TROUBLESHOOTING 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 1970’s. 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). The engine must also be in good tune (spark plugs, wires, distributor cap and rotor, air and fuel filters been recently replaced, etc). The ignition timing and base idle speed must also be adjusted properly.
Complaint #1 – I HAVE AN INTERMITTENT “CHECK ENGINE” LIGHT FLASHING
An intermittent “Check Engine” light is an indication that a service code has been stored in the vehicle’s computer system. First, extract the service codes prior to making auto repairs. A common sensor that creates an intermittent “Check Engine” light is the Oxygen Sensor.
Complaint #2 – MY CAR STARTS HARD
Hard starting can be caused by a number of different items. First, inspect all vacuum lines on the engine and replace them if any cracks or brittleness is found. 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. On fuel injected vehicles, the sensors that can commonly be attributed to this problem include the Coolant Temperature sensor, Manifold Absolute Pressure (MAP) Sensor, or the Mass Air Flow Sensor.
Complaint #3 – MY FUEL INJECTED CAR IDLES ROUGH AND STALLS AT STOP SIGNS
Stalling at stop signs while the vehicle is in gear on carbureted vehicles could be caused by a failing Idle Speed Control Motor. On fuel injected vehicles, you should look for failure in the Manifold Absolute Pressure Sensor, Mass Air Flow sensor, or restricted fuel injectors. If you suspect restricted fuel injectors, we suggest using an in-tank fuel injector cleaner prior to automotive troubleshooting procedures.
Complaint #4 – MY CAR IDLES ROUGH
Rough idle is most commonly caused by a tune-up problem or vacuum leak due to deteriorated vacuum lines. Once these parts have been replaced or ruled out, the sensors that could be at fault include: Coolant Temperature Sensor, Manifold Absolute Pressure Sensor, Air Charge Temperature Sensor, or Idle Speed Control Motor. Check your auto repair manual for the testing of each of the sensors.
Complaint #5 – THE ENGINE SEEMS TO IDLE ERRATICALLY, WAY UP AND THEN WAY DOWN AGAIN
The items that may cause an erratic idle include the Throttle Position Sensor and the Idle Speed Control Motor.
Complaint #6 – THE ENGINE HESITATES ON ACCELERATION, BUT ONLY WHILE THE ENGINE IS WARMING UP
If you own a carbureted vehicle, first make sure the choke system is functioning properly. A hesitation on acceleration during the engine warm-up period may indicate a failure in one of the following areas: Coolant Temperature Sensor or Air Charge Temperature Sensor.
Complaint #7 – THE ENGINE HESITATES ON ACCELERATION AFTER IT IS FULLY WARMED UP
A hesitation or stumble during acceleration is most likely caused by the Throttle Position Sensor (whether the engine is carbureted or fuel injected).
Complaint #8 – I AM EXPERIENCING POOR FUEL ECONOMY, AND OCCASIONALLY SEE BLACK SMOKE OUT THE TAILPIPE
This situation is typically caused by an overly rich air/fuel mixture. This is commonly caused by a defective Oxygen Sensor, Coolant Temperature Sensor, Manifold Absolute Pressure Sensor, or a bad Mass Air Flow Sensor.
Complaint #9 – I HAVE POOR FUEL ECONOMY AND PERFORMANCE, BUT THERE AREN’T ANY SERVICE CODES IN THE COMPUTER
Sluggish performance is often attributed to plugged or restricted injector nozzles, in which case you can use an in-tank fuel injector cleaner to help restore performance.
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 TURN OFF 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.
gave up on sports when the Browns left Cleveland and now spends his
Sundays working under a shade tree in the back yard tuning his son’s
soap box derby car.)