The place to start is by having the diagnostic fault codes read and recorded. The people at many auto parts stores will do that for you for free. The problem is though, a sensor reading has to go outside of a specific voltage range for a code to set. That sensor can sometimes report the wrong value but one that is acceptable to the Engine Computer. In that case your mechanic will have to look at live data on a scanner to see what doesn't look right.
Another thing to consider is a vacuum leak or a leak in the fresh air tube between the air filter box and the throttle body. Every manufacturer other than Chrysler uses a mass air flow sensor to measure the weight of the incoming air. A leak anywhere that lets air sneak in without being measured will result in no gas to go with it. That will cause a lean condition and typically a hesitation. Once the coolant temperature reaches a certain point, the system goes into "closed loop", meaning the computer adds the oxygen sensor readings to its list of things it uses to calculate fuel metering. That can result in the computer requesting extra fuel to get the lean mixture back to where it should be, and the hesitation will be gone.
Friday, June 19th, 2015 AT 8:14 PM