I've been chasing a P0401 code on my 98 2.5. I've fixed several things (DPFE, IACV several vac line leaks, EGR, Riser tube to the EGR) that were verified as bad but the code P0401 returned. Hooked up my home made smoke machine and the only smoke I saw was coming from the newly installed Idle Air Control Valve. Spent quite a bit of time looking for leaks along each vac line and device and saw no evidence of vac leaks with the exception of the IACV where the air would come in. I'm wondering where to go next?? Maybe the MAF? I cleaned it with MAF cleaner already. Any ideas>
Thank you
Jim

 

As a follow-up, I was reviewing what I had done on the truck and realized that when I had the EGR off I did not verify the passage from the EGR to the throttle body was clear. I removed the EGR and probed the passage and it was blocked. I sprayed with carb cleaner then using a .357 barrel brush and more carb cleaner I cleaned the passage. Truck running very good. We'll see if the code returns before doing anything else.
Jim

 

More follow-up; P0401 came back. The truck is idling rough. I'm wondering if the newly installed IACV is bad. I'll be checking component voltages and grounds.
Jim

 

P0401 - insufficient EGR flow
Means the computer is not detecting a big enough drop in pressure from the DPFE(pressure sensor) when it opens the EGR Valve

This can cause pinging/knocking in the engine, after warm up, when accelerating, if code is "real"

There should be no symptoms at idle, unless EGR valve itself is not closing at idle
The EGR system is only used after warm up and only when accelerating or cruising at higher speed, it prevents spikes in cylinder temps that produce NOX gases(toxic gases)

It would be a long shot for the IAC Valve to cause this code, And IAC Valve has its own code set the computer can use if it is having a problem
You can really only use Ford(motorcraft) IAC Valves as replacement, 3rd party IAC Valves are know to have issues on Rangers
Better to get a used Ford IAC Valve at wrecking yard than new 3rd party IAC Valve

The DPFE sensor has 3 wires and 2 hoses
Make sure wire connections are clean
The 2 hoses have exhaust in them so can fill with water, all exhaust has water in it, which is why exhaust systems rust from the inside out, just FYI
So make sure hoses are clean and dry, this is also what 'kills" DPFE sensors, the moisture gets inside the sensor and damages it over time

The 2 hoses connect in a specific order to the EGR tube, the one closer to EGR valve detects the pressure change when computer opens the EGR valve
The one farther away is the comparison pressure

The computer uses the EGR solenoid to open the EGR valve, EGR solenoid has 2 wires and 2 Vacuum hoses
Make sure wire contacts are clean and dry
1 hose goes to EGR Valve
1 hose goes to intake manifold or another vacuum hose that goes to intake manifold

Computer varies the voltage to EGR solenoid to send vacuum to EGR valve that causes it to open a little or alot, the opening cause a change in pressure that the DPFE detects and sends to the computer
P0401 means computer is not getting correct FEEDBACK from DPFE sensor when it sends voltage to EGR Solenoid

EGR solenoid can be tested by applying 12volts/ground, it should CLICK open and pass air, then CLICK closed when power is cut, and block air flow, it has no polarity no +/- terminals, power can be applied in either direction

 

Ron,
Thank you for your detailed and informative response. I hope to do some troubleshooting this coming weekend and I will post the results. The search for the cure(s) for the P0401 has been interesting to say the least. I thank you for sharing your knowledge and experience.
Jim

 

Well the P0401 is still an issue. I replaced the DPFE with a motorcraft unit 6 months ago and the P0401 has been returning for the past few months. I finally had time to check things out and the voltage from the DPFE is a constant 4.95 vdc, regardless of vacuum applied to the EGR or not. It is my understanding that the 4.95 vdc should only be seen when vacuum is applied. Has the DPFE gone bad again after 6 months?

Thanks

Jim

 

It is if that's the center wire you are testing voltage at
Input voltage should be 4.9-5.1volts

Voltage range and operation seen here: http://www.tomco-inc.com/Tech_Tips/ttt36.pdf

Its a good read

 

Thank you. I am going to do more diagnostics today.
Jim