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. If your engine
is idling low with excessive smoke coming out of the exhaust system please visit.
Why is black smoke coming out of the exhaust?
If the exhaust seems normal proceed with the instruction below:
Car Repair Information Guide
Step 1 - Anytime you have a problem with electronically controlled
components such as an engine
inspect all fuses using a
test light. Check
the under hood power distribution center and under dash fuse panels. If all fuses
test okay continue to the next step.
Step 2 - There are two types of low 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 and 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.
Step 3 - Your engine designed to operate with a prescribed amount
of vacuum. If a large amount of vacuum is allowed to leak it can cause a low 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
vacuum line or air intake boot 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 the engine 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 "okay"
proceed to the next step.
Failed Air Intake Boot
Step 4 - 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 can cause the engine to operate at a low 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 "okay"
proceed to the next step.
Step 5 - Some cars have an air intake boot or tube which is
used to transferring the engine's intake 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 cause the system to not monitor the air intake volume correctly
causing a low 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.
Step 6 - 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 lower the engine
idle. 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
Step 7 - The
engine ignition timing for
your engine is very important. This timing controls the combustion process and if
the timing is too far retarded the engine will idle to slow. 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 and 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.
Step 8 - 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 PCM 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.
Step 9 - 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 replacement. If the engine idle doesn't change the booster is ok.
Step 10 - The intake manifold and throttle body are designed
to direct the engine's air intake into the cylinder head intake ports. This manifold
and throttle body are bolted together using gaskets, if these gaskets fail
engine vacuum can leak
causing a low 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.
If further assistance is needed, our certified car repair technicians are ready
to answer your car questions.
Related Car Repair Information
