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P0300 Random missfire code help!!!!!!

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

Postby chem_man » Oct 17 2020, 11:51pm

Reconnect your O2 sensors. Disconnecting them is not the way to "reset" the PCM. To reset the PCM, remove the fuses for the ECM/PCM from the underhood electrical center, or disconnect the negative battery cable for 30 minutes. That will wipe the memory and your ECM/PCM will begin to relearn just as soon as you start driving.

OK on the Autolite plugs. I do know that the 5.3 L V8s are picky about which spark plugs go into them and they really run best using ACDelco 41-962 plugs. Depending on which Autolite plug you are using, that could be part of your problem.

As far as the spark plug wires go, I know nothing about the quality of the Autozone Duralast spark plug wires. However, I do know who manufacturers their Duralast ignition coils for your Silverado, and despite their having an Autozone lifetime warranty, I would not put them on my truck unless I absolutely have no other choice.

I do not know if there is supposed to be anything rattling inside the fuel injection rail.

You mentioned that you need new catalytic converters. That could also be part of your problem. A bad catalytic converter can create excessive back pressure. If you were to remove the upstream O2 sensors and drive around, if the engine settles down and runs much better, then you'll know that clogged cats are a major factor in the poor running.

As I recommended earlier, you should consider getting one of the Bluetooth OBD II dongles (such as the one I referenced) and the Torque Pro app - provided you have an android device. It will really help you in diagnosing problems.

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

Postby Duane132 [OP] » Oct 18 2020, 12:00am

I got a obd reader and the torque pro. Could the crankshaft positions sensor possibly the culprit as well?
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Re: P0300 Random missfire code help!!!!!!

Postby Duane132 [OP] » Oct 18 2020, 12:13am

I did a 02 sensor test and bank 2 sensor 2 is sitting right at 8v. It should be fluctuating right? All the others jump around
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Re: P0300 Random missfire code help!!!!!!

Postby chem_man » Oct 18 2020, 12:51am

Regarding the Bank 2 Sensor 2 O2 sensor voltage, I hope you mean 0.800 volts rather than 8 volts.

Upstream O2 sensor voltages bounce around all over the place. Downstream O2 sensor voltages will vary somewhat, but they will hover around 0.7 - 0.8 volts nominally if things are OK.

I would not worry about the crankshaft sensor unless it gives the appropriate OBD II code.
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Re: P0300 Random missfire code help!!!!!!

Postby Duane132 [OP] » Oct 18 2020, 1:04am

yes i ment 0.8 volts. I pulled up the graph I have on Torque and when i would accelerate the voltage basically stayed the same and after i got off of the accelerator it went down to almost 0.0v and then almost went back up to .8v almost immediately
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Re: P0300 Random missfire code help!!!!!!

Postby chem_man » Oct 18 2020, 1:42am

What you describe is normal behavior for the downstream O2 sensor.
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Re: P0300 Random missfire code help!!!!!!

Postby chem_man » Oct 18 2020, 1:45am

Since you have Torque, you should monitor current and historical misfire counts to see which cylinder or cylinders is causing the P0300. All the PCM knows at the moment is that something is not right part of the time (a threshold value for misfires from a given cylinder has not yet been reached to set a P030X code where X is the cylinder number.
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Re: P0300 Random missfire code help!!!!!!

Postby Duane132 [OP] » Oct 18 2020, 2:53am

Bank 1 sensor is kind of all over the place and not sitting at 0.8v. I swapped 02 sensors and it ended up staying. But i then drove it a few miles and its like the sensors swapped.
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Re: P0300 Random missfire code help!!!!!!

Postby chem_man » Oct 18 2020, 3:12am

Well, you might want to consider replacing your downstream O2 sensors. My recommendation for O2 sensors are Denso or ACDelco OEM. The Denso sensors are a very reasonable ~$26 from Rockauto.com You might also consider replacing the upstream ones as well. Oxygen sensors do age and they get lazy with age and mileage (~10 years/75,000 miles).
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Re: P0300 Random missfire code help!!!!!!

Postby BT Crusier » Oct 18 2020, 4:17am

I have been down this road with a couple 5.3s...

Replace the coil packs. I did the spark plugs, then wires...both times I got the codes...it was always a pair (not sure why) of coil packs.

One only had 25k on it...out of nowhere...throws code. The other had 250k and threw it...coils both times.
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