A fuel injected engine's idle speed is controlled by the IAC motor. This motor controls a plunger or valve that opens and closes a vacuum port in turn controlling the engine idle speed. But there are conditions where the IAC motor cannot control the engine idle speed. Here is a listed the most popular reasons below:
1 - Anytime you have a problem with electronically controlled components such as an engine, transmission, ABS brake, or SRS (supplemental restraint system, Air Bag) inspect all fuses using a test light. Check the under hood power distribution center and under dash fuse panels. If all fuses test ok continue to the next step.
2 - There are two types of high idle conditions, computer controlled and non-computer controlled malfunctions. To check for problems with electronically controlled components such as an engine, transmission, ABS brake, or SRS (supplemental restraint system, air bag) after the fuses have been tested a trouble code scan tool is needed to identify system troubles. Use a simple scanner tool to retrieve trouble codes to check if they relate to the specific problem, like an IAC motor failure code. If a trouble code is present but does not pertain to the immediate problem like an EVAP code ignore it until a later time, after the engine is running properly. The reason we repair non-related codes after the engine is running properly is because sometime false codes can be triggered by a malfunctioning engine. Once the engine is running properly the code might cycle, and turn itself off. If no trouble codes are present proceed to the next step. How do I scan my service engine soon light?
3 - Your engine is designed to operate with a prescribed amount of vacuum. If vacuum is allowed to leak it can cause a high idle condition. Check for broken or dilapidated vacuum hoses on and around the engine. Your car's engine is designed to run on a system that can hold vacuum. Vacuum hose are typically connected to the engine intake manifold or throttle body and will supply engine vacuum to various accessories like power brakes and cruise control. Some manufactures like Ford are designed with a larger vacuum transfer hose that connects the intake manifold to the IAC (idle air control) motor. If broken or dilapidated these vacuum lines can cause the engine to lose vacuum which will allow the engine to idle high. Inspect all engine and accessory vacuum lines to look for missing, torn or dilapidated lines and replace as needed. Also start the engine and while it is running listen for any whistling noise coming from the engine that is not usually present. Follow the noise and inspect vacuum lines in that area. When the engine is running it will pull a broken piece of the hose inward to create a larger vacuum leak. Check the integrity of all vacuum hoses at each end of the hose, typically this is where a vacuum hose fails. If all vacuum hoses check "ok" proceed to the next step.
4 - The throttle body of your engine controls the secondary air intake for the engine. The throttle body becomes dirty with air contaminants that the air filter cannot keep out. This condition is called "choking" and can cause stalling as well as an elevated engine idle condition. The throttle bore in this illustration has been cleaned for visual purposes. Usually the throttle bore and plate are coated with a dark light tar that needs to be cleaned. One of the best product to perform this action is any aerosol choke cleaner and a shop towel.
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Tuesday, February 3rd, 2009 AT 1:20 PM