Understanding the EGR System:
The EGR system reduces NOx emissions by recirculating a portion of exhaust gas back into the engine's intake manifold. This lowers combustion temperatures, reducing NOx formation. Code 71 suggests a malfunction within this system.
Diagnosing the Problem (Code 71):
The specific cause of a code 71 can vary. Here's a systematic approach:
1. Check the Vacuum Lines: The EGR system relies on vacuum to operate. Inspect all vacuum lines connected to the EGR valve and related components for cracks, leaks, disconnections, or kinks. Pay close attention to the line from the vacuum source (likely the intake manifold) to the EGR valve. Repair or replace any damaged lines. A simple visual inspection may reveal the problem.
2. Inspect the EGR Valve:
* Visual Inspection: Locate the EGR valve (usually on the intake manifold). Check for any obvious signs of damage, such as carbon buildup, sticking, or physical obstructions. Excessive carbon buildup can restrict movement. Clean it carefully with a suitable cleaner (carb cleaner is often used, but follow the cleaner's instructions). Be gentle to avoid damaging the valve.
* Operation Test (Manual Check): With the engine off, try manually moving the EGR valve. It should move freely. If it's stuck, you'll need to clean or replace it.
* Vacuum Test: With the engine running, use a vacuum gauge to check the vacuum signal at the EGR valve. This requires connecting the gauge to the vacuum line leading to the valve. Compare the reading to the specifications in your repair manual. Low or no vacuum indicates a problem in the vacuum system or a faulty EGR valve.
3. Check the EGR Vacuum Regulator/Solenoid: This component controls the flow of vacuum to the EGR valve. It could be faulty if the EGR valve is not receiving the correct vacuum signal. Test its operation (often with an ohmmeter and following the specifications in your repair manual).
4. Check the EGR Temperature Sensor (if equipped): Some systems use a temperature sensor to control EGR operation. A faulty sensor can lead to incorrect EGR valve actuation. Test the sensor's resistance with a multimeter, comparing it to specifications in your repair manual.
5. Check for Exhaust Leaks: Leaks before the EGR valve can affect the system's operation. Inspect the exhaust manifold and piping for cracks or leaks.
Repairing the Problem:
* Replace Vacuum Lines: Replace any cracked or damaged vacuum lines.
* Clean the EGR Valve: Thoroughly clean the EGR valve if it's carbon-fouled. If cleaning doesn't solve the problem, replace it.
* Replace EGR Vacuum Regulator/Solenoid: Replace the vacuum regulator if it's faulty.
* Replace EGR Temperature Sensor: Replace a faulty EGR temperature sensor.
* Repair Exhaust Leaks: Repair any exhaust leaks.
Important Notes:
* Repair Manual: Obtain a repair manual specific to your 1995 Toyota Corolla. It will provide detailed diagrams, specifications, and troubleshooting procedures.
* Safety: Exhaust gas contains harmful substances. Work in a well-ventilated area.
* Professional Help: If you are not comfortable performing these repairs yourself, take your vehicle to a qualified mechanic.
After making repairs, clear the check engine light using an OBD-II scanner. Drive the vehicle for a while to ensure the problem is resolved. If the code returns, further diagnosis might be necessary. The underlying cause might be more complex than just the EGR valve itself.
