Chevrolet Cobalt Service & Repair Manual: Powertrain Drivability Diagnosis & Testing

Chevrolet Cobalt Service & Repair Manual / Engine Service / Powertrain Drivability Diagnosis & Testing

A visual inspection must always be performed to confirm the presence of aftermarket components, potential collision damage, or areas of concern.
Detailed information from the customer is key is duplicating and diagnosing a performance concern. Use of a customer interview sheet can assist in pinpointing the cause of the concern.
Whether the customers concern is an emission or drivability concern, the troubleshooting approach is essentially the same. Since emission and drivability concerns are often related and many times originate from the same cause. Concentrate only on the sub-systems that can directly or indirectly cause the emission or drivability symptom experienced.
1. Duplicate drivability concern and document, when they occur, how frequently they occur, as well as other details that may provide information as to cause of concern.
2. Perform following basic inspections:
  a. Verify adequate cranking and engine running battery voltage.
  b. Inspect air filter for excessive dirt or contamination, clean as required.
  c. Inspect and adjust idle speed, as required.
  d. Confirm base timing is properly set to specifications.
  e. Confirm that fuel pressure is within proper range.
  f. Perform inspection of engine mechanical condition.
  g. Perform visual inspection of accessible electrical connections, vacuum and air induction ducting.
  h. Locate and inspect condition of ECM/PCM main grounds.
  i. Inspect for leakage in EGR and PCV valves.
  j. Inspect for unwanted fuel entering intake manifold from EVAP system, fuel pressure regulator diaphragm, or crankcase ventilation system.
3. Connect a suitably programmed scan tool and perform OBDII diagnostic test functions.
4. Retrieve and record diagnostic trouble codes (DTC's). Follow appropriate DTC chart for codes retrieved.
5. Verify engine speed, coolant temperature, load, throttle position, intake air temperature, and oxygen sensor signals are within normal range.
6. Verify feedback system is maintaining normal closed loop control.
7. Common vehicle performance concerns and causes are as follows:
  a. Bucking: This term describes a severe, back and forth jerking motion of vehicle shortly after acceleration or deceleration. Severity of this symptom is much greater than that of a surge, to point of which it physically jerks vehicle back and forth. If bucking condition is severe, movement of vehicle may cause drivers foot to jerk up and down on accelerator pedal further exaggerating condition. Common causes of bucking include fluctuating air/fuel mixture, improper ignition/spark advance system operation and excessively sensitive accelerator pedal linkage.
  b. Poor acceleration: Poor acceleration is a general term used to describe any acceleration condition that is slower than expected. This symptom can originate from many different causes. Most of causes of poor acceleration result from either mechanical or engine related concerns. Common causes of poor acceleration include excessively lean or rich air/fuel mixture, restricted intake air system, incorrect ignition timing advance, restricted exhaust system, improper torque converter operation and dragging brakes.
  c. Hard Starting: Hard starting is a term used to describe an engine that requires an excessive amount of time to start, repeated cranking attempts, or starts but stalls shortly thereafter. Some of more common causes include: excessively lean air/fuel mixture during cold start-up, low battery voltage, poor starter performance, ignition system problems, restricted intake air system, excessively high engine oil viscosity and excessively rich air/fuel mixture.
  d. Rough Idle: Rough Idle describes an engine idle characteristic that is unexpectedly harsh or unstable. An excessively rough idle can even cause engine to stall. Common causes of rough idle include: low engine idle speed, excessively lean or rich air/fuel mixture, improper ignition system operation and improper EGR system operation.
  e. Engine Stall: Engine stall describes a condition where engine unexpectedly stops while running. Engine stall can occur at any time; however, it is most commonly described as a condition where engine dies just after startup, completion of fast idle, decelerating to idle, or shifting. Some of more common causes include: Excessively low idle speed adjustment, excessively lean or rich air/fuel mixture, intermittent ignition system operation, and improper air bypass operation.
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