Chevy Engine Hesitation - What's Going On?

When your engine is hesitating, it is usually a symptom of a problem with the air to fuel ratio mixture. It can be because the mixture is too lean or there is a problem with the ignition system that is creating the misfire when the engine comes under load.

Chevy silverado

As you apply the throttle, the engine draws more air in, and your car’s computer should provide more fuel to the mixture. In a speed-density fuel injection system, the computer gathers its information from the throttle position sensor, air temperature sensor, manifold absolute pressure sensor and the RPMs of the engine in order to determine the correct amount of fuel needed. These types of systems are less sensitive to a vacuum leak than an EFI system that uses airflow sensors.

If any of the data coming from those sensors is missing or incorrect, the Chevy’s computer will be unable to accurately determine how much fuel to mix with the air. This can cause a hesitation or a stumble when the throttle opens or you accelerate.

Additionally, if any of the fuel injectors are dirty or there is low fuel pressure, the mixture can be insufficient. The fuel trim is adjusted by the oxygen sensors that are in the exhaust system. The fuel trim will compensate for a dirty injector or a low fuel problem, but the response is usually too slow to keep the vehicle from hesitating.

A vacuum leak will cause the mixture to run rich because the computer notices the extra air and adds fuel to the mixture.

Possible Hesitation Causes

  • Manifold Absolute Pressure (MAP) sensor is bad
  • Throttle Position (TPS) sensor is bad
  • Fuel injectors are dirty – keeping these clean will eliminate this problem
  • Mass Airflow (MAF) is dirty or bad
  • Fuel pressure is low – possibly from leaks in the regulator or a failing fuel pump
  • Vacuum leak in intake manifold, throttle body, vacuum hoses or EGR valve
  • Bad gas that has been contaminated with an excess of alcohol or has water in it
  • Clogged catalytic converter/exhaust system

There are times when you may think that your vehicle is hesitating when it is really misfiring. A misfire can be caused by:

  • Plug wires are bad
  • Spark plugs are dirty or worn out
  • Ignition coil weak
  • Plug wires are wet

Diagnosing Hesitation

One of the first places to begin your diagnosis is with the engine’s computer. You need a scan tool in order to read any fault codes. Check the various sensors PIDS, and if the values are out of range, use a DVOM or scope on them to determine the cause of the reading. You will want to check for vacuum leaks, measure the fuel pressure and its volume, inspect and clean the EGR valve and check the spark plugs for wear or dirt.

Check the readings of the fuel trim with a scan tool in order to determine whether the engine is running too lean. A lean mixture can be caused by a vacuum leak and will be most noticeable when the vehicle is at idle. It is not noticeable when the throttle is full or partly open since there is already air coming into the engine.

  1. Plug the scan tool in, and then start the engine.
  2. Look at the STFT (Short Term Fuel Trim) and LTFT (Long Term Fuel Trim) values. A normal range is plus or minus eight. Any number plus 10 or more for LTFT and STFT means that the engine is running lean. A vacuum leak is confirmed if you rev up the engine to 1500 to 2000 RPMs for a minute and the STFT value drops to a normal range. If you see no change in the STFT when you rev up the engine, then the problem is probably in the fuel delivery. It could be a restricted fuel filter, dirty fuel injectors, leaking fuel pressure regularly or a bad fuel pump.

P0171 or P0174 Lean Trouble Code Diagnostic Tips

If you see either of these codes, you have a lean running engine.

You can confirm this by using your scan tool to check the STFT at idle. If the reading is greater than 10 or 12, then you are running lean. Rev the engine up to 1600 to 2000 for a minute or two and recheck the value. If it drops three or more points, then there is a vacuum leak. If the value stays the same, then there is most likely a sensor problem. It could be a bad or dirty MAF sensor, or the MAP sensor has gone bad. In addition, it could be due to the fuel pressure being low. Check the airflow and MAP sensor readings with the scan tool. You can also use it to check the fuel pressure.

Reasons for lean fuel conditions:

  • Low fuel pressure – this can be due to a weak fuel pump or a leaky fuel pressure regulator. Check the fuel pressure with a fuel pressure gauge while the vehicle is at idle.
  • Fuel injectors are dirty – clean the injectors.
  • Vacuum leak – the leak can be in the vacuum hose connections, the intake manifold or the throttle body.
  • EGR valve leaks – check the EGR valve.
  • PCV valve or hose leaks – check your hoses and valve connections.
  • MAF sensor is dirty or bad – clean the sensor filament or wires with an electronics cleaner. Do not touch the wires or use anything but the aerosol electronic cleaner.

A fuel related repair includes cleaning the injectors or fixing vacuum leaks. An ignition related repair includes changing spark plugs and wires.