1997 Corolla Egr Flow Insufficient - Pre-1997 Toyota Corolla and Geo Prizm

Guest egoldsmith

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?

Thanks,

Eric

Driving around with an EGR problem will not hurt drivability at first - since the ECM can compensate accordingly - but it is only a temporary fix (allows you to drive and get it repaired laters). Some damage/problems may occur, since the timing is pulled back and more fuel duration dialed in to compensate - you may see increased carbon buildup, loss of power, may not pass emissions (NOx levels will be higher), lower fuel economy, etc.

EGR should flow high at cruising speed and medium acceleration. Should be low flow at lower speeds and light loads. Should be no flow at cold idle, warmup, or WOT.

Chances are it is a carbon build-up inside the EGR valve itself. Most of the time you can get deposits that unseat the valve inside or actually block the intake or exhaust tubes inside. I believe there are four parts to the EGR - exhaust temperature sensor, valve, solenoid, and pressure sensor - they should have been all tested first before you replace components.

I assume that the mechanic put a vacuum on the valve to see if it actually working or not, before he started replacing parts. A vacuum pump pulls the EGR valve open while you monitor RPMs - should be a change in RPMs with the EGR open.

Then he should have cleaned the EGR valve to remove deposits - then after everything is tested out - replace stuff (pressure sensor, valve itself, filter on modulator, modulator itself, etc.)

I'd go back and see if the mechanic followed the correct diagnostic procedure or try a different mechanic altogether. I would definitely not recommend driving around with this issue. Emissions related components can be a nightmare to diagnose and correct - but with so many other systems tied to them, I would not let this slip by.