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What is a Fuel System 'Throttle Body'?

A throttle body regulates how much air flows into a fuel-injected engine. We cover how throttle bodies work, how to recognize when one needs replacing, and how to install it.
Fuel System Throttle Bodies Variety

A throttle body assembly is an integral part of a fuel-injected engine’s air intake system. Its function is to regulate the amount of air that flows into the engine – accounting for factors such as throttle (gas pedal) position, idle speed, cold start warmup, and more. To perform these critical tasks, the throttle body is bolted in place between your engine’s intake manifold and air filter housing. A hinged metal plate inside the throttle body (known as a butterfly valve) opens to let more air in as the gas pedal is depressed. Because fuel injection systems are inherently computer controlled, information from various sensors mounted on the throttle body allows the vehicle’s engine management system to determine the optimum fuel flow rate as based on multiple conditions.

Throttle Body Assembly Location
Throttle body assemblies are located between the engine air filter and intake manifold.

In this article, we'll take a look at how throttle bodies work, how to recognize when one isn't performing properly and needs to be replaced, and we'll cover the steps involved with replacing an OEM-specification unit. We'll also discuss the throttle body assemblies that we offer from aftermarket manufacturers with proven records of quality, fit, and durability.

How Throttle Bodies Work

Throttle body assemblies are known as either "mechanical" or "electronically controlled." On a mechanical throttle body (above left) the butterfly valve is operated by a spring-loaded arm piece that's directly linked to a vehicle's accelerator pedal by way of a manual cable. Newer vehicles that do not have cable linkages to the accelerator pedal will use throttle body assemblies with computer-controlled servo motors to move the butterfly valve as needed (above right). These servo motor assemblies are described as electronic throttle controls.

Mechanical And Electronically Controlled Throttle Body Assemblies
A mechanical throttle body (left), and electronically-controlled throttle body (right).

On either setup, a knob on the outside of the housing rotates as the butterfly valve moves in order to give feedback to an attached throttle position sensor. This sensor then sends an electronic signal to the ECU (engine control unit) so that fuel flow is adjusted to match the amount of air that's now entering the engine.

On the outside of the throttle body assembly will be a hole for mounting a manifold absolute pressure (MAP) sensor. This port connects to a small air duct that runs into the pressure chamber behind the throttle plate. As more air flows in, the MAP sensor will read a higher pressure, which the ECU uses to verify calculations for fuel flow rate.

Another port will exist for mounting an Idle Air Control (IAC) valve, which controls the amount of air the engine gets at idle. On cold starts, this valve allows more air to pass through so that the engine warms up faster when idling.

For regulating overall idle speed at all times (not just during warmup), mechanical throttle bodies will have an idle speed adjuster screw that can be tightened or loosened in its mounting port. As it's loosened, the screw retracts out of the way inside a dedicated air duct so that more air can pass through at all times.

For the purposes of quick winter warm-up, some throttle body housings may be equipped with small nipples that can be connected to the vehicle's coolant lines. These allow just enough warm coolant to pass through so that any moisture that's frozen will quickly thaw away. In order to prevent air inside the throttle body from becoming too heated, the amount of coolant flowing through is kept to a minimum.

Throttle body assemblies for newer vehicles will most likely be equipped with an additional nipple that attaches to a emissions system vacuum line. This nipple will have its own small duct that runs into the throttle body chamber.

How To Recognize A Malfunctioning Throttle Body

A malfunctioning throttle body will typically cause your engine to hesitate and stumble noticeably when you’re accelerating. The engine may also stall completely at startup, when it’s under load, or even at idle. Or, you may notice a consistent delay in throttle response after stepping on the accelerator. In worst-case scenarios, you may lose practically all of your throttle control and the vehicle won’t accelerate. On newer vehicles with more electronics, cycling the ignition key off for a few seconds before starting the engine again will sometimes make the problem go away temporarily – allowing you to drive a little further instead of being stranded.

Broken Vehicle

The metal housing of a throttle body usually isn’t the problem – however, carbon deposits and dirt do build up over time and diminish smooth operation of the butterfly valve. If gunk accumulation is the only issue, a simple removal and cleaning of the throttle body should correct the issues (see below for removal instructions). Throttle bore wear can happen after many miles, and this typically requires replacement of the entire unit.

Typically, problems arise from electronic issues such as wiring circuits that have become compromised with too much electrical resistance due to corrosion, or from an "open" condition where wiring is broken, loose, or no longer grounded properly. Sensors mounted on the throttle body housing may be failing, causing the vehicle's ECU to become confused. When this happens, the air/fuel mixture will be off and the engine will hesitate. Servo motors that adjust the butterfly valve on throttle bodies with electronic controls wear out and can malfunction at the end of their life span.

Fault Codes

The newer your vehicle, the more likely it is that its ECU will pick up on these erratic running conditions and generate fault codes that will trigger a check engine warning light. On some instrument clusters, you may also see a specialized "ETC" or "EPC" warning light, or a symbol that resembles a lightning bolt. These indicate that problems detected are specific to the electronic throttle control system. If you're interested in diagnosing such issues further yourself, a diagnostic scan tool will help you pinpoint the source of the problem. Or, take your vehicle to a qualified mechanic to determine which components need replacement.

Replacement Throttle Bodies We Offer

Whether you diagnose and replace your throttle body yourself or have someone else do it, using quality replacement parts makes all the difference in keeping your vehicle running reliably and delivering all the responsiveness you paid for when you drove it off the lot. Making a quick left turn in front of an oncoming bus is no time to realize your replacement throttle body doesn't cut it. That's why we sell units made by manufacturers with proven track records of quality, fit, durability, and smoothness which match OEM levels. In fact, odds are good that one or more of those manufacturers were contracted by your automaker to build original parts fitted on the assembly line!

If you’ve got a German car from today back to the 1990s, take a look at the Bosch Throttle Body Assembly and Standard TechSmart Fuel Injection Throttle Body Assembly. Bosch units feature only electronic adjustment motors, while the Standard brand units offer mechanical setups when applicable on older vehicles. Note that all of these units also offer a selection for American and Asian vehicles from the 1990s through today.

The Bosch Throttle Body Assembly.
The Hitachi Fuel Injection Throttle Body.

Hitachi Fuel Injection Throttle Body units specialize in Nissan and Infiniti models back to 2002 as well as General Motors brand vehicles including Saturn and Hummer. Additionally, Dorman Throttle Body Assemblies feature additional GM-powered vehicles sold by Isuzu and Saab as well as other American and Asian makes. AC Delco GM OE Fuel Injection Throttle Body units are for General Motors models only from 1996 through today, with both mechanical and electronic setups. If you’ve got a 2006-08 Chevy Aveo (or Canadian model Pontiac Wave), we’ve got hard-to-find throttle body units on our AC Delco Professional Fuel Injection Throttle Body page.

Auto 7 Mechanical Throttle Body
The Auto 7 mechanical Throttle Body.

Spectra Premium Fuel Injection Throttle Body units offer all-electronic setups for select American, German, and Japanese cars from the mid-1990s through today. And for Hyundai and Daewoo-built engines from 1990-2002 (including models sold by GM and Mitsubishi), Auto 7 Throttle Body units offer the replacement mechanical throttle bodies you'll need.

Throttle Body Smaller Replacement Components
We also offer a selection of smaller replacement components that are part of throttle body units.

Replacing A Throttle Body Unit

Unbolt And Disconnect The Air Filter Housing

Air filter assemblies will vary in placement from vehicle to vehicle, so you’ll want to start by removing the cover panel and the air filter itself. Next, locate all bolts that anchor the housing in place. Remove these bolts or screws, putting them aside for later re-installation. Look for any wiring connectors on the side of the air filter housing – unhook these as well.

Disconnecting Air Filter Housing

Locate Your Throttle Body And Remove Any Electrical Connectors

Once you remove your air filter housing from the engine, you'll have a clear view of your throttle body assembly. Typically, you'll see four bolts used to anchor it in place. Like the air filter housing, loosening these bolts will require only simple socket wrenches. You'll also need to unplug the main wiring connector from the throttle body assembly to the vehicle's ECU.

Electrical Connectors Removal

Remove The Cable To Your Accelerator Pedal (On Mechanical Throttle Bodies Only)

If your vehicle is equipped with an actual cable linkage to your accelerator pedal, you'll see the point where it attaches easily. To temporarily remove this cable, you'll need to rotate the linkage arm by hand in order to take tension off the cable and create some slack. With a pair of needle-nose pliers, grab onto the retaining piece (or clip) at the end of the cable. Wiggle the end of the cable loose in order to free it from its mounting point. Your replacement throttle body will have a similarly designed attachment method, so re-installation will be easy.

Accelerator Pedal Cable Removal

Loosen The Bolts In A Cross-Hatch Pattern

When removing the bolts that hold your throttle body assembly in place, make sure to loosen the first two in a cross-hatch pattern. For example, start with the top left bolt, then move to the bottom right one. After that, you can loosen bolts three and four in any sequence you choose. Doing this minimizes stress on any intake manifold parts the throttle assembly is mounted to – which is a good idea, because they’ll probably be made of plastic. You’ll want to follow a similar pattern when re-installing your new throttle body assembly.

Cross-Hatch Pattern Bolts Loosening

Remove Any Factory Gasket Pieces And Inspect Them

Once your old throttle body assembly is off the vehicle, remove any gasket pieces that surround the round butterfly valve opening because your replacement unit will not include a new one. Because the throttle body housing itself doesn't see any movement, the gasket piece should be intact and fit for transferring to your new housing. Use this opportunity to clean it with a de-greasing cleaner.

Factory Gasket Piece Inspection

Check The Gasket Between Your Old Throttle Body And The Intake Manifold

Before installing your new throttle body, it's important to check the intake manifold to see what type of gasket or seal exists in this location. Depending on your particular make and model, you may have a durable rubber O-ring type seal that can be removed and cleaned. Or, your vehicle may have a thin metal gasket that can be removed easily without residue. Both of these types of gaskets are designed to be re-used.

Throttle Body And Intake Manifold Gasket Check

Clean the area on your intake manifold where the old gasket was. If necessary, use a sharp blade with your de-greaser to scrape any remaining gunk away. Should you find yourself in need of new throttle body gasket pieces, we offer a good selection in the Throttle Bodies section of our website also.

Replacement Throttle Body Gasket
Need a replacement throttle body gasket? We've got a great selection.

Install Your New Throttle Body Assembly

Because throttle body assemblies are basically plug-and-play units, installing a new one requires no programming on your part – your vehicle’s ECU will link to it automatically. So, in short, installation is a simple matter of reversing the steps taken during disassembly.

New Throttle Body Assembly Install

We'll note that when you attach your new throttle body to the intake plenum, insert all four bolts and tighten them moderately. Then, make sure the first two you tighten are located diagonally from one other. Finally, tighten the remaining bolts. Reinstall your air filter housing, air filter, and any other components and connections you removed.

Once a faulty throttle body assembly is replaced with a new one, your vehicle will run smoothly again without hesitation. And if your throttle body has been getting slowly worse and not been performing up to par for a long time, the level of performance being restored will be nothing less than refreshing!

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