View Full Version : EGR code 16785 - EGR System: Insufficient Flow - P0401

02-23-2011, 02:32 PM
Car is 1996 A4 2.8V6 2 valve (AFC) auto trans, quattro

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.?

02-23-2011, 04:21 PM
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!