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At Wits End. P0300. Please help.

minorkeylee

New Member
Joined
May 25, 2022
Messages
7
Hello all. First post here. I've got a 2003 Avalanche that has been nothing but trouble since I bought it. I'm looking for some perspective.

I've read ALOT about P0300 and am still struggling to figure what the heck is going on. I've replaced plugs, wires, coils, fuel pump, fuel filter and injectors. I thought I had it solved when I replaced the drivers side bank injectors the other day. It ran much better and was able to get to and from work without misfires. However, yesterday, after finishing the injectors on the passenger side, it reared its ugly head again.

The condition happens pretty consistently. After about 10 minutes of driving, if you downshift and get the revs up to around 2500-3K, the CEL will start flashing, and it runs like garbage. The code rarely sets to solid CEL. If you let is sit for a little bit, it will run fine again. Truck starts, runs and idles fine otherwise.

Any ideas? I assume if it was Cats or O2 sensors, then it would actually throw a different, more specific code?
 
Leaking intake manifold gaskets are common with these older engines and can cause random multiple misfires.

If you decide to change the gaskets, go with the FelPro gasket kit.
 
Sounds like you covered most all the bases I would have it. What octane fuel are you running? What plugs did you put in? (temps) Your mechanic you took it to didnt find anything?
 
The only thing he mentioned was that when the error occurred bank 1 fuel trim was -9% and bank 2 was +17%. I put NGK laser iridium plugs in. Just running 86 gas. Not sure what the "temps" part of your reply is about??
 
The spark plugs have a heat range that depending on the use, fuel, how many toys installed, NOS...all come into play. I attached a chart, Im not sure where your plugs come into play. If you do not have mods on the truck, you may be better off going back to OEM plugs
 

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What are the chances of it being bad exhaust valves on the drivers side head? I can definitely hear rattle at high RPM on the drivers side. I saw a video a while back of a really good mechanic troubleshooting P0300 and it ended up being a leaking exhaust valve...
 
Also worth mentioning. The noise I hear at WOT, engine at highest RPM before shift. When this occurs, the CEL starts flashing.
 
The only thing he mentioned was that when the error occurred bank 1 fuel trim was -9% and bank 2 was +17%. I put NGK laser iridium plugs in. Just running 86 gas. Not sure what the "temps" part of your reply is about??
He wants to know the heat range of the spark plugs you put in.
For example: I have NGK TR55GP spark plugs installed on my Avalanche.
If you post what number is on the insulators of your spark plugs, it will provide an idea as to the heat range they fall into.
Too hot or too cold spark plugs will cause operational issues.
In my experience, high RPM misfires can be caused by a defect in the ignition system.
I worked on a car that ran fine when driven lightly, but ran like crap under load.
Turned out to be the spark plug boots. They were not insulating the spark correctly....and when the engine is under load, it requires more kilovolts of electrical energy to fire the plugs - and as a result of that (because electricity always finds the path of least resistance), the electricity found it easier to jump fire through the spark plug boots to the engine block.
Once I had that situation corrected, the problem was fixed.
That was a tricky one to find.
 
After even replacing a new engine. One should always use either
GM genuine or ACDelco parts. New Acdelco maf air flow sensor ans Acdelco O2 sensors, range technology plus worked for me
 
I wanted to update. It was the CATS. Last thing to touch, obviously. Man. The things I've done to this truck.

I put a Magnaflow Y pipe/CATs this morning. $617 on Amazon. Direct, two piece fit. It took about 3 hours. The truck, in combination with an entirely new ignition system, runs like its brand new.

Thanks for all your input. I was half thinking about making a mega post about all the parts ($3500, including front end) that I've replaced (I did all the work, that number is just parts) on the truck to make it right. Might be a fun project.
 
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