I have a 1997 Toyota Corolla with 100K miles. After a tune up, which included throttle body cleaning and new plugs, the Check Engine light came on. The code is “P0401 – EGR Flow Insufficient”.
The mechanic diagnosed a faulty EGR Vacuum Modulator and replaced it. No change. He tested the system again and claimed it was functioning properly. He suspected an intermittent EGR Solenoid and replaced it. No change.
Not wanting to spend more money guessing, I’ve been driving it this way. I’ve not noticed any change in drivability or gas mileage.
I’ve done some research to better understand the problem, and learned that the EGR valve is only used when the engine is warm. If the PCM determines the EGR valve is not working, it simply switches to a less aggressive timing to reduce detonation due to excessive cylinder temperature.
I reset the PCM a couple of times and the Check Engine light would always come back on after a day or so. However, I noticed that during the time the Check Engine light was off, which presumably is when the PCM is still trying to use the EGR valve, the engine would ping *a lot* under light acceleration when warm.
So, I have two questions:
1) Any tips on further diagnosing this problem?
2) Will any damage be done by just driving is this way?