Engine Stumbles or Hesitates on Acceleration

ENGINE STUMBLES OR HESITATES ON ACCELERATION

Description
Upon accelerating, the engine stumbles, hesitates, backfires, or dramatically looses power.

Explanation
For proper acceleration additional fuel and ignition timing advance are required. If either is not increased, hesitation or backfire through the intake may result.

Causes

Low Fuel Pressure
Low fuel pressure may produce a lean Air/Fuel Mixture. Although the fuel control system may add additional fuel during acceleration, if the initial air/fuel mixture is too lean, acceleration problems may still result.

Testing
- Check Fuel Pressure.

Defective Throttle Position Sensor (TPS)
The engine control system uses the throttle position sensor to determine driver demand. Additional fuel and timing advance are added when a rapid increase in throttle angle is sensed. Glitches or rough spots in the TPS output may produce severe hesitation during acceleration.

Testing
- See Computers and Control Systems for testing. Computers and Control Systems

Ignition Problems
Problems with the ignition system which are not noticed during normal operation may become evident during acceleration. The richer fuel mixture during acceleration requires a stronger spark to ignite than normal combustion. Existing weaknesses in the ignition system may be exposed during acceleration.

Testing
- Check ignition coil output. Spark should be thick and blue. Ignition Coil
- Inspect all spark plugs. Check for fouling, excessive wear, cracked insulator, and proper gap.
- Inspect ignition cables for signs for insulation damage or arcing.
- Inspect ignition cables for high resistance readings or corroded terminals.
- Inspect the distributor cap for signs of cracking or arcing.
- For additional testing see Ignitions System. Ignition System

Defective PCV
During acceleration, the PCV valve is designed to close. If the valve remains open, the engine may hesitate.

Testing
- Remove and inspect the PCV valve, verify it is not stuck open. Positive Crankcase Ventilation
- See Fuel Deliver and Air Induction for further testing. Fuel Delivery and Air Induction

Engine Control System
Problems with the engine control system may prevent proper fuel delivery or timing advance during acceleration.

Testing
- See Computers and Control Systems for testing. Computers and Control Systems