Engine Backfires (Lean Fuel Mixture)

1. Access diagnosis codes. If abnormal codes are present, proceed to ``Diagnosis Code Interpretation.'' If normal code is present, proceed to step 2.
2. Check for vacuum leaks in air intake line. If no vacuum leaks are present, proceed to step 3. If vacuum leaks are present, check the following:
a. Hose connections.
b. PCV hoses.
c. Ensure EGR valve stays open.
d. Oil filler cap and oil level gauge.
3. Check ignition timing. If timing is satisfactory, proceed to step 4. If timing is not satisfactory, adjust as required.
4. On models equipped with 2S-E, 4A-GE, 4A-GEC, 4A-GEL, 4A-GELC, 4Y-EC engines, check curb idle speed. If idle speed is satisfactory, proceed to step 5. If idle speed is not satisfactory, adjust as required.
5. Check cold start injector for proper operation. If cold start injector is satisfactory, proceed to step 6. If cold start injector is not satisfactory, replace cold start injector and/or start injector time switch as required.
6. Ensure fuel pressure is as specified. If fuel pressure is satisfactory, proceed to step 7. If fuel pressure is not satisfactory, check the following:
a. Fuel pump.
b. Fuel filter.
c. Fuel pressure regulator.
7. Ensure fuel injectors are not clogged. If injectors are satisfactory, proceed to step 8. If injectors are clogged, repair as required.
8. Ensure EFI electronic circuit is satisfactory. If circuit is satisfactory, test is complete. If circuit is not satisfactory, check the following:
a. Fusible link, fuse or EFI main relay.
b. Air flow meter.
c. Water thermo sensor.
d. Injection signal circuit.
e. Air temperature sensor.
f. Oxygen sensor.
g. Throttle position sensor.