mort, if you have a 401 code set, the system will abandon futher attempts to operate the EGR system since it has already failed by testing using the computer diagnostic routine.
If you clear out the code, the system will try the diagnostics again for 2 drive cycles before the 'fail' logic sets the fault code.
The low flow test is done by pulsing the EVR vacuum supply to the EGR as a means of controlling the amount of opening the EGR will have.
Since there is an orfice in the metal pipe the DPFE measures pressure drop accross. It expects to read out a certain pressure differencial within narrow limits and is reported to the PCM for table compairison pass or fail result.
This system normnally operates within a road speed range and rpm ranges in Over Drive with light throttle applications as determined by the TPS and other sensors.
The low flow faults that would affect it are a restricted path from the exhaust manifold into the intake, a faulty DPFE sensor that cannot read the pressure. a faulty EVR that cannot pass the correct amount of vacuum to the EGR or electrical connectors and harness.
Since there are other specific tests done on this system for flow, hose failures etc. their absence often rules out certain faults.
The testing limits are very tight and so make this system often test fail for a small fault.
Best I can do is to help you understand the operation for narrowing the fault down.
Think low flow and what would be the causes.
Good luck.

 

Thanks to the members on here for posting the EGR light fix it worked cleaned the two vac ports in the throttle body they were clogged solid.

 

Welcome to the forum Rick

 

great threads for alot of tunning & maint...just getting started on EGR & DPFE checks...EGR operates under maual vacum, picks up, delays on vacum release...(sticky/dirty EGR?)...vac. hose to top of EGR cycles/pulses @ idle cold & hot...no change...DPFE has bat. ref. 12vdc on supply, but backprobe indicates 0-.1vdc @ idle cold, and same when @ operating temp...not the 4-6.ovdc expected...Ohms indicate 31.9...just below range of 32-70ohms...
EGRV reg. comes in @ specs. per manual...Assumption (i know) of DPFE being bad AND dirty/faulty EGR....
idle rpms @ 600-650 at op. temp in a/c,heat,or vent mode...hunting idle was fixed I think due to a/c cycling on low charge, thus PCM would "hunt" & not find defaults...long winded I know, but huge help on these forums...thanks in advance for my 2000 F150 5.4L 7700 w/158,000miles...

 

I just used this sight to repair mine. Part 1 -How to Test the Ford EGR Valve and DPFE Sensor
Great pic's and easy instructions.

 

Thanks Steve for that site...re read my post & boy was I spewin' stats but without reference...that site put me back on track...all was good with EGR, DPFE vacum reg, etc. until the volt test @ operating temp with the DPFE...took it out on the road & vac. test failed too....replaced with new from Ford & took ohms from both...old was higher on Ref. to gnd/Sig to gnd...new road test put a smile on my face...fingers crossed! Idles @ 600rpms w/or without A/C,Heat,Vent,Off...slight drag on rpm when turns is all thats left....off to the pwr/st/pump next time....lol

 

Steeven, thanks also for that site. Printed it all out and will refer to it if/when needed. Trying to avoid buying tools I may never need or use again. When last at the smog cert shop a gauge was put on EGR and got a good reading( truck was in test area so was "driven" as throttle was applied while in drive to simulate driving). Then he told me to take a look at gauge and all of a sudden there was not a reading.(?) He assumed EGR solenoid must be bad, so I replaced it. I already replaced DPFE but it did not solve issue. After spending time on this forum ( more than I 'd like to admit) I assumed I must have the carbon build up a lot of others said was their problem. Took off throttle body and inspected it and it was actually very/fairly clean. Saw really no carbon to speak of, just normal brown/dark brown coloration ( staining?) , certainly not carbon like some pics I've seen posted on here.Thinking maybe I should spend $20 ish and just get a vacumn gauge and do the road test(test 6) on Steevens site. I have inspected all tubing etc. Blown air through all possible small and large tubing to make sure there are no clogs. One of the small tubes ( in harness) admittedly does not blow through real easy but it does blow through. ( long tube ending up in small juncture with a few tubes on passenger side upper corner in eng compartment. Also ,as some have said sometimes its just a small tubing elbow that crumbles apart sometimes. ( usually one behind manifold that is hard to see coming from the PCV valve). Mine is an easy line to get to and is encased in large tubing to protect it from manifold heat. It attaches to inlet in back of throttle body and is in good shape, not the source of an air leak. I have done the spray brake cleaner method on areas looking for an engine surge to show leaks, but cant find any or at least tell a great difference in idle while doing so. Truck doeds not seem to have a real consistant/steady idle anyway, fluctuating a bit. Sorry for the long novel here, any help or suggestions would be greatly appreciated. Temped to go buy a new EGR valve ( more wasted $ maybe for nothing? ) just to know if that is the problem . HELP!!

 

Forgot to note mine is 98 F 150 Triton V 8 4.6. Has 180 k miles.

 

It's a good idea to have a vacuum pump. I can also bleed the brakes with mine. You can put 5-8" of vacuum on the egr and see if it leaks down. I'd also check the dpfe down tubes and make sure they are on the correct tube. Hi-Hi and Ref -Ref. You can also hook up the vacuum line with a t and sit in the truck and drive it to watch for vacuum on acceleration. That way you're not guessing if it's repaired .I was getting vacuum on a part time basis and it ended up being something simple. Drove me nuts trying to find that simple thing. The egr test sight helped alot. Good Luck
P.S. I'm not sure why but when my egr system was corrected I also got rid of that 12-1400 rpm surging. Just some info for the vault.

 

Look at the battery box I found a vacumn port that connects beside the battery on my 4.2l I am working on, the battery acid has deteroriated the plastic harness

 

in regaurds to post by SteevenH on this thread, thanks again...I went through all steps & thought prob. solved by DPFE replacement...ran great for a week, no stalling or rpm drop at idle...Yea!...well, yesterday got below 30degrees & it dumped on me like it did in past at aprox. 55degrees..never found one fix, but multiple svs or replacements...I decided to buy a new EGR valve & a new vacum solenoid to be ready so I could repeat the whole testing again...at cold temp. EGR passed vac. test & solenoid passed, thus another road test w/vac gauge...Never once did the gauge get above .5Hg's...CEL came on right when going into driveway (expected because of test)...rechecked vac. setup to make sure it didn't have seal on road test...all good...but while hot, the EGR valve would not pick up on manual vacum, as it cooled, it would pick up but leak down, then cold it holds 5Hg's for over 40mins...(coffee break)...Vac. solenoid has the correct vdc even at backprobe, and DPFE has correct .9 @ idle and increases to 3.1vdc under vac...I suspect both because vac. solenoid output to EGR gets very weak pulses...Plan: remove EGR to verify orifices & diaphram, then retest...if fails, then replacing EGR vac. solenoid.
so could this DPFE changeout have caused one of the others to show their "weakest link" in the system, or am I off course...any ideas are greatly appreciated and I'll post the results of my action today...

 

Yes the dpfe can cause weakest link, but more than likely the vacumn solenoid was a problem before. I had a hard time checking the vacumn system due to the flow. There is a box on the battery box that can have a break in the line! If the box lines are good and the white line is good going to the box according to the print out, make sure its not leaking from the vacumn elbow at intake (more than likely it's going to throw a po300 code/but it has thrown a po401 code). Check the voltage to the solenoid and check the updated tsb on dpfe voltages.<TABLE cellSpacing=0 cellPadding=0 width="90%" align=center border=0><TBODY><TR><TD align=left>Vacuum and Vapor Hose Diagrams</TD></TR><TR><TD>******** language=JavaScript> <!--var HIGHNONE = '';var LOWNONE = '';// --> *********><TABLE cellSpacing=0 cellPadding=0 width="90%" align=center border=0><TBODY><TR><TD><TABLE cellSpacing=0 cellPadding=0 width=536 border=1><TBODY><TR><TD align=middle width=536 bgColor=#ccffcc>Emission System Vacuum Harness</TD></TR><TR><TD></TD></TR></TBODY></TABLE></TD></TR></TBODY></TABLE></TD></TR></TBODY></TABLE>

 

5" should be enough to keep that cel off. The last I checked mine only had 3" and I've driven about 250 miles with no return of cel. Passed emissions and seems to be running better. Don't want to sound like a smart a## but did you have that code checked to make sure it's P401?
Of course if your egr is leaking down that is definitly a problem. I did notice my vacuum reservoir is leaking down also. I didn't want to unplug the battery and have to go through the driving cycle again before getting
the emissions test. I'm going to look at that this weekend. Although I do have 22" of vacuum at the egr regulator. It does drops to about 3" on hard acceleration. I have a sneaky suspicion it has something to do with the metal vs. plastic dpfe sensor . ( different vref signal )Anyway, Good Luck!

 

Well finally did a fifth road test with a line from top of EGR vacum "T'd" to a V.gauge & the other end to pump...No Pulses to EGR valve once in the warm up...Weird, cause it will bench test all day long & pass...so back to the vacum solenoid and 22Hg from intake right into the vac. port on the solenoid....after watching the vacum gauge at these points, my feeling was they were sticky/dirty but not failed completely @ lower ambient conditions.
Step 1 was replacing DPFE which helped for a week
Step 2 was testing EGR/Vac. solenoid per the link from Steve H. (thanks!)
Step 3 was replacing EGR after seeing condition when removed.
(didn't want to waste time cleaning, which never seems to last)
Step 4 was replacing the Vacum solenoid, even though output @ idle was
approx. 21Hg
My theory is once the engine warmed up, the light buildup in the EGR finally let it open, but the heat warmed the diaphram, which would then leak down rapidly. The Vacum Solenoid was dusted with fine crud/sand that increased the ohms upon reaching operating temp. which would cause failure in that device.
My road tests seemed to confirm this and with new DPFE,EGR,Vac.solenoid, it's hooking the gauge up inline & off for a final test. I have some pics & stats I said I would post the other day, but this whole "Weakest Link" had me goin' around. I also appreciate the diagrams & Scheme's from "Deerslayer73", for they were helped recheck my lines I had done previous to DPFE replacement. One last thing, my A/C clutch started cycling again after svs 2-3 weeks ago, this creates a problem when trying to reset PCM and the drive cycles. I'm going to leave A/C HTR controls off this time too.
Let me know if this sounds like I missed anything or chased the wrong theory. Ya'lls input is appreciated..

 

I got a emissions code p0401 so I was told it was a dirty or faulty eve valve so I replaced It code still there then I was told it could be dpfe sensor changed it code is still there what else can I do I drove the vehicle over 180 miles so I don't know what else to do... Please help asap