Emissions System - Troubleshooting Tips
Classification: EF&EC93-019Section: Engine Fuel & Emission
Reference: SERVICE ADVISORY NTB93-181
Models: All
Date: December 14, 1993
EMISSIONS SYSTEMS DIAGNOSTICS
SERVICE INFORMATION
This service advisory is intended to help diagnose some possible reasons why a vehicle may not pass a vehicle emissions inspection for State vehicle registration.
1. Check all basic engine settings such as; Ignition Timing, Throttle Position Sensor etc, to verify that they are in specification. Refer to vehicle Service Manual for the exact year and model to determine the correct specifications.
2. Check engine oil for fuel contamination. A vehicle with 5-7,000 miles on the engine oil may have accumulated some fuel in the oil, especially on a vehicle that is used primarily for city short distance driving. In addition, severe driving conditions (as defined in the owners and service manual) warrant engine oil changes every 3,000 miles.
3. Inspect air filter condition for excessive dirt or foreign material (leaves, large insects, or paper). If there is a large amount of foreign material on the filter element it may disturb the air flow through the mass air flow sensor causing the mixture to go excessively rich or lean.
4. Check AIV system (if the vehicle is so equipped) for a contaminated air filter. Check for proper seating and operation of the reed valves. A properly operating system will make a pulsating sound when the AIV is activated. If the AIV is not operating, the high RPM test may pass but the idle test will not. Also, make sure that the vacuum lines to the AIV control solenoid are routed correctly.
5. Perform a mixture ratio test to ensure that the oxygen sensor is working correctly. On a CONSULT equipped vehicle perform a mixture ratio test in Function Test.
6. Check the fuel pressure to ensure it meets factory specification at idle and during snap acceleration. A few psi too high and the ECM will not be able to compensate. The higher the fuel pressure the richer the mixture.
7. If any service was performed to the emissions system to correct an out of specification condition, then the ECM's self learning needs to be cleared. This can be easily done on a CONSULT compatible vehicle by clearing self learning in active tests. This will allow the ECM to reset itself to the new conditions.
For a non-CONSULT compatible vehicle produced after 1988 model year the self learning can be cleared by the following three methods:
1. Disconnect the air flow meter connector while the engine is running.
2. Disconnect the 02 sensor connector while the engine is running.
3. Disconnect the battery cable.
Refer to "Injector Leak Diagnosis" (Code 45) in the Service Manual for detailed information for the three methods.
It is necessary to drive the vehicle with constant speeds under the following conditions for at least fifteen minutes without acceleration and deceleration before attempting to measure the emissions again.
A. No load 2500 +/- 300 RPM (in Neutral)
AND/OR
B. Loaded test condition only for the state of Arizona.
8. Check for leaking injectors. Install a fuel pressure gauge near the fuel injector rail and turn the ignition on to cycle the fuel pump and build fuel pressure. The fuel pressure should stay the same for a while on a cool engine. If there is a sudden loss of pressure after the fuel pump stops then try clamping the fuel rail inlet and outlet hoses to insure that it is the injectors and not a check valve in the fuel pump or a leaking fuel pressure regulator.