cats.


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## sledrider2005 (Nov 8, 2008)

Well I also have the same problem above 2500 rpms the lights starts blinking scanned it with a snap on scanner with the misfire counter and #1 cylinder had the most . I replaced all plugs and wires with ac delco parts . That didnt work so we replaced #1 coil since it had the most misfires. Still the light blinks .I was told by a gm mechanic it was fuel pressure change the fuel filter did the no change. Then a buddy told me he fixed his misfire problem by replacing the up draft o2 sensors so I did that with denso sensors. Still have a misfire . So we decided to check fuel pressure it was at 46 lbs. which is low for this truck. So over this.past weekend I replaced the fuel pump all the fuel lines and replaced all the brake lines with a ss kit since the tank was out. And guess what it still flashes and has a misfire according to the snap on.scanner what do I do now. We are very stumped btw its a 03 ccsb 6.0 with 88000 on it thanks if anyone.can help


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## Whiffyspark (Dec 23, 2009)

sledrider2005;1675792 said:


> Well I also have the same problem above 2500 rpms the lights starts blinking scanned it with a snap on scanner with the misfire counter and #1 cylinder had the most . I replaced all plugs and wires with ac delco parts . That didnt work so we replaced #1 coil since it had the most misfires. Still the light blinks .I was told by a gm mechanic it was fuel pressure change the fuel filter did the no change. Then a buddy told me he fixed his misfire problem by replacing the up draft o2 sensors so I did that with denso sensors. Still have a misfire . So we decided to check fuel pressure it was at 46 lbs. which is low for this truck. So over this.past weekend I replaced the fuel pump all the fuel lines and replaced all the brake lines with a ss kit since the tank was out. And guess what it still flashes and has a misfire according to the snap on.scanner what do I do now. We are very stumped btw its a 03 ccsb 6.0 with 88000 on it thanks if anyone.can help


Check the valve springs.

I had one grind a can flat with no broken springs. 4 shops missed that


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## sledrider2005 (Nov 8, 2008)

The truck runs very good you would never know it has a misfire . Also it has a wheatly tuned computer.


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