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P0300 Random missfire code help!!!!!!

P0300 Random missfire code help!!!!!!

Postby Duane132 [OP] » Oct 15 2020, 10:04pm

Hey all,
I have a 2004 Silverado 1500 5.3l Vortec. I have the dreaded Random misfire code. No other codes. I have replaced all the spark plugs, wires, and coils. I have put in new fuel injectors, new fuel pump/filter, everything. Lifters were checked, rockers were checked, there is no metal in the oil or oil filter paper. I took it to a trusted mechanic and he said to change the injectors. I did have a knock sensor code and changed that out and it got rid of that code. Compression is good in all cylinders. Im curious if i have a wiring issue.
The electric fan in front of the radiator (escapes my mind on what its actually called) will turn on with the headlights and headlights only and it doesnt automatically shut off. Could that be the culprit? There are also no vacuum leaks. I need to replace the catalytic converters but i want to figure out this misfire issue first. Please help! I'm about to lose my mind and dont want to waste more money on parts I potentially dont need yet.
Original Poster
Duane132
Firing on 2 cylinders [L1]
Firing on 2 cylinders [L1]
Posts: 21
Topics: 1
Joined: Oct 15 2020, 9:49pm
Location: WA
Year: 2004
Make/Model: Chevrolet Silverado
Variant: Z71
Extra Info: 1500 5.3l vortec
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Re: P0300 Random missfire code help!!!!!!

Postby chem_man » Oct 16 2020, 4:03am

If you have an android smart phone, you might want to consider purchasing a Bluetooth OBD II dongle and get the Torque Pro app (cost is ~$5.00). That will allow you to monitor virtually every thing that is going on. For the P0300 code, I would suggest you look at the misfire counts, both current and historical for all 8 cylinders to try and narrow down which cylinder is misbehaving.

You can get a very good Bluetooth dongle for under $15 from Amazon.com. Here is the link:

https://www.amazon.com/gp/product/B011N ... ghtbars-20

There is a gentleman on another forum I participate in who uses the Torque Pro app along with the Veepeak dongle and it's absolutely amazing all he can do with it.

Good Luck!
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chem_man
Combustion on 1 piston [L2]
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Re: P0300 Random missfire code help!!!!!!

Postby Duane132 [OP] » Oct 16 2020, 4:16am

I took it to my mechanic and he put it on his reader. The reader said 2 and 7 were misfiring. Once it has more rpm the misfires go away. That's why i changed the injectors even after I had cleaned the injectors by removing them. He used his stethoscope and put them on each injector and it was 2 and 7 that were not putting fuel in consistently. Could it maybe be the fuel rail needs replaced?
Original Poster
Duane132
Firing on 2 cylinders [L1]
Firing on 2 cylinders [L1]
Posts: 21
Topics: 1
Joined: Oct 15 2020, 9:49pm
Location: WA
Year: 2004
Make/Model: Chevrolet Silverado
Variant: Z71
Extra Info: 1500 5.3l vortec
Group Membership:
Registered users

Re: P0300 Random missfire code help!!!!!!

Postby Duane132 [OP] » Oct 16 2020, 4:18am

Now I haven't had it back on the computer since i replaced the injectors.
Original Poster
Duane132
Firing on 2 cylinders [L1]
Firing on 2 cylinders [L1]
Posts: 21
Topics: 1
Joined: Oct 15 2020, 9:49pm
Location: WA
Year: 2004
Make/Model: Chevrolet Silverado
Variant: Z71
Extra Info: 1500 5.3l vortec
Group Membership:
Registered users

Re: P0300 Random missfire code help!!!!!!

Postby chem_man » Oct 17 2020, 4:45pm

I would think that if the fuel injector rails were dirty, then all your injectors would have been messed up.

Are you still experiencing problems since replacing the two injectors?
User avatar
chem_man
Combustion on 1 piston [L2]
Combustion on 1 piston [L2]
Posts: 180
Joined: May 19 2012, 1:51am
Years of membership: 8 years of membership8 years of membership8 years of membership8 years of membership8 years of membership8 years of membership8 years of membership8 years of membership
Location: Maryland
Year: 1999
Make/Model: Chevrolet Silverado
Variant: LS
Extra Info: Ext Cab 5.3L V8 2WD
Group Membership:
Registered users

Re: P0300 Random missfire code help!!!!!!

Postby Duane132 [OP] » Oct 17 2020, 6:39pm

I replaced all the injectors and its still doing the same thing. I've cleaned the throttle body as well and still doing the same thing. The only thing I haven't done to it is the fuel rails.
Original Poster
Duane132
Firing on 2 cylinders [L1]
Firing on 2 cylinders [L1]
Posts: 21
Topics: 1
Joined: Oct 15 2020, 9:49pm
Location: WA
Year: 2004
Make/Model: Chevrolet Silverado
Variant: Z71
Extra Info: 1500 5.3l vortec
Group Membership:
Registered users

Re: P0300 Random missfire code help!!!!!!

Postby chem_man » Oct 17 2020, 8:18pm

So, you are still getting a P0300?

Did your mechanic check the spark plugs, spark plug wires and ignition coils for cylinders 4 and 7 before stating that it was the fuel injectors?

I still think more diagnostics need to be done before you replace anything else despite what the mechanic did.

I do see where you replaced all your spark plugs, plug wires and ignition coils. What brand and part number of spark plugs did you use? What brand of spark plug wires and ignition coils did you use?
User avatar
chem_man
Combustion on 1 piston [L2]
Combustion on 1 piston [L2]
Posts: 180
Joined: May 19 2012, 1:51am
Years of membership: 8 years of membership8 years of membership8 years of membership8 years of membership8 years of membership8 years of membership8 years of membership8 years of membership
Location: Maryland
Year: 1999
Make/Model: Chevrolet Silverado
Variant: LS
Extra Info: Ext Cab 5.3L V8 2WD
Group Membership:
Registered users

Re: P0300 Random missfire code help!!!!!!

Postby Duane132 [OP] » Oct 17 2020, 9:45pm

Spark plugs were Autolites and the coils and wires were Durolast from Autozone. I got the autolites for budget reasons and when i am able to i will install a better brand instead. All these brands have a lifetime warranty. The spark plugs i removed were not at the proper gap. The ones i installed i put to 0.406 i believe is what its supposed to be. Before any of that was done though the computer read 2 and 7 were misfiring. Truck was due for a tune up anyway. so i didnt mind doing new plugs and wires and coils.
Original Poster
Duane132
Firing on 2 cylinders [L1]
Firing on 2 cylinders [L1]
Posts: 21
Topics: 1
Joined: Oct 15 2020, 9:49pm
Location: WA
Year: 2004
Make/Model: Chevrolet Silverado
Variant: Z71
Extra Info: 1500 5.3l vortec
Group Membership:
Registered users

Re: P0300 Random missfire code help!!!!!!

Postby Duane132 [OP] » Oct 17 2020, 9:52pm

One thing i did do and was told by someone else was to unplug the O2 sensors and drive it for a couple days. He said it would basically put a blanket on the ECU and could basically try and reset the computer. Ive done it for about a day now and nothing has really changed yet. I know i need new cats and putting a camera inside you can see splotches from raw fuel. That leads me to believe its not the cats
Original Poster
Duane132
Firing on 2 cylinders [L1]
Firing on 2 cylinders [L1]
Posts: 21
Topics: 1
Joined: Oct 15 2020, 9:49pm
Location: WA
Year: 2004
Make/Model: Chevrolet Silverado
Variant: Z71
Extra Info: 1500 5.3l vortec
Group Membership:
Registered users

Re: P0300 Random missfire code help!!!!!!

Postby Duane132 [OP] » Oct 17 2020, 10:24pm

One thing i forgot to mention, when i replaced the injectors the fuel rails had a rattle. Are they supposed to have a raddle? I can get new ones but there is nothing around close to me in stock that i can grab a new one and check. Should i maybe replace those?
Original Poster
Duane132
Firing on 2 cylinders [L1]
Firing on 2 cylinders [L1]
Posts: 21
Topics: 1
Joined: Oct 15 2020, 9:49pm
Location: WA
Year: 2004
Make/Model: Chevrolet Silverado
Variant: Z71
Extra Info: 1500 5.3l vortec
Group Membership:
Registered users
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