I'm getting two codes on my '96 4.0 -

P0401 - EGR Flow Insufficent
P0402 - EGR Flow Excessive

Should I be looking to change out the EGR valve? Or is there something else I should be checking first?

Truck idles really rough...

 

100 to 1 its the DPFE. That's simple to test though, if you have a vacuum pump (necessity for a modern car) see if it will hold vacuum, if not then its a sure be the DPFE is toast.

Are the stickies back yet guys? There is excellent advice in the stickies on the DPFE sensor. Read it if you can find them. I printed them out and keep them in my toolchest.

 

Hey Agryphon,
Did you solve your problem? Was it the DPFE sensor, like Eigenvector predicted? I've been through both error codes, and it was the DPFE sensor in both cases. The part I bought at Pep Boys lasted, but the one I bought at AutoZone failed within months.

 

Actually, I have not worked on it yet. Got a back injury last week that's slowing me down considerably. Truck idles very "lumpy", seems to run ok, but lousy gas mileage. Age and condition of the plugs and wires are unknown, so trying to get those changed out first. The wires look pretty old, and seem to have a loose fit in the coil pack. They have only one clip per wire, not two clips like other wires I have seen...

Also wanted to do a little testing before I run out and hang new parts. How did you determine that the differential pressure sensor was bad? Or did you just change it?

 

Test it first. The test is trivial. Sure wish the tech stickies were front and center like they used to be.

I can't rattle it off the top of my head, but basically you apply vacuum (5 Hg) to the exhaust port and if it won't hold vacuum it's toast. One of the ports needs to hold vacuum, the other doesn't, so worse comes to worse if both don't hold vacuum you know the issue. If one does, either do more testing or swallow the 110 bucks and hope its the DPFE. Some of the other tests a bit more involved, like back probing and all that. At 109 bucks a module it's worth it to do a basic exhaust port test. If you can get to it, you might also try replacing the exhaust hoses that go from the exhaust tube to the DPFE (follow the hoses from the DPFE back). I replaced the exhaust hose on mine, it was totally melted to the exhaust tube and DOA, but it took an hour of fighting it wedged in there under the intake manifold. I didn't bother with the other DPFE hose, my bear paws wouldn't fit in there.

Where are the stickies moderators?

 

I used my mityvac to check the sensor, and neither port would hold vacuum. It would leak down fairly quickly. I also put vacuum on the EGR valve as well & and it sounded like it opened and closed like it should. So I replaced the DPFE sensor ($50 @ pep boys). I had the older style metal cased sensor, the replacement was a plastic bodied unit. The hoses are two different sizes, so it was easy to make the swap. Both hoses were in good shape.

I also replaced the PCV, which was cruddy looking, and the Idle Air Control, which had begun to make a howling noise after I attempted to clean it.

Runs much better now - I will clear the codes and see what happens.

It still has a slight miss, but hopefully that will clear up after I change the remaining plugs and wires tonite.

 

I did not pinpoint my problems quickly, but they were invariably traced to DPFE failure. In fact, the vaunted OBDC-II threw me a curve, upon the failure of the original DPFE that came with my vehicle (at about 70K mi). I was receiving P401 (insufficient EGR flow), when in fact, my vehicle was suffering from excessive EGR flow. This was quite disconcerting, considering that at that point, I had never heard of a DPFE, IAC, or much of anything else in modern engine design.

Second failure:

Upon the failure of AutoZone's DPFE (I can't remember the actual mfgr), I received a P402 (excessive EGR flow), which was accurate. Incredulous at the short lifespan of this DPFE, I hesitated to replace it. Of course, I was delighted and also disappointed that a sensor like that could fail after only a few months and a few thousand miles.

Autozone's part had a 90-day warranty. The Pep Boys DPFE (mfgr: Borg Warner) has a 1 yr warranty. My van is just barely starting to ping again, so my suspicions are returning...8 months later.

Yes, pinging was a salient characteristic of my DPFE failures. I reason the engine was running too lean (or at least with some improper laminar flow), thus introducing pinging, especially at freeway speeds, going uphill.

A more direct observation was that offered by a vacuum gauge, which I placed "as a tap" to the output of the DPFE. How could I do this? You say the output of the DPFE is electrical? Well, it is, but a solenoid follows the DPFE, in series with the EGR. The solenoid uses the DPFE's electrical output to control a vacuum circuit, whose "inches of vacuum" controls the EGR. This vacuum tap was easily made; much easier than baring delicate electrical wires, for a voltmeter tap-in!

I could see spikes in vacuum, coincident with engine flat-spot (near stalling). Thus I concluded that I had hyperactive DPFE, regardless of deceptive P401 code.

The cure was instantaneous, and the part was easy to replace, even on my monstrously inaccessible van engine. So I recommend you swap your DPFE, for about $50.

You might want to employ a vacuum gauge, too, to keep an eye on things. Mine is still in place, many months after DPFE replacement. One more ping, and I will start looking at that vacuum gauge.

>>>>>>>>>>
Like you, I have also replaced my IAC (Idle Air Control) valve, which muddied my DPFE problems, briefly. This part is non-repairable. Another $50 went to Pep Boys, and the replacement has worked perfectly.

I believe EVERY IAC is doomed to fail within $100 kmiles, but I am admittedly a novice on this. Still, I can see that engine "crud" will kill any IAC eventually.

 

While I'm competent to hook up my vacuum gauge, I'm not particularly educated in using it to diagnose engine problems. No one will ever mistake me for a mechanic! I don't necessarily even understand all that I DO know... That's where this forum comes in! And in case I haven't said it before - Sure do appreciate the help I get here!

 

Well, you DO have a vacuum gauge, so that says something about you!

You're not so interested in absolute vacuum levels, as you are in whether there's any vacuum at all. On my car, at least, there is a solenoid (essentially, an on-off valve) ahead of the EGR vacuum "signal." Given the "capacitance" of the downstream (EGR and vac lines) infrastructure, there will be intermediate levels of vacuum observable.

But here's what to look for.

Your EGR is supposed to be turned on (i.e., with clear vacuum level present) when you're barreling along on the freeway. That's because NOX is produced at high engine rates. You are not supposed to be inhibiting NOX at near-idle speeds. Yet that is what I was observing, thanks to my vacuum gauge, and my malfunctioning DPFE. More interestingly, to my novice eyes, was the clear correlation to elevated vac levels, and an exactly-coincident flat spot in engine performance, just as I would gently accelerate at very low speeds. THAT, to me, was the smoking gun that prompted me to replace the DPFE.

The solenoid could have been at fault, because I was observing its output, rather than the actual electrical output of the DPFE, but I made a correct educated guess to replace the DPFE, instead.

And, for the record, the vac levels I observed, under both failing and good DPFE devices, were definitely attenuated from manifold vacuum levels. That EGR would probably rupture, if subjected to severe vac levels.

As others have advised, I also subjected my EGR to manually-induced vac levels, and received coincident engine reactions. That told me the EGR valve, itself, as well as its associated gas piping, was operational. This is not a definitive test, but is a pretty good one, just the same.

 

After changing the DPFE, clearing the codes, and driving for a day or so, I am now getting P0402. With the engine running, I cycled the EGR valve a few times with my mityvac. Sure seems like it opens and closes - although if it closes all the way I have no way of knowing. Applying vacuum to the valve causes the engine to run a little rougher, and I can hear "flowing" noises from the area...

Think I need to be looking at replacing the EGR valve now?

 

Just noticed there is a small hole in the EGR body, just below the large disk... Guess thats bad, huh? What are the chances of unscrewing the valve from the pipe without the whole thing getting destroyed... dang.

 

A month or so back I posted a whole series of threads on my life of heartbreak dealing with my truck that just wouldn't stop pinging when the weather got hot.

From my experience the EGR, at least on the 2000 3.0, rarely needs to be replaced. It might be cleaned once in a while, but only if you clean it using harsh solvents will it need to be replaced. The diaphram is what you don't want to rupture, putting carb cleaner on it will probably do that. However I sprayed carb cleaner in my EGR, both with vacuum appled and without and got all that sooty crap out of the chamber and smoothed the valve seats out. All of this I did by removing the EGR and cleaning it. I didn't do it while the thing was still installed.

So my advice is to concentrate on the DPFE, the vacuum hoses, the MAF elements, and maybe the IAC solenoid (if yours has one). The EGR maybe, the exhaust tube - brushing it out wouldn't hurt, the EGR port on the intake manifold - brushing it out wouldn't hurt either.

Think about how the DPFE works, that's your key. It works by pressure differential - between the big hose and the small hose attached to it. If that works good, then something else is to blame. Remember the computer isn't diagnosing the problem - only reporting sensor data out of range.

 

About that hole. It's hard to believe there would be a hole in it. The body for the EGR is heavy cast iron.

Sure wish you post a pic of it.

As to removing the EGR, it takes a 21??mm open-end wrench and a rememberance of "righty-tighty, lefty-loosey" I almost destroyed mine after I forgot that.

 

Ok, here's the hole:

 

That answers that, replace.