Well, here goes. For Sale: 1998 F-150, 146k miles, cold A/C, Has ABS and CEL. $1700.00


Reader says P0401. Cool! My favorite. Pay the man $1500.00, and off I go.


Pull a vacuum on the EGR valve, no change in idle. Uh-oh, gonna be a dirty job. Pull the throttle body to clean it, and immediately notice it doesn't much need it. Reassemble.


Pull a vac on the valve again, no change in idle.


Check voltage at the DPFE. 1.3v, but doesn't change with a vac on the EGR valve. AH-HA! Pick up a new DPFE and start removing the old one. Sensor is oddly clean, like the throttle body. Somebody has been chasing this thing already. I put the new one on, know full well nothing is going to change. It didn't. 5v reference voltage IS present on bottom wire. Phew.


Went ahead and checked voltages on The Vac Reg Solenoid, and drove the truck with Vac gauge connected. Nice, positive drop to zero when throttle is released. Voltages all checked out.


I am completely stumped. Any suggestions regarding the next direction of my suspicion would be appreciated.

 

Did you pull the EGR valve and check if it is opening when you draw a vacuum?

 

Leave everything else alone .
Pull the EGR off and open the ports in the manifold behind it.
401 code is a low flow test failure. That path is obstructed.
The only other fault would be the DPFE can't report the test results.
Good luck.

 

Thanks for the replies.


Yes KD, I tested the valve. The first thing I usually do is pull the tube off and pull a vacuum on the valve. If there is no change in the idle, the valve comes off for a cleaning and bench test. It was good. (Opens and closes properly, and diaphragm hold a vacuum)


Hello again Bluegrass. That was my train of thought too. The throttle body has been pulled and cleaned. Like I said in OP, it was suspiciously clean. There was no change in the voltage from the dpfe when I applied a vacuum to the egr valve. That's where it started getting strange. The lack of voltage change says "Bad DPFE" to me, but the one on the truck was obviously new.

 

Hmmmmm. I googled some engine compartment pics on line. It would appear the hoses running to the DPFE were swapped. As the nipples are two different sizes, the previous owner had to work at that. I have driven the truck a couple of times and the fault hasn't returned.

 

The hoses can't be reversed very easy but the hose sizes can be.
If the idle did not change when vacuum was applied to the EGR and the EGR does open, the passage is Blocked.
It has to be because the combination of exhaust pressure and intake vacuum will draw from the exhaust manifold toward the intake.
This exhaust will cause the motor to run rough.
If it does not, the passage is blocked.
Good luck.

 

Okay fellas. Time for the "Post of Shame". In my defense, it was dark and I had been at work all day.


Apparently, when I pulled the throttle body off, I must have been holding it backward from how I thought I was. The "suspiciously clean" ports I spoke of in my OP were NOT the EGR ports, but vacuum ports. >insert forehead slap here<.


This evenings activity revealed the EGR ports to be firmly packed flush with the inside of the throttle body.


A pick, a bore brush, and half a can of TB cleaner and my $1500.00 treasure will pop a wheelie. Well, kinda.


Thanks guys. Off to change a light bulb in the pantry. Hopefully I'll remember to take the old one out first.

 

Good for you.
It may sound a bit strange to say this but the system and it's codes are very precise such that a system 'self' mistake almost never happens but misunderstanding of the codes can happen.
Very often a code sets due to an associated remote system fault that initiated the final code as the only way the system had to inform of a fault.
A case in point is the 171/174 codes that is initially detected by the Ox sensors.
This causes a shift in fuel tables that in turn is caused by a lean condition in the intake system.
Thus the code description tells you of a lean condition bank 1 and 2 when it is not faulty Ox sensors as they are only reporting the fault.
When....the fault is remote to the detecting part of the system..
In this case you have to be awhere of this possibility and sort of look backwards for the cause but it is still considered accurate otherwise the codes would have very little value.
This is why I directed you back to the EGR passages in the intake.
If the engine does not respond to an open EGR, there had to be a blockage.
Good luck.

 

Exactly! The old "Trust Your Instruments" motto.


I was thinking about it on the way home this evening, and realized I had answered my own question. I had convinced myself that the passages were open, partially from a brief visual inspection, and partially because there was a strong vacuum when I started the truck with the EGR valve REMOVED! Uhhh, wait a second.


Funny how we can skip steps, and dismiss logic, when performing a fairy routine bit of troubleshooting. I would liken it to diagnosing a bad fuel pump without ruling out the gas gauge by pouring a gallon of fuel in the tank.


Lesson learned. Thanks Bluegrass.