Dreaded code P0300

Tiny
TONYNEAL
  • MEMBER
  • 2002 GMC ENVOY
  • 4.2L
  • 6 CYL
  • 4WD
  • AUTOMATIC
  • 155,154 MILES
Purchased used for $2,000.00 at an auction. When I brought it home I changed the oil saw it needed plugs and changed them and one of the coils had to be replaced, so I did that. Now it runs really badly! Hooked up my code reader and I got the dreaded "P0300"! So I rechecked all the plugs and coils and their all good. So got any ideas?
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Wednesday, May 8th, 2019 AT 3:50 PM

5 Replies

Tiny
ASEMASTER6371
  • EXPERT
Good afternoon,

The most common cause is low fuel pressure. I would check the fuel pressure first.

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

You have done the basics but it is time to diagnose it and find the issue.

Roy
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Wednesday, May 8th, 2019 AT 4:20 PM
Tiny
KASEKENNY1
  • EXPERT
Hi Tony,

A P0300 is just saying that random cylinders are misfiring and there is not one that is consistently missing. This is commonly referred to as a general misfire code and that is not accurate. Basically, the PCM is not seeing crankshaft accelerations on different cylinders at different times. So what can cause a cylinder to misfire on one stroke but not the next and cause others to do the same. Normally, this is something that effects all cylinders equally like fuel, air, or a sensor issue.

Let's start with hooking a vacuum gauge. You should see a very steady needle or you have a leak causing your poor running condition and it could be the cause of the P0300.

Next you need to monitor o2 sensor data and see what it is telling you. Short term o2 should be bouncing around 0. Send me these numbers and what the vacuum gauge is doing.

This will point us down the road or fuel or something else.

Lastly, it needs to be said that if it was not running poorly prior but now it is, that you would think there is something left off or disconnected. Try coming back to this and rechecking everything after you have had some time away and your fresh eyes may see something. If nothing, then lets get that information and go from there. Thanks

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge
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Wednesday, May 8th, 2019 AT 4:39 PM
Tiny
ASEMASTER6371
  • EXPERT
Good information, but I would start with a fuel pressure test to verify sufficient fuel to the injectors.

Roy
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Wednesday, May 8th, 2019 AT 4:52 PM
Tiny
TONYNEAL
  • MEMBER
Where are the o2 sensors located on my vehicle? And is there a gauge I can borrow from the parts store or can I monitor the o2 with my OBD II?
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Wednesday, May 8th, 2019 AT 9:06 PM
Tiny
ASEMASTER6371
  • EXPERT
You can rent a fuel pressure tester at a parts store.

Yes, you can monitor the sensors with a scan tool, not a code reader.

Roy
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Thursday, May 9th, 2019 AT 3:20 AM

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