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Trouble code p0300?

I have a 2006 pontiac grand prix base model. The other day at the red light my car started vibrating and it didn't want to pull off. Then it sounds like I have loud pipes now. I immediately took it to autozone  and got trouble code p0300. 

I have changed the coil packs, spark plugs, spark plug wires, and replaced one of the fuel injectors. However, this has not fixed the problem. Do anyone have anymore suggestions or something that my mechanic is not looking at. His advice was to go buy a new one. I always do proper maintenance on my vehicle. I change oil EVERY 3,000 due to the miles. No other history of engine or transmission issues. Any help is appreciated. 

8 Answers

  • 3 months ago
    Favorite Answer

    Likely have a vacuum leak perhaps the intake manifold gasket if this is a 3.8L or similar engine. Upper or likely the lower intake manifold gasket. Check for vacuum leaks. Since you had a noise right away, I'd also check the exhaust from the exhaust manifolds, donut gaskets, down to the catalytic converter. Cat converter can plug up on these models and create a lot of back pressure on the engine and exhaust forward of the cat converter. When that occurs, you can blow an exhaust manifold gasket or donut gasket on the pipe(s) and this would create a "loud exhaust" noise and exhaust leak.

    You basically eliminated the electrical side of the possible problem so I'm leaning toward mechanical which could be a gasket (intake, exhaust, head gasket) or even something that fell off like an O2 sensor. Check your ECM wire bundle. I had a mouse nest in my ECM next to my air filter box but thankfully found it before the mouse chewed through the wires. Had to wrap a dozen individual wires it chewed or ate the insulation on.

    Possible causes of P300 or similar code.

    Low Fuel Pressure– If there isn’t enough fuel getting to the engine, this will cause combustion to be less than optimal. Diagnosing low fuel pressure can be tricky. Typically, if you do have low fuel pressure, the vehicle will act fine when it doesn’t need a lot of fuel. But, it’ll sputter and act like it’s going to die at speed or under heavy acceleration. Here’s some information on how to tell if you have a bad fuel filter.

    Vacuum leak– If your Grand Prix has a vacuum leak, it can be very difficult for it to get the right air/fuel mixture. This will cause the cylinders to misfire and it’ll throw the P0300. Also, since a vacuum leak almost always affects each cylinder the same, you’ll typically get P0300 with it and not any cylinder specific misfire codes. Here’s a great article from Popular Mechanics on how to detect a vacuum leak. It’s easy (and kind of fun) to chase one down. Popular Mechanics: How to find a vacuum leak.

    EGR Problems– If the EGR system is not able to recycle the engine gasses right, it’ll throw P0300.

    Ignition Problems– Bad plug wires (if equipped), bad coil packs, and spark plugs can cause misfires to occur. This isn’t higher on the list because typically you’ll get a misfire in one cylinder specifically, and not a P0300 only. If you got a P302 or something similar with the P0300, it may be a good idea to check and see if there is any damage or failure from your ignition components. Here’s how to test a coil pack, how to tell if a spark plug is bad (video), and how to test plug wires (video).

    Cam or Crank Sensors– This one is very unlikely, but it does happen. If the ECU is not getting the right signal from these sensors, the vehicles timing is not going to sync up and it’ll misfire.

    Low Compression– If you have a leaking head gasket, bent valve, cracked head, etc.. that would cause compression to not be as high as it should, you’re going to get P0300. You should also feel the vehicle is down on power as well.

    Most Common P0300 Fixes

    A lot of the time, P0300 is going to be fixed by something obvious, such as an EGR leak. When it’s not glaringly obvious what is wrong, a tune up is a great place to start.

    • Tiffany w3 months agoReport

      Thank you so much for responding. I do have the 3.8. The problem was fixed today. I had to replace the Ignition control module, which is the module the sits directly under the coil packs. 

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  • 3 months ago

     Check fuel pressure with a gauge.

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  • 3 months ago

    Do a compression check.

  • Anonymous
    3 months ago

    P0300 is random misfire.  Check for a vacuum leak.  Clean or replace the MAF sensor.  Check for a bad EGR valve.  Check fuel pressure with a gauge.

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  • 3 months ago

    That code indicates a random misfire. A good mechanic can fix this. Seek help from another mechanic that actually knows what he is doing. 

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  • Dan
    Lv 5
    3 months ago

    Take it to a dealership, your so called "mechanic" is guessing.

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  • 3 months ago

    Take a gasoline sample and check the ethanol content. You can't run 88 octane in that motor. That's only for Flex Fuel vehicles.

    Check the fuel pressure.

    Source(s): Mitsubishi Master Tech
  • Barry
    Lv 4
    3 months ago

    Go to a different shop. Your mechanic sounds either disinterested or incompetent. 

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