Code P0300?

CAITLYN BRIMER

MEMBER
1996 GMC JIMMY
4.3L
V6
2WD
AUTOMATIC
211,000 MILES

I’ve got the p0300 trouble code for multiple cylinder misfire. I’ve got rough idles, misfires, backfiring especially while driving, engine reviving a few hundred RPMs. I’ve checked, spark plugs, plug wires, distributor cap, rotor button, camshaft position sensor, timing, MAP sensor, and mass airflow sensor, throttle position sensor, and fuel filter and came up empty. My friend suggested checking the ignition coil and ignition control module, but they were replaced within the last year so I’m leaning more towards fuel. Any advice?

5 Replies

JACOBANDNICKOLAS

MECHANIC
108,192 POSTS

Hi,

It can be a few things. For example, if the engine has 211K miles, it could simple be low compression due to wear. On the other hand, you could have a faulty fuel pressure regulator that is allowing too much pressure to build and causing the engine to run rich along with other things such as an engine vacuum leak, catalytic converter issues, and so on.

So, I have a few questions. Is this the only code you found? If it is, have you checked for engine vacuum leaks? If not, here is a link that explains how that is done:

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

Next, it would be a good idea to check fuel pressure. Have you ever noticed any black or dark gray smoke from the exhaust? If so, that would indicate a rich fuel mixture., If not, here is a link that explains in general how to check fuel pressure:

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

I attached the manufacturer's fuel pressure specifications below for you.

Also, if you have a live data scan tool, I need to know the short-term fuel trims as well as the engine coolant temperature sensor's temperature signal. Do this when the engine is cold.

Let me know.

Take care,

Joe

See pic below.

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Thursday, November 17th, 2022 AT 8:19 PM

CAITLYN BRIMER

MEMBER
4 POSTS

Yes, it’s the only code we have, we’re planning on checking for vacuum leaks tonight and we can update later. We have seen no black or dark grey smoke, but we do plan to check fuel pressure soon as well. I can’t remember exactly what the short-term fuel trends were but they were in between the -10 and positive 10 like it’s supposed to be. Also not sure on the coolant temperature sensor but we can check that again and get back to you.

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Friday, November 18th, 2022 AT 9:12 AM

JACOBANDNICKOLAS

MECHANIC
108,192 POSTS

Hi,

If the fuel trims were as mentioned, that is where they should be. Chances are the fuel pressure isn't the issue. However, take a look through this link and see if it helps:

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

Let me know what you find. Also, check the coolant temperature signal. That could be throwing things off as well.

Let me know.

Joe

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Friday, November 18th, 2022 AT 7:08 PM

CAITLYN BRIMER

MEMBER
4 POSTS

So, we checked fuel pressure, it’s supposed to be between 60 - 66 PSI and it was running at around 53 PSI and after letting it sit within 15 minutes it had dropped to 39 PSI. I have no external leaks that I can find, I’m leaning towards the regulator under the intake letting fuel leak into the back two cylinders.

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Saturday, November 19th, 2022 AT 9:40 AM

JACOBANDNICKOLAS

MECHANIC
108,192 POSTS

Hi,

That is very possible. It could even be the pump is starting to fail or a restriction in the fuel line itself.

If you want to nail it down to the actual issue, I attached the diagnostics below which walks you through testing the regulator, pump, restriction, and nearly anything that can cause this.

Let me know if this helps or if you have questions.

Take care,

Joe

See pics below.

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