Did You Know Your Car Can Fail For...? pt2

Did You Know Your Car Can Fail For...? pt. 2


This is a follow-up to my November 10, 2007 post describing scenarios for Smog Check failures that may not be immediately understood.

1. The gas cap is missing.
Explanation:
The gas cap is part of the fuel evaporative (EVAP) system. The EVAP system prevents raw gasoline from escaping the vehicle and evaporating into the atmosphere. As part of the inspection, the technician visually inspects the EVAP system and associated plumbing for defects or modifications, including the gas cap. If the gas cap is missing, fuel vapors from the gas tank will reach the atmosphere. The gas cap integrity is also checked. The technician will either pass or fail the EVAP system based on observations made during the test.

2. The gas cap is loose.
Explanation:
A loose gas cap will allow fuel vapor from the gas tank into the atmosphere. On 1996 and newer vehicles a loose gas cap can cause the "Check Engine " light or "Service Engine Soon" light to come on. When either of these lights come on during the Smog Check, the vehicle will fail the test.

3. The "Service Engine Soon" light is on.
Explanation:
This also applies to the "Check Engine" light. The vehicle's on-board computer monitors various engine management and emission systems on the vehicle. It looks at and receives information from sensors and actuators. The on-board computer uses this information to adjust the air-fuel mixture on the vehicle. It can also tell by the data it sends and receives if a particular emission system is working as it should. When the light comes on, the computer will store a code or codes telling the technician which emission systems are affected. This is part of the "Functional" test of the Smog Check.

4. The "Check Engine" light does not work.
Explanation:
This also applies to the "Service Engine Soon" light. During the Smog Check, the technician will move the ignition key to the "on" position. The "Check Engine" light or "Service Engine Soon" light should illuminate. If it does not, then the vehicle owner will not know when an emission system is not working correctly and the vehicle may not be performing optimally. This important warning light MUST work.

5. The engine RPM is too high or too low.
Explanation:
This applies to vehicles that require the ignition timing be checked as part of the Smog Check. The ignition timing is described as: the timing of the spark relative to the operation of the valves and the placement of the piston in the cylinder. BTDC stands for "before top dead center" a reference to a mark on the main pulley on the front of the engine. For most engines, when the timing is set to 0 degrees TDC, the number one piston will fire at the very top of the stroke. For vehicles where the manufacturer specifies a timing setting, the Smog Check inspection allows plus or minus three degrees from that setting. For vehicles where the manufacturer specifies a timing range, the vehicle must be in that timing range.
If the manufacturer specifies an engine RPM, the Smog Check inspection allows plus or minus 100 RPM.
For instance, if the timing spec is 10 degrees BTDC @ 800 RPM, the engine has to be at 7-13 degrees BTDC @ 700-900 RPM
If the engine RPM is below 700 or above 900 RPM it will fail the Smog Check.

6. The alternator was just replaced on your 1999 Honda Civic.
7. You just replaced the battery in your 2000 Buick Regal.
Explanation:
Statements #6 and #7 are related and apply to 1996 and newer vehicles.
The on-board computers on these cars monitor emission systems and components such as:

• Fuel evaporative system
• EGR system
• Oxygen sensor
• Catalytic converter
• Misfires
• Fuel system
• Secondary air system

There may be other or even more of these "monitors" on some cars. As the on-board computer looks at these systems, it decides if each one is working or not. If all is well it will call each system "ready". It takes time and driving time for the computer to complete this readiness check. This is called a "drive cycle" .

As part of the Smog Check the readiness monitors are checked, If not enough of these systems are "ready", the car will fail the Smog Check.

If the battery is disconnected, the on-board computer has to start these "readiness monitors" all over again, requiring a "drive cycle" to complete the checks.
So, if you get your Smog Check shortly after you have replaced you battery or for one reason or another disconnected it, the readiness monitors may not have run.

8. Your neighbor just turned off the "Service Engine Soon" light on your 2001 Chevy truck.
Explanation:
This is related to the previous two statements.
The "Service Engine Soon" light was on because the on-board computer saw a problem with one or more of the emission systems. By turning the light off , (requires special equipment) the computer has to start over and complete a "drive cycle" and enough "monitors" have to be ready. If not enough monitors are ready, the vehicle will fail the Smog Check.

More to come...
Thanks,
Bob