Engine Hesitation – What Is It – What Can Cause It – Diagnosing It

Engine Hesitation - What Is It - What Can Cause It - Diagnosing It
Engine Hesitation - What Is It - What Can Cause It - Diagnosing It

Engine hesitation can make your vehicle seem like its; bogging down, misfiring or stumbling when you hit the gas pedal.

When you step on the gas pedal; you will suddenly feel like the power has been lost in the engine.

Consequently, After the engine hesitation it goes back to normal; but it may take a second or two to respond.

Everyone wants their vehicle to accelerate smoothly; so it can be frustrating when that does not happen. Engine hesitation is a symptom that shows, an imperfect internal combustion in your engine.

Insufficient Burning
Insufficient Burning

Insufficient burning inside the engine, will make the vehicle hesitate when driving. This could happen during acceleration, driving downhill, or when changing gears.

The Diagnostic Process

When you’re trying to diagnose a hesitation or stumble; it’s important to work your way through, the diagnostic process carefully and logically. Don’t jump to conclusions and don’t assume the worst. Start with the simple stuff first (i.e. the air filter) and continue from there.

Also, remember that when interpreting any trouble codes; you need to be able to read the signs, and determine why a sensor is sending an out-of-spec reading. Sometimes, a problem might trigger a string of several trouble codes; all of which need to be analyzed.

Common Causes Of Engine Hesitation Include:

  • Dirty Air Filter
  • Stuck Or Failed Mass Airflow Sensor (MAF)
  • Clogged Or Leaking (EGR) Valve
  • A Failing Ignition Coil
  • A Failing Throttle Position Sensor (TPS)
  • Clogged Fuel Injectors
  • Dirty Or Clogged Fuel Filter
  • A Failing Oxygen Sensor (O2)
  • Catalytic Converter

Dirty Air Filter

Your engine needs air, spark, and fuel to run. But, the air part of that equation can be compromised if the air filter is clogged and dirty. It will literally smother the engine, hurting fuel economy and acceleration. Fortunately, this is the easiest one to fix. Just take the old air filter out of the air box , inspect it, and drop a new one in.

Stuck Or Failed Mass Airflow Sensor (MAF)

This is the next part to look at if you’ve ruled out the air filter. The (MAF) sensor monitors the amount of air entering the engine. It then sends that information the engines computer, so it can set fuel metering accordingly. A stuck or failed (MAF) sensor will throw these readings off, and may result in stumbling and hesitation. This is one that may or may not register a trouble code.

Clogged Or Leaking (EGR) Valve

The exhaust gas recirculation (EGR) valve is an emission-control device. It directs a portion of exhaust gases back into the engine to be reburned. When this valve becomes too saturated with carbon, it can begin to stick.

Carboned Up (EGR) Valve
Carboned Up (EGR) Valve

Also, its position sensor can fail and send an out-of-spec reading to the engine computer. Consequently, activating the Check Engine light (this commonly registers as trouble code P0406).

So, When this sensor is sending voltage readings that are too high or too low, it can result in:

  • Engine stumbling
  • Engine hesitation
  • Surging at part throttle and light load
  • Poor fuel economy
  • Rough engine idle
  • Heightened emissions

A Failing Ignition Coil

Just about all vehicles from model year 2000 on are equipped with ignition coils on each spark plug. When a coil starts to fail, it can cause hesitation, stumbling, and surging. And, as it gets worse it can result in bucking and an extremely rough-running condition. The good news is that coils are easy to access and replace. Usually requiring nothing more than a Phillips-head screwdriver or a small nut driver.

A Failing Throttle Position Sensor (TPS)

This sensor informs the engine computer of how much pressure is being put on the accelerator pedal. And, how open the throttle itself is. The computer then adjusts ignition timing and fuel metering accordingly. When this sensor is starting to fail, the computer’s strategies are thrown off. So, the engine will have trouble maintaining an idle or responding to the accelerator. This one will usually trigger a trouble code and illuminate the (CEL).

Clogged Fuel Injectors

These tiny nozzles deliver a spray of fuel into the cylinder; where it mixes with air for compression and combustion. Usually, fuel injectors last for the entire life cycle of an engine. But, they do become clogged or start to fail in some instances. A failing or dirty fuel injector can cause a regular misfire; which can eventually escalate to stumbling and hesitation.

Dirty Or Clogged Fuel Filter

The fuel filter can cause loss of power, poor fuel economy, stumbling, hesitation, or even a no-start condition. Unfortunately, this part is located in the fuel tank on most newer vehicles. But, If the vehicle has an external fuel filter  it’s inexpensive just replace it. Likewise, a failing fuel pump (also mounted in the tank) can cause fuel starvation; especially when the vehicle is heading up an incline.

A Failing Oxygen Sensor (O2)

The oxygen (O2) sensors) are positioned in the exhaust stream. Usually with one closer to the exhaust manifold and one further down the exhaust pipe.

Engine Hesitation - What Is It - What Can Cause It - Diagnosing It
Engine Hesitation – What Is It – What Can Cause It – Diagnosing It

It monitors the oxygen content of the exhaust in proportion to the other gases. Consequently, sending this information back to the engine computer; for emissions control and fuel-metering decisions.

A failing (O2) sensor will cause:

  • A rich-running condition
  • Black smoke from the tailpipe
  • Poor fuel economy
  • Loss of power
  • Engine hesitation and stumbling

Catalytic Converter

Since the ’70s, the catalytic converter; has been one of the primary emission control systems. The catalytic converter is located in the exhaust pipe; and runs at extremely high temperatures. As a result, it can literally incinerate certain noxious exhaust gases; before they can make it out the tailpipe. Catalytic converters usually last the entire service life of the vehicle; but they’ve been known to fail. A failing catalytic converter will result in loss of power; poor fuel economy, and stumbling.

Diagnosing Engine Hesitation

Start by checking the engine computer with a scan tool for any fault codes. A code P0171 or P0174 (or both) indicate the engine is running lean. This means there is too much air and/or not enough fuel. Consequently, the air/fuel mixture is not being properly enriched or is going lean. Possibly, the ignition system is weak and is misfiring, when the engine comes under load.

Automotive Sensors - Why Are Sensors So Important Today
Automotive Sensors – Why Are Sensors So Important Today

Check sensor responses with a scan tool, by looking at the various sensor PIDS. Consequently, if the inputs from any of these sensors are inaccurate or missing; the engine computer may not add enough fuel. As a result, causing the fuel mixture to go lean. And, causing a misfire that produces a hesitation or stumble; when accelerating or opening the throttle.

Other Considerations:

  • The amount of fuel added by the computer when the throttle opens; may also be insufficient, if the fuel injectors are dirty or fuel pressure is low.
  • The oxygen sensors in the exhaust monitor the air/fuel mixture; so the computer can adjust fuel trim as needed to maintain the proper air/fuel ratio.
  • Fuel trim adjustments can compensate for dirty injectors and/or low fuel pressure to a certain extent; but occur too slowly to offset a throttle hesitation problem.

Additional diagnostic checks may include searching for vacuum leaks; inspecting/cleaning the (EGR) valve, measuring fuel pressure and volume; removing and inspecting the spark plugs, etc.

Image Of Four Used Spark Plugs With Black Tips
Image Of Four Used Spark Plugs With Black Tips

Conclusion

Repairs will depend on what is causing the hesitation. If the cause is fuel related, it may require cleaning the fuel injectors or fixing a vacuum leak. If the cause is ignition related; it may require replacing the spark plugs and plug wires.

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