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Join Date: Apr 2009
Location: Cape Cod
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If the spark plugs are sooty, most likely you are running rich, It makes sense that it runs better cold when it needs a rich mixture. If that is true, you need to find out what makes it run rich / too much fuel. The O2 sensor code might be telling you something. It could have coded earlier but now the parts has fully failed and is giving worst data to the computer. I'm old school and I read spark plugs to tell me a lot of motor issues.
You need good clean spark plugs for a motor to run smooth.
I also use fuel injector cleaners in the gas tank a few times a year, It helps keep the motor running great and keeps the MPGs up to a new car level..
Does your car have an " Idle air control valve?? Is the throttle body clean? Air filter clean?