Step by step automotive repair guide on how to troubleshoot and fix an engine that is idling high (fast) or low (slow). (Note: The engine idle speed is controlled by the computer and is NOT adjustable.) The troubleshooting procedure for high and low idle conditions are similar, which are included in the following:
Difficulty Scale: 4 of 10
Step 1 - If the check engine or service engine light soon has illuminated retrieving the diagnostic trouble codes could lead you in the right direction of what the problem is, or is related too.
Step 2 - The
idle air control motor (valve) is responsible for controlling the engine idle
speed, if this part fails or becomes weak the engine idle speed could become erratic,
additionally, this condition may or may not trigger the MIL (check engine, service
engine soon light.) Because of the nature of this valve (large air volume
processed) it should
be serviced
when a tune up is performed to remove "coking" which will hinder the valve's performance.
To perform a rough check of the idle air control, remove the valve while reconnecting the wiring
harness. While observing the valve with a flashlight, turn the key to the "ON" position,
(DO NOT START ENGINE.) the valve should move slightly.
Step 3 - Engine vacuum is a sealed system and must
hold a specific amount to maintain proper idle conditions, inspect the
engine
vacuum hoses and intake gaskets.
Step 4 - The
air intake
boot or tube is used to deliver air to the intake system of the engine. If this
tube becomes loose or broken it can leak un-metered air into the engine effectively
causing an unbalance (via the mass air flow MAF sensor) in the computer system which
can produce fast or slow idle.
Step 5 - A throttle bore is used to regulate the
volume of air flow the engine consumes. Controlled either by computer or by manual
foot pedal this device must be
clean and
free from coking build up (light tar substance.)
Step 6 - The crankcase ventilation system or PCV
valve of an engine is used to scavenge vapors from inside the engine while running,
if this system malfunctions erratic idle conditions may result.
Step 7 - Engine timing, both ignition and camshaft
orientation can effect the engine idle speed. When this condition exists the computer
is rendered unbalanced and will not react correctly. These items must be checked
and corrected before normal engine operation will resume. Step 8 - The mass air flow sensor uses a hot wire
that changes in resistance as engine intake air is passed by it. The computer uses
this information to make adjustments to the operating system. When this wire becomes
contaminated which naturally occurs during usage, the sensor must be
removed and cleaned.