97 F150 EGR Insufficient Flow - DTC P401 (EGR Vacuum Regulator solenoid)

Jason, It's good to see someone who tries to troubleshoot a problem instead of asking "what should I replace next...". The EVR has +12 volts to it all the time, the PCM modulates the -12V (ground) to vary the EGR operation. You're on the right track, just a little backward in your thinking. ;) Most PCM outputs control the negative (ground) side of the circuit with the +12volts being constant. Don't feel bad, most everything in a car is the opposite and the concept can be a little hard to wrap your head around at first. You have worked the EGR valve and verified that you have flow with the valve open. Take a test light connected to ground and probe the -side (negative, ground, control side) of the EVR, this will usually energize the solenoid and apply vacuum and cause a change in the idle. That will verify the EVR. If that checks ok, then look at the DPFE. Test if you want to, but the DPFE is a pretty high failure item and most likely the problem. Check the 2 hoses for cracks and make sure you reconnect them correctly.

Hope this helps, Tom Adkins

Something else you may want to check. My '95 4.6 has a nasty habit of clogging the EGR ports in the intake with guck. The engine burns a bit of oil since it is a higher mileage one. The goo in the EGR ports eventually clogs them, giving the P0401 reading. I have to remove the throttle body and elbow to the intake, then scrape out the channels. I should probably replace the valve seals but I don't have the time or $$ right now. I only have to do it every 40-50k. I have seen instructions and pictures of the process on the web somewhere but I can't seem to find them now.

If your engine is using oil, even without any smoke, chances are good the ports are clogged. I have to do it again soon myself.

PoD

Paul, you hit the nail right on the head. The EGR ports on the throttle body (TB) were plugged solid. They were so bad that gunk was protruding out of the ports and looked like one of those "black pellets" that you can light and they produce a black snake. For those interested, the ports are about the diameter of a pencil (hard to tell since I had no reference point).

I pulled the throttle body, cleaned the ports, and replaced the rubber end of the hose which connects from the pcv to the back of the TB. Replaced the gasket at the bottom of the TB where it mates with the intake and reassembled. So far, no CEL (check engine light), so I'll keep my fingers crossed.

Thanks again.

Thanks for posting the outcome. I wish more folks would do so.

Frank