Engine Hesitation Problem: Car Hesitates When Accelerating

Engine Related Issues

By Rodney (Mechanics)

Hesitation is when the engine stumbles, misfire or when you step on the throttle or you accelerate it lacks of power. The issue occurs most generally due the air/fuel mixture is going lean or isn’t being enriched properly. Or when the air/fuel mixture going to lean or the engine comes under load, the ignition system is weak and misfiring. There more air will be suctioned by the engine when you step down on the accelerator and the throttle opens. The computer must respond by adding more fuel.

If a speed-density type of fuel injection system (don’t have airflow sensor) is in the machine, to estimate how much fuel the engine needs and airflow, the inputs from engine RPM, air temperature sensor, manifold absolute pressure sensor and the throttle position sensor will be used by the computer.

NOTE: When compared to EFI systems that use airflow sensors, the speed-density system is much less sensitive to vacuum leaks.

When the engine comes with mass airflow or vane airflow, from the airflow sensor, it looks primarily at the airflow signal, but likewise takes to account what MAP sensor (when equipped) and the throttle position sensor are telling it.

NOTE: The sensitivity of airflow EFI system’s to air leakage downstream of the airflow sensor and the vacuum leak is very high.

This will result in the engine’s computer probably not adding enough fuel, due to the missing or inaccurate input of one of these sensors, it can causing misfire due the fuel mixture going to lean, so when opening or accelerating the throttle will produces a stumble or hesitation.

If fuel pressure is low or the fuel injectors are dirty, the amount of fuel added by the computer when the throttle opens may also be insufficient. To maintain the proper air/fuel ratio, the air/fuel mixture will be monitoring by oxygen sensors in the exhaust, so fuel trim can be adjusted as needed by the computer.

When a leak occurs, the computer tries to compensate for the extra air being sucked into the engine because the vacuum leak usually causes the fuel trim to run rich.

Fuel Trim Diagnose
Fuel Trim Diagnose

Engine Stumble Or Hesitation Causes:

BAD Gasoline

It can be the fuel too much alcohol or contaminated with water

Vacuum Leaks

Including EGR valve, throttle body, vacuum hoses, intake manifold.

Dirty Fuel Injectors

To fix this problem, you should clean the injector as often as possible.

Fuel Pressure is Low

Including the fuel pump running slow due charging voltage or low system voltage or weak fuel pump, fuel pressure regulator leak.

Dirty or BAD Mass Airflow Sensor (MAF Sensor)

Need to run a complete computer diagnostic, a particular code will be generated if there bad MAF sensor found. low fuel pressure, low vacuum or low compression is most likely causing this problem.

BAD Throttle Position Sensor (TPS Sensor)

If TPS Sensor found faulty, remove and replace it.

BAD Manifold Absolute Pressure Sensor (MAP Sensor)

Sometimes need professional technicians to do a complete engine diagnostic to find out the cause of problem.

4 Things That Causing The Ignition Misfire:

Sometimes, rather than lean misfires, ignition misfire feels like a hesitation.

  • Wet plug wires
  • Weak ignition coil
  • Bad plug wires
  • Worn or dirty spark plugs

How To Diagnose Problem Of Engine Hesitation

To diagnose the problem need a scan tool too checking the engine computer for misfire codes and any other fault codes. By looking at the various sensor PIDS (the scan tool will display sensor values) using a scan tool to checking a sensor response. If the sensor is not responding normally or the values are out of range use scope or DVOM to testing sensors. Inspecting and removing the spark plugs, measuring fuel volume and pressure, cleaning/inspecting the EGR valve, including searching for vacuum leaks is another diagnostic check that needs to be done.

To see the engine is running lean, check the fuel trim readings using a scan tool. The most noticeable effect at idle caused by vacuum leaks is lean mixtures. A vacuum leak that results in a little extra air entering into the engine does not have any meaningful effect because there is so much air entering into the engine when at part and full throttle.

Start the engine and into the diagnostic connector plug the scan tool. then take a look and note the values of Long Term Fuel Trim (LTFT) and Short Term Fuel Trim (STFT). Plus or minus 8 is typically a normal range. Engine is running lean if values of the STFT (Short Term Fuel Trim) and LTFT (Long Term Fuel Trim) higher than +10. For a minute or so rev and hold the engine at 1500 to 2000 RPM, take a look on STFT, if the values is drop back to the more normal reading, it tell you at idle there a vacuum leaks on the engine. However, if there are no change on STFT values, fuel delivery is probably the problem that causing the lean fuel condition (a leaky fuel pressure regulator, weak fuel pump or dirty fuel injector).

Tips For Lean Trouble Codes P0171, P0174 (Diagnostic Of Lean Trouble Codes)

Engine is running lean when you have both or just one of the code P0171 or P0174 showing on your diagnostic tool. It telling you there is not enough fuel or/and to much air.

You can test and confirm the engine is running lean by checking n the STFT at idle, make sure your scan tool can display Short Term Fuel Trim (STFT). The engine is running lean if STFT values output is 10 to 12 or higher. You can suspect the vacuum leak that causing lean problem if the STFT values dropped 3 or 4 points or more. However, if no change on STFT values (still same values), probably low fuel pressure or sensor-related (bad MAP sensor or bad or dirty MAF sensor) is the problem. Check the fuel pressure and next take a look at the airflow and MAP sensor readings.

Fuel Trim Diagnose Code P0171 or P0174
Fuel Trim Diagnose Code P0171 or P0174

What causing lean fuel condition?

Defective or Dirty Mass Airflow Sensor (MAF).

Use aerosol electronics cleaner to clean up the filament or MAF sensor wires. Do not touch the sensor wires or do not use another to clean the sensor.

PCV Valve or Hose Leak.

Check the connection of valve and hose.

EGR Valve Leak

Check the EGR valve operation.

Vacuum leaks

You need to check the vacuum leaks at throttle body, the intake manifold or vacuum hose connections.

Dirty Fuel Injectors

Clean the injectors should help fix the problem.

Fuel Pressure is Low

Leaky fuel pressure regulator or weak pump may be causing the low fuel pressure, at idle check the fuel pressure using fuel pressure gauge.

INFO: Differential Pressure Sensor (DPFE) causing lean code P0171 or P0174, or both sometimes appear on many Fords. EGR flow monitoring by DPFE sensor, the sensor located near EGR valve on the engine. from the exhaust manifold to the EGR valve, the sensor connected to the tube uses two hoses. The computer will increase the EGR which has leaning effect on the fuel mixture if the sensor misreads the EGR flow. Replacing the DPFE sensor is way to fix it.

Hesitation caused by many things, so to repair the problem it depend on source of the problem. For example, if hesitation caused by fuel related, fixing a vacuum leaks or cleaning the fuel injectors is required to repair the problem. And replacing the plug wires and the spark plug is required if the cause is ignition related.