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Engine Hesitates, Stumbles, Lacks Normal Power

The computer uses this information to determine how much fuel is needed to maintain the correct air/fuel mixture, and when extra fuel is needed if the throttle suddenly opens wide.
Symptoms:
  • Engine Hesitation
  • Lacks Power (Figure 6)
  • Throttle Position Sensor (TPS) full range test (Figure 7)
  • Mass Airflow (MAF) Graph and Fuel Trim Readings
Probable Causes:
  • Lean Fuel (P0171, P0174)
  • Throttle Position (P0120 or P0124) or (P0222 or P0229)

An engine that hesitates, stumbles or misfires when accelerating or when it is under load, is an engine that is either sucking too much air, not getting enough fuel, or misfiring. If the Check Engine Light comes on, you may find any of the following codes:

  • P0171, P0174 Lean fuel condition codes
  • P0120 to P0124 Throttle position sensor codes
  • P0222 to P0229 Throttle position sensor codes
  • P0400 to P0409 EGR related codes

If there are no misfire codes, a common cause of acceleration stumble is a bad throttle position sensor (TPS). The TPS tells the computer how far the throttle is open. The computer uses this information to determine how much fuel is needed to maintain the correct air/fuel mixture and when extra fuel is needed if the throttle suddenly opens wide. Figure 6 shows a properly functioning TPS.

Figure 6

Figure 6

Another common cause are dirty fuel injectors. If varnish deposits have built up in the tips of the injectors, they won’t spray as much fuel as they normally do, or will “dribble” fuel instead of spraying a fine mist. This creates a lean fuel mixture and conditions that are ripe for stumble and hesitation (also misfire). Look at short term fuel trim (STFT) and long term fuel trim (LTFT) with your scantool. If the numbers are high, it tells you the engine is running lean and the injectors need cleaning. Treat mild cases with a high quality fuel-injector cleaner additive. Severe cases require professional cleaning equipment. Other problems that cause acceleration stumble include vacuum leaks, low fuel pressure, a weak spark caused by low coil voltage or bad coil(s), retarded ignition timing, and contaminated gas. Look at the following with your scantool (as shown in Figure 7): throttle position, mass airflow (MAF), short term fuel trim (STFT), long term fuel trim (LTFT), ignition timing, and fuel pressure (if a PID is available). Throttle Position Sensors (TPS) typically wear in the idle and just above idle positions, but they may also have dead spots at any point in their range of travel. With the key on, engine off, graph the sensors output while slowly opening the throttle all the way. The graph should look like a relatively smooth ramp, with no suddenly drops or flat spots.

Figure 7

Figure 7