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Q: losing acceleration

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My husband's 2009 GMC Acadia is stalling and loses acceleration when he drives anything over 45 or 50 mph. He got the codes read which indicated he needed a tune up which has been done (replaced the spark plugs ect..) and still continues, any idea what it could be? Fuel filter or fuel pump ?

My car has 120000 miles.
My car has an automatic transmission.

If the codes say he needs a tune up, I will have to assume the engine is misfiring and is sending codes to the misfiring cylinders. Also, the system may be in a failure mode which will limit speed. You should have the vehicle diagnosed by a certified mechanic to check the system to determine the cause of the misfire and why your speed is limited. You may have had a tune up but the problem may be in the ignition coils or a sensor is failing.

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