I have been trying to fix a P0401 Code on my 98 2.5 in sig. What I believe may be a symptom to the code is an intermittent noise that sounds like a Cicada, the noisy flying bug of summer. It has the same volume when cruising, not accelerating or decelerating. I'm thinking it is vacuum related. Has anyone had this and fixed it?
Thank you
JIm

 

P0401 Exhaust Gas Recirculation Flow Insufficient Detected

This is generally a DPFE sensor issue

EGR system is pretty simple
There is an EGR Valve on the intake manifold that is connected, by a metal tube, to the exhaust manifold, this valve is opened by applying Vacuum
There is an EGR Vacuum solenoid valve that applies vacuum to the EGR valve, this solenoid is controlled by the computer(PCM)

There is a DPFE(differential pressure feedback) sensor connect with two rubber hoses to the Metal tube between exhaust manifold and EGR valve

When the computer opens the EGR Valve, applies vacuum, it monitors the DPFE difference between the two hoses, i.e. the hose closer to EGR valve will have a bigger drop in pressure than the one farther away, this tells computer how much exhaust is traveling up the metal tube and into the intake.

P0401 means the "reported" pressure drop is not high enough, so Insufficient Flow

So first test that the vacuum lines on the EGR Valve and the EGR solenoid are OK, no cracks or breaks

Then check the TWO exhaust hoses from the metal EGR pipe to DPFE sensor are OK, no cracks or breaks, also make sure there is no water in these hoses, yes water

If all are OK then replace DPFE sensor on speculation, because these do wear out from moisture intrusion

If that doesn't get rid of the code then you will have to clean EGR valve and tube as they are clogged up with carbon build up, can be a big job

 

Thank you for your response.
I have replaced the DPFE sensor and the lines to it. as it showed bad with the VOM testing. I'll verify the lines are oriented correctly. I'll inspect the lines you mentioned. I am thinking a clogged line in the EGR and intake manifold, so removal and cleaning may be in order.
Thank you again
Jim

 

With engine idling you can put a longer hose on the EGR Valve and suck on that hose, apply vacuum, EGR valve should open and engine should start to stumble, suck harder and engine should stall
If engine doesn't run that badly then yes, EGR valve and/or tube is clogged up

 

After further checking, I ran a line from the line that goes to the EGR valve to a vacuum gauge mounted on my wiper blade and drove the truck. The gauge responded as expected, going to > 5" Hg when accelerating and dropping to 0" Hg immediately when off the accelerator.
I attached my hand vac to the EGR and upon applying and releasing vacuum, I could hear the pintle operate, but the vacuum would not hold. I believe I have a torn diaphragm on the EGR. I'm thinking that under higher vacuum conditions the tear is allowing the diaphragm to vibrate causing the noise. Regardless I'm going to replace the EGR and when open check the intake port for carbon build-up

On this 2.5 what works best to get the nut off the EGR Valve? It looks like it is 1 1/16". A crow's foot? A pipe wrench? I already started soaking it in PB Blaster

Thanks
Jim

 

Anything that works is best, lol

EGR system nuts are notoriously hard to get off

Crows foot flare nut is probably best because you get that wrap around which hopefully prevents stripping off material on the nut

 

I bought a 1 1/16" crows foot and it did make the removal easier. Since I was replacing the EGR valve, I tore the top off to give me more access. Also put anti seize on the tube nut. Took it for a quick test drive and the cicada noise is gone! Hopefully the P0401 code also.
Jim

 

Good work

Thanks for posting the FIX