My '96 4L developed the infamous P0401 insufficient EGR gas flow code 6 months ago. Rough running. Forgot to license it, now the St. Emissions Test was due before I could get new tabs issued. Finally got some warm dry weather to work on it. Started with the electrical measurements per DPFE graph of EGR flow vs electrical output to the PCM.
Showed the DPFE sensor failed, low V output max 2v. The old OEM aluminum body DPFE sensor was still in the old girl after 16 years. Corroded internally and ext., one hose nipple almost corroded off. Eureka.
Put in a new Motorcraft plastic body upgrade. Same darn code.
Retested everything from the EGR vacuum valve that feeds the EGR valve to the new DPFE and then the EGR tube from the exhaust pipe. All tested ok electrically and leak wise. No fresh air leaks in the EGR V or EGR tube.
All pointed to restriction in EGR gas flow when PCM demanded it. Ran small stainless steel wire down the upper EGR tube line nipple to the EGR orifice trying to clear carbon build up, still the same P0401. Poured several cups of paint store acetone down each EGR tube hose with engine off and at 2k rpm to emulsify and dissolve the exhaust carbon out. Engine vacuum sucked the acetone up thru the EGR valve also cleaning it internally.
Let set overnight and took out for test drive.
Still P0401. Almost ready to drive the green beast to the crusher of death.
Poured more acetone in the EGR and let set for 2 days prepared for the emissions station test failure then the $150 repair at emissions cert. shop with a shop waiver for no effective repair. Decided that the EGR tube or EGR valve were leaking fresh air, rusted and corroded. The end nuts on the tube were rusted away. EGR tube NLA from any source.
Went to the emissions test today and passed flying colors. NO P0401, drove all day on business. Got relicensed.
The old girl runs and drives like a dream, she's actually parked and sleeping in the garage tonight all pretty and shiny with a wash and wax job, got a reprieve from the Jaws of Death
Glad someone is up and running these days. Good sleuthing on your part. I would write a longer congrats note but I gotta go crawl back under my van and do more tranny push ups, i.e. take that pig out yet again.
I thought about smearing the EGR valve and tube in white lithium grease after all the work and failed attempts at repair, but gave up that on that idea when I realized all the neigbors would think I was roasting the Pope under my Aero with all the white smoke rolling across the neighborhood.
The old Aero ran too good today, just waiting for the next headache.
Sure love the seat on the old girl, comfy.
I went to emissions last week and faild due to a P0113 which is, Intake air temp circuit high input. I opened my chiltons and started reading. I found the IAT had been unplugged. I plugged it in and the moter started running smother than it had in a while. I figured I had the problem fixxed so back to emissions I went. This time it failed again but with two codes.... P0113 and P0401. Exhaust gas recirculation flow insufficent detected. on last weeks test this all passed but, not this week. will replacing the EGR valve fix this or, are these two things connected in some way? thanks for any info you can give.
Rarely will replacing the EGR valve fix it, as the EGR valve on these rarely every actually fails. The problem is most often either the DPFE has failed, a line is plugged with carbon deposits, or there is a leak in the EGR system.
The first two issues are actually closely related to running rich, which could be related to your previous issue, and is often related to tune ups (or rather lack of tune ups), and old faulty O2 sensors which cause rich operation.