DIY

Bad Throttle Body Symptoms

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The throttle body plays a vital role in regulating the amount of air that goes into your engine. When it goes bad, your car may not even start. It’s important to identify a bad throttle body quickly so you can have it fixed as soon as possible.

What is a Throttle Body?

The throttle body is composed of a slightly oval shaped plate mounted on a shaft within a special housing so that changing the angle of the plate by rotating the shaft regulates airflow, thus controlling the speed of the engine. When the plate is at rest very little air can get past it. Some throttle plates have a small hole in the plate. The oval shaped plate-on-a-shaft is called a throttle plate (also known as a butterfly valve).

car throttle body
A car with a dirty or damaged throttle body is likely to have poor fuel economy.

There are two primary types of throttle bodies: mechanical and electronic.

A mechanical throttle body connects to the accelerator pedal via a cable. When the driver pushes the pedal, the throttle plate opens a certain amount, thereby letting air into the engine. This system was used for decades.

  • Most mechanical throttle bodies include an idle air control valve (IAC) that regulates the amount of air bypassing the throttle plate. The Idle Air Control is manipulated by the ECM/PCM, not only to control idle speed, but also for no-touch starting (it’s wide open on engine start), and also for “dashpot” function, temporarily holding the engine speed slightly above idle during sudden deceleration to prevent stalling. The air bypassing the throttle plate is measured by the Mass Airflow Sensor so the ECM/PCM can maintain proper fuel delivery.
  • Electronic throttle control (ETC) systems are found in most newer vehicles. An ETC system uses an electric motor to operate the throttle plate.
  • On ETC systems, a redundant pair of throttle position (TP) sensors is mounted to monitor the position of the throttle plate, completing an important part of the feedback loop so the ECM/PCM can apply pulses to the plate motor to drive the plate to its desired target. Other parts of the feedback loop the ECM/PCM monitors are engine rpm, coolant and air temperature, etc.

On an ETC system, instead of a mechanical linkage connecting the accelerator pedal to the throttle body, there are redundant accelerator pedal position (APP) sensors—usually two or three—that monitor the position of the pedal. The connection between the APP sensors and the ETC Throttle body is always a control module.

When GM first began using the ETC system in their 2000 model year, the PCM couldn’t process APP/Throttle Control data fast enough and so a stand-alone module was used. Ford didn’t begin to use ETC until a PCM could be developed with enough processing speed to handle ETC functions.

Typically, there is no idle air control valve (IAC) in an ETC system. Instead, the control module manages idle speed by adjusting the throttle opening. There is also no cruise control module needed on ETC platforms, only the steering wheel Speed Control buttons and the software programmed into the PCM, which controls the throttle plate anyway.

What are the Signs of a Bad Throttle Body?

When the throttle body is not working properly, your car may stall, run rough, and/or lack power. Below is a detailed overview of the common symptoms of a bad throttle body. Most throttle body systems will, after detecting a fault, illuminate a special warning light (wrench or throttle body shaped light), and limit throttle opening to half or not at all. This is definitely something you’ll notice.

mechanic holding a car's throttle body
You may be able to restore the throttle body’s operation by cleaning the throttle body can properly restore its operation.

Stalling

Since there’s so much PCV steam floating around in the intake, it tends to coat the inner surfaces of the intake with sludge, which can also gather around the edges of the throttle plate. This blocks airflow and/or causes the throttle to stick closed on cable operated systems so that when you apply the throttle, the pedal won’t initially yield but then will pop loose suddenly. This can be dangerous if it happens in reverse, because the sudden acceleration can throw your weight against the accelerator pedal, causing a runaway situation.

However, the plate doesn’t always stick. What does happen on cable operated and ETC systems is that the ECM/PCM “learns” that it needs more air to reach its idle rpm target and applies more IAC flow or Throttle Angle to reach that target. Thus, when you replace the battery sometimes the vehicle’s ECM/PCM “forgets” what it learned over time and you may encounter a situation where the engine won’t idle until it “relearns,” which can take awhile. Cleaning the throttle body is a good practice, but here are a couple of very important cautions on ETC systems.

  1. Don’t ever open the throttle body with your fingers, even with the engine switched off or with the throttle body removed. On some vehicles, this can ruin the throttle body mechanism so that it has to be replaced. Always have an assistant hold the throttle pedal down to open the throttle plate.
  2. NEVER clean the throttle body with your fingers; always use a brush, because if the throttle body plate is driven closed (like if your the assistant releases the pedal), the throttle body will cut your finger off. KEEP YOUR FINGERS OUT OF THE THROTTLE BODY AT ALL TIMES!!!

The car’s primary computer, which is often referred to as the powertrain control module (PCM), expects to see a certain amount of airflow through the throttle body. When that doesn’t happen, because the throttle body is dirty or faulty, the engine may stall.

Unstable idle

Problems with the throttle body can also cause an unstable idle. The PCM bases its idle calculations on a predetermined amount of airflow through the throttle body. When that predetermined value isn’t met, the engine may fail to idle properly.

A problem with the idle air control valve, which is usually mounted to the throttle body, can cause an unstable idle as well.

Rough running

A faulty throttle body may disrupt the engine’s air/fuel mixture, resulting in misfiring and a rough running condition. This is pretty rare, however.

Illuminated check engine light

When the control module detects a problem with the throttle body (or a problem caused by the throttle body), it will turn on the check engine light.

Reduced power warning message on the dash

When there’s a problem with the ETC system, some vehicles, particularly those made by General Motors, will display a “Reduced Power” warning message (or something similar) on the dash.

What Happens When You Have a Bad Throttle Body?

A bad throttle body can cause all of the problems mentioned above. What’s more, a failed throttle body can reduce fuel economy and even cause damage to other components—such as the catalytic converter—if left unaddressed.

It is important to note that a throttle body that’s acting up doesn’t always require replacement. What you’re experiencing could just be dirty throttle body symptoms brought about by the build-up of dirt and carbon deposits.

In such a scenario, cleaning the throttle body will restore proper operation.

Is it Safe to Drive with a Bad Throttle Body?

You might be wondering—is it safe to drive with a bad throttle body? The quick answer is no. Once it starts to show signs of going bad or being clogged, you should address the problem right away. Otherwise the vehicle may stall or fail to accelerate, creating a safety hazard.

Both types of throttle body assemblies (mechanical and electronic) are located between the engine air filter and intake manifold. Most cars use just one throttle body, though some engines (usually those with twin turbochargers) use two throttle bodies.

Educating yourself on the car throttle body symptoms outlined above will allow you to make informed decisions when it comes to repair—and that’ll help you keep your vehicle running better and longer.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic.

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