Q: My car won't go over 20 mph. It will start to shake vigorously once it hits 20 mph

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The codes read p0300, p0304, p306, p0305, p0400. I've run the codes several times after resetting. It consistently comes up with p0300, p0304, p0306, p0305. I have replaced all of the spark plugs and wires. The MAF has been replaced, as well as the air filter. I can't wrap my head around whats going on. It idles beautifully and revs the engine (in neutral or parked) perfectly. That being said, When I put it in drive, it will go fine for a short time (until I get to 20 mph). You can tell the entire engine shakes along with the misfire engine light flash while going 15-20 mph. My rpms won't go over 2000-2100. By the time i get back, the engine is barley running. It usually stalls when going in reverse. My next task is checking the EGR valve. Is this a good staring point?


My car has 151000 miles.
My car has an automatic transmission.

Hello. When an issue like this turns up with this many codes I usually start by checking the fuel pressure. If the fuel pump is failing and not creating the right pressure then you will get multiple misfires. If the fuel pressure is fine then it may be another issue. I have seen a failing catalytic converter in the exhaust cause this and I have also seen a failing timing chain cause this. Once the fuel pressure is tested then I would test the exhaust and check the cam and crank readings. A compression test may also be needed. This is something that you should have an expert mechanic look at, like one from YourMechanic, that can come to you to [diagnose why the check engine light is coming on] https://www.yourmechanic.com/services/check-engine-light-is-on-inspection

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Hello. The codes that you have listed are for engine misfires. P0300 is for random misfires, while P0304, P0305, and P0306 are for specific cylinder misfires. Misfires can be caused by many things. If you have already replaced the MAF, spark plugs, and wires then the next things I would move on to are the EGR valve and the fuel pump. If there is an issue with the EGR valve then it may be possible that it is allowing unmetered air into the engine, which may be upsetting the air fuel ratio and causing the misfires. I would check to make sure that the valve is operating properly, and that any EGR tubes, hoses and pipes are all intact and in good condition. The same can also be caused by a vacuum leak, if it is large enough. If the EGR checks out then the next thing I would do is look at the fuel pump and fuel system. If the fuel pressure runs low, due to a worn or weak fuel pump then it may cause the engine to misfire due to a lean condition. Also keep in mind that the check engine light flashes when the computer detects an issue that could potentially harm the vehicle’s emissions components, such as the catalytic converters or oxygen sensors. If the light is flashing then continuing to drive the vehicle may result in damage to these components. As random misfires can sometimes be difficult to diagnose, I would recommend having a professional technician, such as one from YourMechanic, come to your location to diagnose what may be causing your issue.

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