EGR code 16785 - EGR System: Insufficient Flow - P0401

[vtraudt]

Car 1996 A4 2.8V6 2 valve (AFC) On the engine side, this is now my priority, particularly after reading the potential effects, some of which I seem to observe: * Malfunction Indicator Light (MIL) ON * Reduced Exhaust Gas Recirculation (EGR) * Reduced Power Output/Engine running rough * Increased Emissions * may notice drivability problems such as pinging (a.k.a. pre-ignition knock) when the engine is under load or the vehicle is at higher speeds. So, a first glance under the hood revealed it is not a too easy/self explanatory fix. I had hooked up a manual vacuum pump to check that the EGR moves (it does; idle changes). What is next? Take the EGR out? Are there electric sensors (from other cars I know the so called DPFE (differential pressure) sensor was a culprit. On the Audi, too?) Are there good instructions how to remove and inspect EGR, sensors, etc.?

[AudiMick]

Originally Posted by vtraudt

Car 1996 A4 2.8V6 2 valve (AFC)

On the engine side, this is now my priority, particularly after reading the potential effects, some of which I seem to observe:

* Malfunction Indicator Light (MIL) ON
* Reduced Exhaust Gas Recirculation (EGR)
* Reduced Power Output/Engine running rough
* Increased Emissions
* may notice drivability problems such as pinging (a.k.a. pre-ignition knock) when the engine is under load or the vehicle is at higher speeds.

So, a first glance under the hood revealed it is not a too easy/self explanatory fix. I had hooked up a manual vacuum pump to check that the EGR moves (it does; idle changes).

What is next? Take the EGR out? Are there electric sensors (from other cars I know the so called DPFE (differential pressure) sensor was a culprit. On the Audi, too?)

Are there good instructions how to remove and inspect EGR, sensors, etc.?

It's probably a blocked EGR passage.

Remove the EGR valve , throttle body clean out passage that runs thru intake manifold.

Very common problem on the 12 valve V6, $20-30 (for gaskets and a can of Gunk) and about 2 hours DIY.

If you search their should be a procedure, the question comes up all the time.

[vtraudt]

The best I found with my forum searches was this write up. Are newer/better ones out there? Photos, particularly of the TB, the blocked passage, etc.

1) Remove the plastic battery cover. Using the 7/8 flarenut wrench, squeeze your arm past the ABS unit, and loosen the flarenut connecting the end of the EGR tube to the rear of the drivers side exhaust manifold. Remove the battery if you need more reach-in room. This is the hardest part of the whole operation.

2) Remove the plastic engine cover, remove the MAF to throttle body duct black rubber duct tube (loosen the two hose clamps).

3) Remove the black plastic throttle body duct: One hose clamp for the breather tube (drivers side), one squeeze-to-remove breather tube to the passenger side valve cover, and one vacuum line on the rear. Remove the two 10MM (11?) bolts on top, and then pull the duct off, by pushing it straight back towards the firewall. Careful not to lose the rubber bushings on the two guide pins!

4) Disconnect the cruise vacuum servo (one bolt, one hose, and a twisty (?) pin/wire at the linkage ball joint)

5) Remove the rearmost drivers side spark plug wire, and unplug the rearmost fuel injector (depress the wire spring, and pull straight off) for maximum access. Also, similarly unplug the square and rectangular plugs just rearward, mounted to a metal bracket, facing straight up.

6) Remove the two 10mm (11?) bolts which hold the EGR valve to the intake manifold. Use a 1/4 inch drive ratchet and short extension for the front bolt, and a universal swivel for the rear.

7) Pull the EGR valve off the intake manifold, and wiggle back and forth until the pipe to exhaust manifold comes loose. Now just set the EGR valve to the side, to gain access to the passage hole on the intake manifold.

8) Remove the blue vacuum line, and the two TORX head screws which hold the intake manifold changeover vacuum servo to the manifold. I did not pop the linkage ball off, just let it the servo hang.

9) Remove the 4 hex socket head bolts ("Allen key") which hold the throttle body to the intake manifold. If you want to remove the throttle body (for cleaning), disconnect the throttle position sensor plug, throttle cable (two clips), vacuum hose on the right side, and evap hose on the left side (clamp). Otherwise, you could just let the throttle body hang against the firewall. (Amazing how small the "Primary" throttle butterfly is!!)

10) The plugged EGR passage is now visible (!?). It is located on the intake manifold "floor" just behind the location of the smaller primary throttle butterfly was located. It is probably just a darker black circle, about the diameter of a pencil eraser.

11) Clean it out with a sharp instrument. Use a shop vac to try to suck up the carbon debris. Then clean the rest of the passage, which makes a 90-degree turn, and runs to the now exposed EGR valve hole. I used a foot long piece of aircraft/bicycle cable, attached to my cordless drill, as sort of a power-cleaning snake.

12) Reverse everything, and put it back together. Double check that you have not forgotten to reconnect any hoses, connectors, spark plug wires, vacuum lines...
Reset the computer by leaving the battery disconnected for 15 minutes. I did not use any gasket sealing compounds.

13) Enjoy code free, ping free motoring for another 80K miles!

[AudiMick]

Thats a very good writeup, I dont have pics, it would be hard to take pics of the egr passage.

[moloveshisaudi1254]

He said when vacuum was applied via hand vacuum pump the EGR moved and idle changed. How can that indicate a blocked passage?? Isn't that what it is suppose to do? It is one of the tests to check for a stuck EGR or blocked passage. The temp. sensor at the end of the passage is suppose to pick up the increase in temp and pass that info to the ECU.

[AudiMick]

Originally Posted by moloveshisaudi1254

He said when vacuum was applied via hand vacuum pump the EGR moved and idle changed. How can that indicate a blocked passage?? Isn't that what it is suppose to do? It is one of the tests to check for a stuck EGR or blocked passage. The temp. sensor at the end of the passage is suppose to pick up the increase in temp and pass that info to the ECU.

That's true, but in my experience the passage doesn't need to be totally blocked to have the problem.

Opening the EGR valve fully at idle should cause more than a slight rPM change, it should probably almost stall.

The OP could run VAG.com tests on the EGR temp sensor to see if it's functional and also check there is enough vacuum from the plenum / intake to operate the valve.

Chances are the EGR passage is blocked as this is a common problem.

[4Driver4]

Well, I guess the upside of no one using the search feature anyone is that no one complains about how crappy it is.

[turbo510]

A couple things in that writeup I'd question: I have no idea why the battery is mentioned, but the EGR's exhaust manifold connection is easily reached through the driver's side wheel well. Then, I wouldn't just lay the EGR to the side, you want to take it and the pipe out for inspection and cleaning. It will come up and out with some twists here and there. Make some scratch marks on the pipe and EGR valve for alignment when reassembling.

The valve itself shoud be cleaned in a way that doesn't get any kind of solvent in the diaphram area. I used "Easy-Off" to loosen the gunky stuff, carb cleaner to finish it up.

The clogged passage is closed off at the passenger side by the temp sensor. I didn't take mine off, would have made cleaning easier if I had. You'll need the appropriate crow's foot wrench and ratchet extension.

The throttle-body was held by Torx screws on my '96, T45 I think. They were pretty tight too. Buy the proper tool because you don't want to deal with a stripped screw back there.

While you have the throttle off, clean the bores and plates, also check your valve cover breather hoses, and use some sealer on the plastic manifold where it fits on the throttle body rubber gasket.

[dcautomech]

Working on my A4 Quat 1997 2.8L. P0401 code. Have done all the checks of the EGR valve for operation. Everything functioning. Vehicle does not stall with vacum applied to valve directly, but sure stumbles and runs rough. I'm thinking that this is enough flow so I am looking toward the EGR temp sensor. Only problem is I don't have accurate wiring diagrams that include connector pin outs. I have ECU connectors, (5) a 12/24/24/24/12 pin connectors. Diagram says temp sensor, but don't identify it as an EGR sensor. Haynes manual idents as intake air temp sensor, with a green/red wire. I'm hoping that the intake air sensor as it's called in Haynes is also the EGR sensor. I really need help with pin charts so I can accurately check voltage at ECU. A little background on me is that I am a 25 year ASE/20 Chrysler Master Tech, and spent 20 years in the So. California area as a licensed Smog Test and Repair technician.
I'm in process of cleaning out the EGR passage and see if anyone can give me some direction on the electronics as I mentioned for pin outs, etc. I have found that it is a 5 volt sensor and should change with temp, just as I am used to.

[turbo510]

See if the code comes back after the cleaning. Carbon plugs up against the temp sensor, which might somewhat insulate it even if some exhaust is getting past. If you can get that sensor out, the passage cleaning will be easier. You'll need the throttle body out too, to clean the short intersecting passage.

I looked at the schematic that I've got for a '96, no mention at all of the EGR system.