Mechanics

MODELS ENGINE BUCKS AT 40 - 60 MPH

2009 Chevrolet Other

Engine Mechanical problem
2009 Other Chevrolet Models 6 cyl All Wheel Drive Automatic 9000 miles

This is a 6-speed auto, and has two problems. Erratic I might add. Traveling at a steady speed and in 6th gear the car will " buck" a few times until I step on the accelerator a little more, then it is fine.
It also will hesitate to downshift to 5th at times.
All this comes about when the vehicle has been run steadily at highway speeds. Note, no problem until you require a downshift. It is not a fuel problem, that has been eliminated.
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Zhangman
August 24, 2009.



Dirty fuel injectors (cleaning the injectors often fixes this).
Bad MAP (manifold absolute pressure) sensor
Bad TPS (throttle position) sensor
Bad or dirty MAF (mass airflow) sensor
Low fuel pressure (leaky fuel pressure regulator or weak fuel pump)
Vacuum leaks (intake manifold, vacuum hoses, throttle body, EGR valve)
Bad gasoline (fuel contaminated with water or too much alcohol)

Sometimes, what feels like a hesitation is actually ignition misfire rather than lean misfire. The causes of ignition misfire may include: Dirty or worn spark plugs
Bad plug wires
Weak ignition coil
Wet plug wires

Rasmataz
Aug 24, 2009.