Top 5 Causes of Engine Hesitation

Manifold Absolute Presure Sensor
Auto Facts Repair Manuals
Auto Facts Repair Manuals

We decided to review the top five causes of engine hesitation, because it’s a far-reaching problem. It also lands high on the list of the most annoying automotive problems. Drivers push on the gas pedal and expect the engine to respond the way it did when they first got the automobile.

Even if this engine accrued a lot of miles, this remains a reasonable expectation. We compiled this unscientific engine hesitation list by interviewing several professional auto mechanics. Together we discussed the meaning and common symptoms associated with a hesitation complaint. Then they supplied us with the top five causes of engine hesitation they’ve seen in real life.

Definition of Engine Hesitation

Throttle Body Assembly
Throttle Body Assembly

Many automotive educational materials describe an engine hesitation as a tip in or stumble as the throttle plates open. As a quick review when you push down on the gas pedal you’re opening the butterfly on the throttle body. This allows additional air to enter the engine.

The power train control module sees this additional air and then increases the injector on time to add more fuel to the mixture. The larger the charge ignited in the combustion chamber provides additional engine power.

The cylinder expels the spent mixture through the exhaust valve and then out through the tailpipe. All of these factors must work in perfect harmony or the automobile will react poorly when the driver requests additional power by pushing on the gas pedal.

Engine Hesitation from Exhaust System Problems

Catalytic Convertor
Catalytic Convertor

In order for the engine to create more power on demand it needs to be able to inhale as much fuel and air as possible. If lingering exhaust gases occupy the cylinder, when the intake valve opens to pull in the new charge, efficiency is reduced.

It’s like if you tried to take a deep breath without completely exhaling first. It’s the exhaust systems job to allow spent exhaust gases to freely flow from each cylinder. Higher-than-expected back pressure from the exhaust system stops this from happening. As another example, this is why the car runs so poorly when someone stuffs a banana or potato in the tailpipe.

As for the common causes of excessive exhaust system back pressure we can often look to failure of the catalytic converter. It’s the job of the catalytic converter to scrub the exhaust system of pollutants. The exhaust flows through a honeycomb ceramic type filter. If the small holes in the honeycomb ceramic clog or start to collapse the exhaust flow efficiency is reduced.

In addition, these ceramic filters can break apart and get deposited in the muffler. The exhaust system muffler uses a series of baffles to redirect the sound and reduce the level of noise. These very same baffles can catch pieces of the catalytic converter causing an exhaust system restriction.

Restricted Engine Air Filters

As we compared an exhaust system clog to the vehicle’s inability to exhale properly, we’ll apply a restriction to the air filter as its inability to take a deep breath in. When the driver requests additional engine power they open up the throttle plate to allow more air to flow.

All of this air passes through a filter to make sure no debris enters the combustion chamber. If this filter becomes clogged then the efficiency is reduced. It’s like if you put your hand over your mouth and pinched your nose closed. You’re not going to breathe very well and your ability to perform physical tasks becomes reduced.

Vehicle manufacturers outline maintenance requirements of the air filter in the owner’s manual. However, mechanics consider this a guideline and not a rule. If you live in a dry, dusty region or on a dirt road the air filter needs replacing more often. In the end, physically inspecting the air filter and replacing as needed remains the best advice.

TPS (Throttle Position Sensor) Issues

Throttle Position Sensor
Throttle Position Sensor

The throttle position sensor, lovingly called the TPS by professional mechanics, reports real-time throttle position to the PCM. This way the computer knows what the driver wants at all times. The part itself utilizes a variable resistor and a 5 V reference signal. The amount of voltage flowing back to the main engine computer translates into the current throttle position.

The two extreme positions reported from the TPS become an idling situation or a wide open throttle request. In an idle situation a minimum amount of fuel flows into the combustion chamber. In the wide open throttle position the computer commands maximum fuel enrichment. One of the most important pieces of information this sensor provides is when we crack the throttle from idle. In other words, if the vehicle was stopped at a traffic light and then that light turns green.

Delayed or incorrect reporting from the throttle position sensor provides the computer with false information about the current engine load demands. That’s why this component has made the list of the five most common causes of engine hesitation. Fortunately, when this part fails, it often sets a check engine light code and diagnosis becomes a straightforward process.

Manifold Absolute Pressure Sensor

Manifold Absolute Presure Sensor
Manifold Absolute Presure Sensor

A manifold absolute pressure sensor provides the power train control module with real-time engine load information. The sensor uses manifold vacuum to provide this measurement. A gas engine builds maximum manifold vacuum with the throttle closed at engine idle. Under a wide open throttle condition the vacuum reading falls to zero.

As with the throttle position sensor the manifold absolute pressure sensor sends this important information to the computer with a variable voltage signal. What makes this part a little more vulnerable than others becomes its means of gathering an input signal. Often a vacuum line feeds the sensor. As automobiles age these vacuum lines break or leak. In other situations the sensor fits on top of the intake manifold where it’s exposed to heat and engine vibration. These extreme situations make this sensor unreliable as it ages on many popular models.

Fuel System Delivery Problems

Automotive Fuel InjectorAs we mentioned in the opening section when increased power is requested from the driver, additional fuel needs to be added to the mixture quickly. Problems in the fuel system can obviously create an engine hesitation. We provide a detailed article on diagnosing fuel systems on our sister site.

With that said, the ability to add additional fuel starts in the fuel tank. The in tank fuel pump pressurizes the fuel rail so when the injectors open, gas flows freely into the combustion chamber. This means a failing fuel pump, clogged fuel filter, defective fuel pressure regulator and a problem with the fuel injectors themselves can all cause an engine hesitation.