Recently picked up a 2000 standard cab long bed 2wd 4.6 xl truck. Truck has 175K miles, PO claims transmission was replaced 30K or so by a shop. When I got it had 1 code (P0401) and 2 pending (P0125, and P0402) I cleared all codes to start with to see what came back. P0401 came back after a couple days. I applied vacuum to the EGR Valve and did not have much reaction so I figured it was the TB elbow EGR ports that were clogged. I pulled the Throttle body and elbow off and of course snapped both bolts that hold the EGR. Had a machine shop get the bolts out. The ports were clogged solid with carbon so with a screw driver and blow gun and a bottle of carb cleaner later I got the ports clean. Put everything back together just used some hardware store bolts to get the EGR back on. Put vacuum on the EGR valve and now truck really runs rough with vacuum. Cleared the code and took it on a bit longer drive and CEL came back on. P0401, P0402, and P0125. Looked on here and it looks like the first step with the P0125 is check coolant level. Coolant level looked good so I pulled the thermostat and tested it with hot water. It started to open about at 160 degrees and barely stayed closed. Replaced the thermostat and reset all the codes. Temp gauge went from staying right near the bottom all the time to about in the middle. Drove to work this morning and no surprise P0401 came up again. What should be my next step with the P0401? Replace the DPFE sensor? Also truck has a vibration when I let off the gas at around 65 or so. I pulled the driveshaft to check the u joints and they both seemed ok although the back was a bit crusty looking so I replaced it. Still have the vibration.

 

The 401 is a low flow test failure.
At this point I would replace the DPFE sensor as it may not be able to report the results of a good flow test.
The vibration is another issue.
Go to Neutral and let off the throttle as a test to see if it goes away then decide from that which way the issue is likely to be.
And if braking has any effect.
Good luck..

 

Replace the DPFE sensor with a new Motorcraft unit.

 

Put a new Motorcraft DPFE sensor in and have been driving the truck the last two days and no code has come back so I think I have the codes cleared for now. The vibration does not seem to go away if I throw the truck in neutral nor does it seem to be caused by the brakes. I honestly though am not not sure yet exactly where its coming from or what is causing it. Sometimes I think its coming from the rear end sometimes it seems to be more engine trans. Sometimes I think its letting off the accelerator that causes it but I wonder if its more a specific speed range that it vibrates at and letting off the accelerator just gets into that speed range. Hopefully driving it more will help me figure out that.

 

After driving and experiencing the vibration, park the truck, and walk around checking each of the wheels for excessive heat. Just hold the back of your hand as close to the caliper as possible. If one of more is excessively hot, a caliper is intermittently dragging. My SD was doing something similar and that's how I found the RF caliper that was bad (again).

 

Just drove home tried to make it vibrate as much as possible. Felt both front calipers they were cold. Driver's side rear drum was cold. The passenger side drum was warmer than everything else but not hot.