Code 0P0300, random misfire

Tiny
MORGRETBUSHIDO
  • MEMBER
  • 2005 PONTIAC BONNEVILLE
  • 3.8L
  • V6
  • 2WD
  • 167,000 MILES
Came up with 0P0300 code. I changed coils, plugs, TCS, MAP, injectors, cleaned EGR valve, and still has severe misfire. Also, I lost acceleration going up hills, and engine shudders when I push the gas. Help!
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Friday, July 8th, 2022 AT 2:46 PM

12 Replies

Tiny
AL514
  • EXPERT
Hello, it looks like you have changed a lot of parts, have you cleaned the Mass Air Flow sensor? Most engines have either a MAP or a MAF, but it looks like your car has both. The Mass Air Flow sensor is in the air intake tube going to the throttle body. I'll put a diagram of it below, but the MAF has a resistor on a hot wire that measures air flow into the engine. If it is dirty, it will under report the amount of air coming into the engine and there will be a lack of fuel delivery. You can buy Mass Air Flow cleaning spray to clean the sensor. If cleaning the Mass Air Flow sensor doesn't help, we can look into some other issues. But what did the spark plugs look like when you took them out? Were they all black with soot and carbon on them?

https://www.2carpros.com/articles/mass-air-flow-service

https://www.2carpros.com/articles/engine-has-low-power-output
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Friday, July 8th, 2022 AT 4:03 PM
Tiny
MORGRETBUSHIDO
  • MEMBER
All plugs were black, and I pulled the MAF out today and I'll get cleaner tomorrow. Check engine light has been on and flashing.
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Friday, July 8th, 2022 AT 4:10 PM
Tiny
AL514
  • EXPERT
Okay, the flashing means that the misfires are bad enough to cause Catalytic Converter damage. That might be a concern as well. This car has each set of three fuel injectors powered up together, were there any plugs that looked worse than the others? I would inspect the throttle plates while you have the air intake boot off. But if there is a Catalytic Converter failure here preventing the engine from being able to take in enough air, it will cause the plugs to be covered in soot like that. Doing a back pressure test at the front Oxygen sensor location would tell if the Converter has melted down and is blocking off the exhaust. If you don't have the means to do a back pressure test, just removing the Front Oxygen sensor and starting the car will release some of that pressure and you will notice a difference in the car running condition. It will be loud and you can't drive it that way, but if you can rev the engine up better and it seems to run better, then that might point to a Catalytic Converter failure. I'm not sure how many Converters this car has being a V6 but having the front Oxygen sensor out should be a noticeable difference.
If you don't notice any difference, doing a fuel pressure test would be next. But I think with so much carbon build up on the spark plugs, it sounds like a lack of air or too much fuel. A bad Catalytic Converter will cause a lack of air because the exhaust is having trouble escaping. Start with cleaning the MAF sensor first though.

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

https://www.2carpros.com/articles/bad-catalytic-converter-symptoms

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator
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Friday, July 8th, 2022 AT 4:16 PM
Tiny
MORGRETBUSHIDO
  • MEMBER
Yes, but I changed them over a week ago so I can't remember which were worse. But some were blacker than others. I know cylinder six was probably the worst.
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Friday, July 8th, 2022 AT 4:24 PM
Tiny
AL514
  • EXPERT
Okay, here is a wiring diagram of the Fuel Injectors, you will notice they are powered up by each engine bank. These can be checked with a 12-volt automotive test light. Power and control. I'll put the guide for testing the injectors too. Just to make sure you're not having a wiring issue to the injectors.

When cranking the engine over, does it sound like an equal cranking type sound, nothing noticeable like a weak cylinder? With compression issues you can notice an engine cranking that sounds a bit off.

https://www.2carpros.com/articles/how-to-test-a-fuel-injector

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

https://www.2carpros.com/articles/engine-has-low-power-output
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Friday, July 8th, 2022 AT 4:35 PM
Tiny
MORGRETBUSHIDO
  • MEMBER
I do know that I pulled the connectors off of each injector and the front right and back left injectors made no difference in the engine performance.
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Friday, July 8th, 2022 AT 4:36 PM
Tiny
AL514
  • EXPERT
Okay, good, so you did a cylinder balance test. I was going to ask you to do that.
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Friday, July 8th, 2022 AT 4:39 PM
Tiny
MORGRETBUSHIDO
  • MEMBER
Would those 2 not making a difference be an issue with the wiring?
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Friday, July 8th, 2022 AT 4:41 PM
Tiny
AL514
  • EXPERT
It means that those two cylinders are not contributing to engine power output. Checking them with a test light to verify they're getting power and are being controlled by the ECM will at least rule that part out. Do you hear them clicking if you put a long screwdriver up to them and listen? They should sound the same as the other injectors. So, you know so far that you have two cylinders down. But follow the Fuel Injector test guide and see what you get. Since you replaced all the injectors, you may have had a couple of the old ones that were shorted out, and when fuel injectors short out they can burn the ECM driver for those injectors. This will cause the new ones you put in to not function at all. When you do the control test with the test light, it will not flash.

Do you have the old fuel injectors still?

https://www.2carpros.com/articles/how-to-test-a-fuel-injector
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Friday, July 8th, 2022 AT 4:55 PM
Tiny
MORGRETBUSHIDO
  • MEMBER
Yes, I still have the old injectors.
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Friday, July 8th, 2022 AT 7:01 PM
Tiny
AL514
  • EXPERT
If you have a multimeter, you can check the old fuel injectors' resistance and make sure they are all reading about the same, somewhere around 14 ohms, although sometimes an injector will show its problem when it is hot, but a static check couldn't hurt. If there's one that's reading very low resistance, that could point to an issue.
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Friday, July 8th, 2022 AT 7:11 PM
Tiny
MORGRETBUSHIDO
  • MEMBER
I believe my brother-in-law has one. Thank you for all your help and insight. I'll dig into all those potential solutions in the morning.
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Friday, July 8th, 2022 AT 10:20 PM

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