Free Car Repair Advice by Certified ASE Technicians.
Troubleshooting
/ Check Engine
Light On
Check Engine Light is On
The system that controls the operation of the
engine,
transmission and
emissions is called an ECM -
Engine Control Module. The ECM is a small dedicated computer loaded with an
operating program. The "Check Engine Light" is illuminated when the ECM has detected
a malfunction somewhere in the system. A
trouble code scanner
will retrieve a numbered
code that when identified will lead you to the system malfunction. If a failed
sensor is indicated or an "out of range" condition exits additional tests that can
effect this reading need to be considered.
Example:1 If scanning indicates a MAF (Mass
Air Flow Sensor) problem. Inspect the intake boot or PCV valve vacuum feed hose
for cracks or tears. A vacuum leak between the throttle body and the mass air flow
sensor will cause a false reading and illuminate the check engine light.
Example:2 If a
code scanner
indicates a EVAP (Evaporative Emissions System) system failure. Inspect the gas
cap seal, and replace if the "o" ring if it has been damaged or is missing.
Your ECM pressure tests the fuel vapor recovery system as part of the emission testing.
When the test fails do to a leak the "Check Engine" light illuminates. A trouble
codes identifies the system malfunction but not the actual problem. It is up to
you to inspect the system and repair the obvious leaks. (Technical note: If after
inspecting the system all seems ok, try replacing the storage canister (charcoal
can). This canister can develop hairline cranks that leak and are hard to detect.
Replace the canister with a new unit, clear codes and recheck system. Toyota, Honda,
GM had problems with the charcoal canister.
Service Engine Soon or Check Engine Light
If the check engine light has illuminated your vehicle enters into
"limp mode" this means it is running on a predetermined program that causes poor
mileage and increased emissions. An
trouble code scanner
reader is an easy to use communication tool. All 1996 vehicles and newer utilize
a "D" style plug in connector that connects to the code reader. Most vehicles connector
is located at the lower drivers compartment. Once the code reader is plugged in
you can retrieve trouble codes the ECM has stored has in it's memory. These codes
are the same codes the dealer and repair shops use to replace sensors and clear
codes. To locate your vehicles computer connector (ALDL) most vehicles are on the
drivers side lower. Some vehicles you might need to look around a little, on the
passengers side, and around the center console under a plastic cover.
ALDL or Data Link Connector
Connect the
trouble code scanner
to the under dash OBDII ALDL connector. The code reader will automatically turn
on and prompt you to the next step for code retrieval. Then, identify the
trouble code here, or on the trouble code chart supplied with the code reader,
if you do not have a code chart consult a
repair manual. After the repair has been made, clear trouble codes and recheck
system. After the codes have been cleared the computer system is ready for internal
testing, this means that while the car is operation the onboard computer is performing
tests. These test are called "monitors" most cars have between 4 and 8 monitor systems.
The computer initiates testing procedures through the next 40 to 60 miles. Only
after all monitor systems have run successfully will the system be ready for a smog
inspection.
Common Problems
- After a car repair has been performed the MIL indicator will
illuminate because the technician neglected to plug in a sensor.
- A blown fuse can cause the MIL to illuminate.
Troubleshooting Procedure
A. Testing for DTS's (Diagnostic Trouble Codes) for this you need
a ECM Scanner this is a small hand held electronic device that plugs in to your
cars OB2 (OBD11) diagnostic connector (under dash on left side, mostly). This device
can tell you the trouble code that was stored in your cars Main ECM. (Engine Control
Module). Read Trouble Code Chart
B. Testing Fuel Pressure: Test for
proper fuel pressure with a fuel pressure gauge. Connect the gauge to the test port
on the fuel rail. Fuel pressure reading's very depending on the system your in our
vehicle, but most throttle body injection cars (TBI) are between 13 psi and 17 psi.
and most (DPI) direct port inject systems are between 40 psi and 55 psi. To find
out your vehicle system fuel pressure consult a
repair manual. If little fuel pressure is present the fuel pump needs to be
replaced.
C. Check Engine for Vacuum Leaks:
If an
engine
vacuum leak is present it will cause the sensor input readings to the
ECM to be incorrect causing an engine serge. The system is programmed to work
at prescribed value. When a vacuum leak is present these readings or incorrect causing
the engine to surge under power and idle rough. Inspect the air intake boot for
tears and vacuum feed lines to all accessories. Replace torn or dilapidated hose
with a new hose and recheck system. Also an IAC (Idle Air Control) valve bypass
hose it can develop tears/holes mid way through the hose, inspect thoroughly (common
problem on Ford trucks and SUV's) Sometimes a vacuum leak can be detected by opening
the hood, start the
engine
and allow to idle, then listen for an audible whistling sound. Then inspect that
area of the engine compartment to locate leak.
D. Check for Plugged Exhaust System.
The exhaust system in your car does three jobs, first it transfers poisonous exhaust
gases from the engine to the rear of the car next, it quiets down the engine sound
while running. Finally, it converts unspent fuel into spent fuel with the help of
a
catalytic converter. The
exhaust system consists of an exhaust manifold, exhaust pipe,
catalytic converter,
muffler
and tail pipe. There are endless configurations of
exhaust systems from the very loud free flow low restriction system to the barley
audible system depending on application and usability. When an exhaust system becomes
plugged it can cause low power and high emissions. This causes the "check engine"
light to illuminate. To test for this condition attach a backpressure feedback gauge.
When the engine is loaded it should not read more then 3 PSI. If a restriction exits
remove the exhaust system and inspect. Look for damaged catalytic converter malfunction
and heavy soot.
Plugged Catalytic Converter with Broken Catalyst
E. Test for Weak Ignition Spark or Failing System:. A weak
ignition system can be a sign of a failing ignition component and cause the
engine to surge. Remove spark plug wire and insert an extra spark plug into end
of wire, attach spark plug to engine ground then crank engine. You should observe
blue spark between spark plug gap. If a yellow spark is observed, your
ignition coil is weak and needs to be replaced. Also the spark should in a constant
rhythm, if the spark is inconsistence suspect the crank angle senor, ignition coil
or ignition module.
G. Check for Automatic Transmission
Clutch Slippage: When the clutches inside a
automatic transmission start to wear they can malfunction. This condition will
deliver a surging or a tugging sensation to the driver. Without taking the transmission
apart it is impossible to check the wear of the clutch discs. But there are tests
that can help you in the repair diagnosis, first remove transmission dip stick and
inspect the condition of the fluid, if the fluid has a burnt smell and is dark color
instead of the normal red color the fluid is burnt. The transmission fluid is a
hydraulic fluid that when extreme heat is applied to the fluid condition becomes
burnt and smelly. The extreme heat is generated from the clutch discs slipping.
Sometimes a
transmission service will help the operation by removing the burn fluid and
replacing it with new.
Automatic Transmission cut-away Image
Troubleshooting
/ Check Engine
Light On
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