Code PO300

Tiny
SNOWMAN65
  • MEMBER
  • 2006 DODGE RAM
  • 4.7L
  • V8
  • 2WD
  • AUTOMATIC
  • 170,000 MILES
My check engine light came on so I had it scanned and it's showing code po300.
So I Googled the causes of code po300 and it's a random/multiple cylinder misfire. So I went trough all the possible causes of the problem.

I have already replaced the spark plugs.
I replaced the coil packs.
I replaced the EGR valve.
I replaced the crankshaft sensor.
I replaced the camshaft sensor.
I replaced the throttle positioning sensor.
I cleaned the fuel injectors.
I checked the catalytic converters which they are good.
I checked for vacuum leaks. There were none.
I dropped the gas tank and checked the fuel pump which is good.
And replaced one of the four o2 sensors.
And after all of that it still keeps throwing the code.

So if you have any idea what this is or might be the problem with it I am all ears. Lol. I can't affordably take it to a mechanic right now I was just laid off. So any suggestions that might help me.
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Tuesday, November 17th, 2020 AT 6:23 PM

7 Replies

Tiny
JACOBANDNICKOLAS
  • EXPERT
Hi,

You have done a lot. I have to ask if you notice the misfires when they happen? Does the check engine light flash?

Also, how did you check for vacuum leaks? Here is a link that explains how to do it. See if it helps:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Also, is the fuel pressure within the manufacturer's specs? Here are the specs:

FUEL SYSTEM PRESSURE

58 psi 2 psi

Note: The fuel pressure regulator and filter are one component on this vehicle. Fuel Pressure Regulator Operation: The pressure regulator is a mechanical device that is not controlled by engine vacuum or the Powertrain Control Module so it could have failed and causing issues. If you look at the attached pic, it shows the filter/regulator. Is that what was checked?

Next, if you don't notice a misfire when driving, it could be happening due to engine compression. Compression variations beyond 10% can cause random misfires.

Let me know. We'll figure it out.

Take care,
Joe

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Tuesday, November 17th, 2020 AT 6:55 PM
Tiny
SNOWMAN65
  • MEMBER
Thank you for your response.
Yes, it does miss while driving and when slow down and come to a stop. It starts to lose power. I end up having to pump the gas to keep it from shutting off, then when I start to accelerate there is a huge lack of power.
The way I checked for vacuum leaks was I sprayed starting fluid on and around the vacuum lines, pretty much the entire top of the motor, lol. And as I was spraying it I was listening to the motor to hear for it to rev up which it didn't.
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Wednesday, November 18th, 2020 AT 6:46 AM
Tiny
SNOWMAN65
  • MEMBER
And while I'm driving every now and then it does backfire.
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Wednesday, November 18th, 2020 AT 6:49 AM
Tiny
JACOBANDNICKOLAS
  • EXPERT
Is the fuel pressure within the manufacturer's specs? A rich mixture can cause a backfire.

https://www.2carpros.com/articles/engine-backfires-while-running

Let me know.
Joe
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Wednesday, November 18th, 2020 AT 8:21 PM
Tiny
SNOWMAN65
  • MEMBER
Well, that's one thing I have not tried yet.
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Thursday, November 19th, 2020 AT 10:03 PM
Tiny
SNOWMAN65
  • MEMBER
What kind of gauges do I need to test the fuel pressure?
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Friday, November 20th, 2020 AT 4:43 AM
Tiny
JACOBANDNICKOLAS
  • EXPERT
Hi,

You need a fuel pressure gauge. Most nationally recognized parts stores will lend one to you. If you get one and have questions, let me know.

Take care,
Joe
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Friday, November 20th, 2020 AT 8:44 PM

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