P0400 Code on 1997 Mitsubishi Montero

P0400

Hello all:

I have a 1997 Mitsubishi Montero, FWD, 3.5L engine, not the sport. It is giving me a P0400 code indicating a flow proplem in the EGR system. I have replaced the EGR valve, cleaned (as best as possible without dis-assembling the throttle body, etc.) the EGR ports. I also took the vehicle to the local Mitsubishi dealer that working on it for about 3 hours, took it for a "drive cycle" and then pronounced it ready for emission inspection. However, when I took it back for the re-inspection it showed that the drive cycle had NOT been completed. Approximately 20 miles later the Check Engine light came back on and, guess what, that pesky P0400 code is back. Any ideas on other things that might cause this? Which method does Mitsubishi use to monitor the EGR operation, is it exhaust temp, O2 temp, throttle body/manifold pressure, or some other method I don't know about. I think they use the MAP sensor for the manifold pressure but am not sure.

ANY help ASAP will be appreciated.

Randy Abernathy