Is throttle body service necessary?
Throttle body cleaning is often recommended by auto repair shops, but is it really necessary? Toyota, Subaru, Honda, and Mazda don’t have throttle body service listed in the scheduled maintenance. So why do some mechanics recommend throttle body service even when there’s nothing wrong with the car?
In this article we’ll cover:
- What a throttle body is and does
- Dirty throttle body symptoms
- Where the throttle body is located
- How to clean a throttle body
- Special considerations with an electronic throttle body (drive by wire throttle body)
- Throttle body cleaning cost
- How to reset the throttle body learned value for electric throttle bodies
- How often should you clean your throttle body
What is a throttle body?
According to Wikipedia, a throttle body meters either air or fuel into an internal combustion engine. I’ve never seen a design that meters fuel. Even diesel engines that control power output by metering fuel rather than air use a throttle body to meter air. However, just because I haven’t seen it doesn’t mean it doesn’t exist.
For the sake of simplicity, let’s agree that a throttle body meters air into the engine. In the old days, a cable was connected to the “gas” pedal and the throttle body. When you stepped on the pedal, it opened the throttle body and let more air into the engine. The more air you allowed into the engine, the more power the engine made.
Symptoms of a dirty throttle body
You probably won’t ever have an issue with a throttle body not letting enough air into your engine. On the contrary, when throttle bodies have problems, it’s at idle, when the throttle valve is closed. When a throttle body is closed, the air leaking past the disc valve is what controls the idle speed. Over time sticky residue clogs the gap between the disc and the throttle bore and decreases the air flow. The throttle plate can become “stuck in the mud”, making it hard to press the gas pedal. This is especially true if the car has been sitting for a long time.
Failing to clean the throttle body can cause stalling, a check engine light and trouble codes, or even a failure to start and run.
Why do throttle bodies get dirty?
Here’s the issue. All engines have “blowby”. Blowby is gas that moves past the piston rings into the crankcase. Some of the gas is unburned fuel, so it dilutes the motor oil and can even explode.
Before emissions standards, cars had draft tubes, which was a crankcase vent that allowed vapor to escape from the crankcase to atmosphere. These tubes were under the engine near the street. When the car moved the air passing the draft tube created low pressure and helped empty the crankcase. If you Google “70s freeway” you’ll see black stripes in the center of all of the lanes. This was from the draft tubes.
To reduce pollution, all cars now have “sealed” crankcases. Crankcase vapor is still bad, but instead of venting it to the atmosphere we suck it back into the engine through the intake. The gross sticky mixture of soot, oil, and unburned fuel is suck right past the throttle body. That’s why throttle bodies get dirty. We deliberately run nasty vapor past it whenever the engine is running.
A low quality or ripped air filter can also add to the problem, allowing dirt into the intake to mix with the sticky smoke which allows faster build up. Bricks to go with the mortar if you will.
Where is the throttle body located?
All multi-cylinder cars have an intake manifold. A manifold typically sits between intake ports on the cylinder head and a throttle body, although some cars have multiple throttle bodies. The throttle body will be between the intake manifold and the intake tube, which will be coming from the engine air filter. So, to locate your throttle body, find your air filter and follow the tube leading from it to your throttle body.
Most modern cars have electronic throttle control. Instead of a cable from the gas pedal to the throttle body, there’s a sensor at the gas pedal to measure position and an actuator on the throttle body to move the throttle valve. Redundant throttle position sensors monitor the throttle valve position to make sure it’s doing what the driver and ECM want it to.
How to clean a throttle body
Cleaning a throttle body isn’t difficult. Depending on your mechanic ability, you may have difficulty removing the necessary air intake system plumbing to access the throttle body, but the actual cleaning can be done by anyone capable of brushing their teeth. In fact, you’ll need the same tool – a toothbrush. You’ll need to hold the throttle valve open with one hand and brush the deposits on the bore and disc with the other. However, it’s going to be sticky, and you’ll need a solvent to break it down.
Be careful when working with electronically controlled throttle bodies!
Acetone, which is a major component of throttle body cleaner, destroys many types of plastic. The throttle actuator gears and throttle position sensor (TPS) are plastic. Be careful not to spray where the throttle shaft enters the throttle body because it can wick into the actuator or TPS and cause problems. Instead, wet the toothbrush before scrubbing.
How much should throttle body service cost?
Cost depends on the type of car and geographic location. You can’t compare California to Idaho, and you can’t compare a Toyota Corolla to an NSX. Call around in your location to find a price you can live with or consider doing it yourself. Everyone has worn out toothbushes and a can of throttle body cleaner is less than $20.
The ECM learns how to control the throttle valve for the engine speed it wants over time. If you clean the throttle body everything it has learned is now different. It will eventually relearn, but it will take a while. It’s better to erase its memory by disconnecting the battery and then following the idle learn procedure (often just letting the car idle for a while).
How often should you clean the throttle body?
Hopefully you now realize throttle body cleaning is necessary. How often? Google it and you’ll find hundreds of different answers, many from people who have no business talking about auto repair. But in this case, it doesn’t really matter. There is no one right answer. It depends. What kind of car? How well do you maintain the motor oil? How worn is the engine? Do you do a lot of stop and go driving? We’re going to say every 30K if you want to be very sure you never have a problem and every 60K if you’d rather not spend money you don’t need to.