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Q: My car is jerking back and forth at high speeds

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Hi, I recently bought a car from a private seller and the check engine light has been on. When I go on the highway and speed up the car jerks back and forth. It does it almost every time I speed up but after I travel about 20 miles or maybe a little bit more it doesn't do it. What could possibly be wrong with it? Will it cost me a lot of money?

My car has 110800 miles.
My car has an automatic transmission.

A: The first thing that needs to be done is to...

The first thing that needs to be done is to connect a scanner to the car and retrieve the code. This information will lead a technician down a diagnostic path. There is an important distinction that needs to be made here. Does your car "jerk back and forth" while accelerating or at a cruise when the throttle and speed are held steady? These are different circumstances in regards to what could be wrong.

Surging at a cruise is usually a sign of a lean running condition caused by fuel system components. This could be the MAF, an injector, fuel pressure regulator, or fuel pump. Bucking and jerking while accelerating is usually do to ignition components. If there is a vacuum leak or the fuel pump can’t supply enough fuel, you can get similar symptoms. The problem could be a mixture of several of these.

As for why it doesn’t happen after twenty miles, this could be because a motor needs more fuel to run when it is cold. If the fuel system is performing as it should, this could easily be the cause. Several components have warm up cycles that may not be working correctly. If they warm up to slowly, numerous drivability issues can arise.

I recommend having your Check Engine Light diagnosed by a qualified technician, like one from YourMechanic, to help you figure this out.

Good luck!

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