I get an intermittant engine code (about once a week but sometimes more often) that is P0401 - EGR Low Flow. The first think I did was to warm up the engine, then with a finger touching the EGR diaphram, I would increase RPMs to about 2000 rpms. I'd notice the valve would open but within a few seconds it would shut and stay shut until I let off the throttle. This was repeatable every time I tried it with the EGR valve opening each time for a few seconds then closing while I was still holding the throttle at high rpms (about 2000 rpms).
I figured I'd just replace the EGR valve and gaskets, the vacuum hoses, the modulator thing next to the EGR valve and the VSV valve and I cleaned out the vacuum fittings into the manifold using pipe cleaners going pretty far in. That should do it right? Wrong - I still have the same problem.
I've checked the following - If I manually open the EGR valve with a vacuum pump, the engine stalls immediately. If I put my finger over the EGR vacuum hose and increase rpms, I can feel the suction then it cuts off after that few seconds and stays off until I cycle the RPM's again.
I have also tested for exhaust blockage by watching the vacuum to see if it drops at high rpms - it stays high. Also, there is no vacuum to the EGR at all when the engine is cold. I also bypassed the VSV valve and when I do that, the EGR system works and keeps working. Something is shutting off the vacuum supply by applying power to the VSV valve (incorrectly).
I've read that the MAP sensor can fool the system and trigger this error but I am tiring of just replacing parts.
Has anyone experienced this and solved it - or does anyone have some suggestions? If someone helps me solve this, I will pay for the help. I am good with meters and electronics. This problem started suddenly about a month after replacing the front O2 sensor. At that time, I was getting an O2 heater failure and that went away when the 02 sensor was changed.
The parts used have been all geniune Toyota parts brand new. I am not getting any other MIL codes. My scanner can't do live monitoring that I know of but I do know how to use an ohm meter.
Any help is appreciated. This is a Japan made 4 cylinder version with 70k original miles on it. It runs great except for this problem.
Please help if you can - I'm out of ideas.