2002 pontiac Grand Am SE

Im a women so bear with me please 🙂
Okay, so i bought the car 6 months ago. For the first 2 months it drove beautifully. Then the bad upstream o2 sensor came on and I lost about 5 mpg. Replaced it, 2 weeks later a new code popped up, bad catalytic converter, and more mpg loss and power loss. I just now got it replaced (The last guy (illegally) put a pipe through it.) I had not known this until replacement. The lost hp is defiantly back, she sounds great. Although I am literally getting 10 mpg. Its been a slippery winter Michigan but something is wrong. And when I fully stop at a light, foot all the down on brake, the car tries to lurch forward. It is a tiny little jump and could have gone unnoticed, I’m not sure.
The air filter is good, the spark plugs are new, it needs a oil change soon. Everything else is mystery.
Why does it jump forward?
Why did the car run so good if there never was a catalytic converter?
Why is my mileage so terrible?
I have the Haynes manual and no car lifters, I can squeeze under the car, but there’s no room to work.
Please and thank you very much!!! I am poor and have no one to help me with car stuff so I don’t want to go and have a repair guy lie to me.

1 thought on “2002 pontiac Grand Am SE”

  1. 1) Why does it jump forward?

    My first thoughts make me wonder if the brakes seem to stop normally or if you have to put everything you have on your foot to stop the car. If the brake pedal is hard and doesn’t stop the car normally, i would look for a vacuum leak at or around the power brake booster.
    If the brakes are working properly, then i would think that the Lock Up converter in the transmission is not disengaging properly. To test this theory, when you first start it up and go to drive, put the shifter in low and drive a short ways up the street and stop and see if it no longer jerks forward. then put it into Drive and drive over 35 mph and then stop as you normally would and see if the jerking forward is there. If it is, you are probably looking at having the converter replaced.

    2) Why did the car run so good if there never was a catalytic converter?

    The catalytic converter does nothing to help the performance of the engine. It is there only to conform to Governmental Regulations for Emissions. It is actually a restriction on the performance(power not fuel mileage) of an engine. The upstream O2 Sensors control how much fuel to inject into the engine based upon how efficient the fuel is being burned. The down stream, or after the converter, O2Sensors job is just to monitor the functionality of the converter and has no bearing on fuel mileage either.

    3) Why is my mileage so terrible?

    Could be a number of things. If you have a vacuum leak on the engine. (normally would see a slightly higher idle). Letting the car run and warm up uses more fuel. Extremely cold weather uses more fuel to keep the Air/Fuel Ratio constant as colder air more dense it requires more fuel to achieve this ratio. If your check engine light is still on, you may still have something else going on under hood. if you post the codes here in a comment below i would be happy to help you address those as well. Make sure when you turn your key of before you start it that the light comes on and then goes off a few seconds after you start the engine.

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