Free Auto Repair Advice by Professional Mechanics
Troubleshooting
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Why Does My Engine
Idle to high?
Why Does My Engine Idle High?
Introduction
An engine's idle condition is important for proper vehicle operation. 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. We have listed the most common reasons below:
Car Repair Guide - READ COMPLETELY BEFORE BEGINNING!
- Step 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.
How do I
check my car fuses?
- 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?
How to Scan for Trouble Codes Video
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Step 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.
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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 "coking" 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 thin tar that needs to be
cleaned. One of the best product to perform this action is any aerosol choke
cleaner and a shop towel.
Cleaning Throttle Body
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Step 5 - Some cars have an air intake boot or tube which
is used to transfer the engine's air intake. This tube is mounted
between the throttle body and the
MAF sensor. If your engine does not have a MAF or an intake tube you can
skip this test. This tube and boot is subject to engine torque, heat and
vibration. If this tube or boot develops a tear or crack it will allow the
system to become lean causing a high idle condition. Using a flashlight,
inspect the intake tube and boot for any malfunction. Most intake boots will
tear inside the pleat so look carefully. If a crack or tear is discovered
replace the failed part with a new unit and recheck system.
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Step 6 - An IAC (idle air control) motor is designed to adjust the engine
idle RPM speed by opening and closing an air bypass passage inside the
throttle body. An IAC motor can fail one of two ways; either the motor short
circuits and stops working or the motor will develop high resistance and
cause the
IAC control motor to react slowly.
Either failure will cause the engine
to operate at an elevated idle. When a trouble code scan is performed it sometimes won't
always detect a failed or weak IAC motor. To check the
IAC motor remove the unit.
With the wires connected turn the key to the
"on" position without starting the engine. The IAC should move in or out. If
the IAC motor does nothing it has probably failed. Replace it with a new
unit and recheck system. Note: while the IAC motor is removed clean (use
aerosol carburetor cleaner) the passages the IAC uses to control idle air
speed. If the
IAC valve checks "ok" proceed to the next step.
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Step 7 - The PCV valve (positive crankcase ventilation)
is designed to pull engine crankcase vapors from the inside of the engine
and burn them through the
internal
combustion process. This valve is designed with a pressure spring that
determines the amount of vacuum allowed through the valve. If this valve
malfunctions such as the pressure spring breaking or carbon holding the
valve open it will cause the engine to "hunt" of idle or a steady high idle
condition. To check for this condition remove the valve and inspect it. The
valve should have a spring loaded plunger that open and closes with little
force. If the valve has failed replace the PCV valve with a new unit and
recheck system.
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Step 8 - The
engine
ignition timing for your engine is very important. This timing controls
the combustion process and if the timing is too far advanced the engine will
idle too fast. To check for this condition use a "timing light" and follow
the timing setting procedure for your car. Every engine is different so you
will need to consult your under hood sticker or a
repair
manual to perform this procedure. (Note if the timing is off but if
returned the timing marks to specification the engine has no or little
power. Either the timing dampener has moved due to a keyway issue or the
outside ring of the damper had "spun"). Also for timing belt style of
engines, if the timing belt has jumped a tooth or has been installed
incorrectly. The cam to crankshaft correlation timing is incorrect causing
low power. Some mechanics will advance the timing to try and make up of this
condition. An engine with a timing belt "timing" issue will have elevated
emission outputs.
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Step 9 - The sensors that monitor your car's engine are
delicate. If a sensor is malfunctioning intermittently it might not
set a trouble code and this is why. Let's say the sensor is failing
intermittently about every 2 seconds. The PCM program is not designed to set a
trouble code unless the sensor fails at .5 milliseconds or shorter.
Now the PCM is collecting data that is false and adjusting accordingly. This false data convinces the
ECM to raise the engine idle. Unfortunately it takes a little more detective
work to
detect these types of sensor problems. Using a
multi meter
with a zero BOD rate (real time) connect the leads to the sensing wire of
the sensor and ground. You will need a
car repair
manual to perform this test. I can tell you this, the
oxygen
sensor (02),
MAF or MAP
and coolant sensors are prone to this type of sensor failure.
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Step 10 - Most cars are equipped with a power brake
booster. This booster is designed to assist the driver by helping add
pressure to the brake pedal mechanism. As the driver pushes the brake pedal
down the brake booster uses a diaphragm and releases engine vacuum to help
apply
pressure to the brake system. When this booster fails it can allow an engine
vacuum leak causing an elevated engine idle condition. To check for this
condition start the engine and allow to idle. Next use needle nose pliers to
pinch the vacuum feed hose to the power brake booster. If the engine returns
to the normal idle range the brake booster has failed and needs replacing.
If the engine idle doesn't change the booster is ok.
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Step 11 - The intake manifold and throttle body are
designed to direct the engine's air intake into the cylinder head intake
ports. The manifold and throttle body are bolted together using gaskets, if
these gaskets fail engine vacuum can leak causing an elevated engine idle
condition. This condition can be easy or difficult to find depending on the location of the gasket failure. Try this; with the engine cold start
allow the engine to idle. Next, spray aerosol carburetor cleaner around the
gasket area of the intake manifold or throttle body. If the engine speeds up
or slows down you have a gasket failure.
For exact specific repair procedures for any of the steps above visit our
car repair
manual page.
WARNING! Always have the vehicle under inspection on level
ground, in park with the emergency brake on. Always wear protective eyewear, gloves
and necessary clothing before inspection or work begins. Never crank an engine over
when anyone is near the battery or engine. Always have an operational fire extinguisher
close by, obey all first aid instructions in the event of an injury. Never stand
in front or behind a vehicle when starting or running. When engine is cranked over
keep hands and clothing away from rotating components. Anyone with a heart pacemaker
should not perform these tests.
Troubleshooting
/
Why Does My Engine
Idle to high?
Additional Resources
Check Manufacturer Specific High Engine Idle Questions and Answers
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