ROUBLESHOOTING DRIVABILITY PROBLEMS by Mark Davidson To control vehicle
emissions and improve performance and fuel economy,automobile manufacturers
began installing"on-board"computer systems on vehicles in the late 1970s launch
creader crp129. This provides a monitoring system that continuously checks
the readings from various sensors and turns on the"Check Engine"or"Service
Engine soon"light when a sensor is not providing the correct information to the
computer. Whenever the"Check Engine"or"Service Engine Soon"light appears,there
is a service code,often called the trouble code,stored in the computer's memory
circuit that can be extracted and used to help troubleshoot the problem. Today's
computerized vehicle systems can actually help you diagnose and troubleshoot
themselves if you have the right information and a few basic hand tools. If you
are experiencing a drivability problem,your first step is to extract any"trouble
codes"from the vehicle's computer. The following are the most commonly
experienced drivability problems and a short description of what components may
be affected. In each case,these suggestions are assuming the engine is
mechanically sound (camshaft,pistons,timing chain or belt,valves,etc). First,inspect all vacuum lines on the engine and replace them
if any cracks or brittleness is found launch
crp129. If your vehicle is carbureted,the choke system must be functioning
properly before any sensor testing is performed. If it is operating
correctly,the Coolant Temperature Sensor could be affecting how well the vehicle
starts.Complaint #10 - THE ENGINE SEEMS TO"PING"OR"KNOCK"AFTER IT IS WARM
The first item to check is the base ignition timing. If that is correct,the
electronic component to test,if your vehicle has one,is the Knock Sensor. An
inoperable knock sensor will not send a signal to the computer to retard timing
therefore pinging may result. Complaint #11 - MY CAR SURGES AT HIGHWAY SPEEDS
Surging at highway speeds is commonly caused by a lean air/fuel mixture. This
may be due to a failing Coolant Temperature Sensor,a defective Manifold Absolute
Pressure Sensor,or a Mass Air Flow Sensor problem. Complaint #12 - MY ENGINE HAS
A CARBURETOR AND IT DIESELS ("RUNS ON") AFTER I TURNOFF THE KEY Engine"run on"is
commonly attributed to a high idle speed setting caused by a defective Idle
Speed Control Motor,an incorrect adjustment,or excessively advanced ignition
timing. Complaint #13 - I AM EXPERIENCING A"ROTTEN EGG"SMELL OUT OF THE TAILPIPE
Rotten egg smell out of the tailpipe is caused by an overly rich air/fuel
mixture or by running leaded fuels in a vehicle equipped for unleaded. Overly
rich fuel mixtures can be caused by a leaking fuel injector,a bad Oxygen
Sensor,or defective Coolant Temperature Sensor. Complaint #14 - MY CAR"BUCKS"AND
OCCASIONALLY BACKFIRES Other than a mechanical or ignition type of
problem,bucking or backfiring can be caused by a defective Manifold Absolute
Pressure Sensor,a failing Mass Air Flow Sensor,or a bad Throttle Position
Sensor. As you can see,many of the sensors of a computerized vehicle system can
create the same types of drivability problmes. But with a few inexpensive hand
tools and the right troubleshooting information you can diagnose and repair the
electronic computerized systems on today's vehicles. Remember the above are only
to be used as a guideline for testing. Many times,other components or sensors
can cause the same symptoms. Consult the manufacturer's auto repair manual
covering your specific vehicle before proceeding with auto repairs.
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