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EGR code 16785 - EGR System: Insufficient Flow - P0401

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Old 02-23-2011, 01:30 PM   #1
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Default EGR code 16785 - EGR System: Insufficient Flow - P0401

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.?
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1999 Passat 1.8T manual; Kraftwerk Turbo TM3, 550cc Bosch EV14, FSI coils, 3" MAF, 3" DP, custom TIP, FMIC, Kraftwerk Tune

2002 Audi Allroad 2.7T quattro manual; Kraftwerk Turbo TM4, 750cc Bosch EV14, FSI coils, stg 5 clutch, FMIC

2002 A6 3.0 bad (auto), waiting for 2.7T to be swapped in

2000 A4 ATW manual, blown 1.8T, waiting for build block, AEB heads, big turbo, water/meth.

www.regulatorfix.com
www.kraftwerkturbo.com
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Old 02-23-2011, 02:08 PM   #2
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Quote:
Originally Posted by vtraudt View Post
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.
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Old 02-23-2011, 03:19 PM   #3
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Default EGR (passage) cleaning - better write up? Photos?

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!
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1998 Passat 1.8T manual; Kraftwerk Turbo TM2, 550cc Bosch EV14, FSI coils, 3" MAF, 3" DP, high flow manifold, S4 clutch, single mass FW, FMIC, custom TIP, Kraftwerk Tune

1999 Passat 1.8T manual; Kraftwerk Turbo TM3, 550cc Bosch EV14, FSI coils, 3" MAF, 3" DP, custom TIP, FMIC, Kraftwerk Tune

2002 Audi Allroad 2.7T quattro manual; Kraftwerk Turbo TM4, 750cc Bosch EV14, FSI coils, stg 5 clutch, FMIC

2002 A6 3.0 bad (auto), waiting for 2.7T to be swapped in

2000 A4 ATW manual, blown 1.8T, waiting for build block, AEB heads, big turbo, water/meth.

www.regulatorfix.com
www.kraftwerkturbo.com
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Old 02-23-2011, 04:06 PM   #4
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Thats a very good writeup, I dont have pics, it would be hard to take pics of the egr passage.
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Old 02-23-2011, 05:41 PM   #5
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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.
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Old 02-24-2011, 04:55 AM   #6
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Quote:
Originally Posted by moloveshisaudi1254 View Post
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.
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Old 02-24-2011, 05:20 AM   #7
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Well, I guess the upside of no one using the search feature anyone is that no one complains about how crappy it is.
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Old 02-25-2011, 06:16 PM   #8
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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.
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Old 06-18-2011, 09:15 AM   #9
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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.
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Old 06-18-2011, 09:49 AM   #10
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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.
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Old 06-19-2011, 04:15 AM   #11
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Quote:
Originally Posted by turbo510 View Post
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.
Makes sense. Once I get to the area of the sensor, I can double check the wiring and be able to check that after re-assembly.
Thanks for the info!!
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Old 06-19-2011, 09:12 AM   #12
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Makes sense. Once I get to the area of the sensor, I can double check the wiring and be able to check that after re-assembly.
Thanks for the info!!
Why don't you check the sensor using VAG.com? Group 17 IIRC

That sensor is a pain to remove, I really doubt carbon buildup would cause the sensor to react slowly.
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Old 06-19-2011, 01:27 PM   #13
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Thanks guys. The sensor isn't really that bad to get to. Once the TB is removed, it's about 1" forward of the TB, just to the right of center. A 14mm box end and there you go. Once you break it loose, it will finger turn out. I don't really believe the Haynes manual does anything but make a job more difficult. It you want to do it right, get the original Audi Service Manual.
As to the sensor and checking it, The connector is just below the Vacum diaphram for the cruise control. There are two connectors there, it is the rear most connector. Under the boot you will find two wires, White/black and tan/red (or brown/red) DVOM connected to each terminal, (positive to white wire, neg to brown) and you'll read somewhere between 4.25 to 4.40 volts with the connector connected. I cheated a little and took the manifold vacum line of one of the solenoids (I didn't bring home a vacum pump)and touched it to the vacum line going to the EGR valve. Of course the engine started running rough, but the DVOM's voltage reading at the sensor connector started dropping immediatly indidcating good sensor operation.
As to carbon "insulating" the sensor, by way of design, I can see this happening. The line bore goes accross the bottom of the plenum with an intersected bore for entrance to the intake system. After looking at the sensor, I don't believe the exhaust flow does more then just reach the tip of the sensor. (direct heat from the exhaust may burn the sensor) Residual carbon build up, along with oil vapor that moves around in there could easily cover the tip of the senor.
On this model, the nut on the EGR tube where it enters the valve. Loosen 1/2 to 1 turn, remove the bolts from the EGR to manifold, and work the valve back and forth a couple times and it will lift off the tube without having to remove the tube from the exhaust manifold/cylinder head connection.
As to removeing the sensor, I would recommend a 14 mm crows foot. I mentioned that I used a box end, but this involves cutting the wiring to remove the connector so you can get the box end on the sensor. These wires CANNOT be soldered back together, so I used a type of "butt" connector to crimp the wires together and used a GOOD quality heat shrink to seal the crimp.
As to whether this has fixed my problem? Only a road test will tell for sure. Since all the functional and electrical tests of the EGR system passed, I am looking forward to a good repair.
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Old 06-13-2012, 07:36 AM   #14
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Does anyone have a detailed diagram or photo of the egr system for an A4 quatro 1997 v6
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Old 06-19-2012, 03:20 PM   #15
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Quote:
Originally Posted by moloveshisaudi1254 View Post
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 is true and a good observation on your part. However if you haven't seen the EGR temp sensor and it's passage and position, then that is a valid question. Due to design, you can have an active EGR system and still have the fault due to an insufficient temp change with EGR flowing. Carbon can biuld up on, and "insulate" the temp sensor and allow the ECU to "think" the EGR is not flowing, even though it actually is.
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Old 06-19-2012, 03:43 PM   #16
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On going now, it's been a year now since I've done my repair, cleaning the EGR passage and temp sensor port, and all is well. As to someone thinking that carbon build up could not insulate the temp sensor, I would have to say that statement is inaccurate since that is what has happened with my A4. Once I found the EGR temp sensor connector, I back probed the sense wire and observed the voltage change prior to cleaning it. Very little to no change at all. After removing and cleaning the passage and making sure the EGR valve itself was clean, along with the sensor and it's port/passage, then checking the voltage change with the system clean and seeing a nice voltage sweep tells me that carbon build up CAN affect the sensors operation. I had the same symptoms, checked the code, then checked the EGR valve operation, actuating the EGR solenoid and watching a vacum gauge, verifying operation. Then manually opening the valve and watching RPM change and idle quality deteriorate told me that EGR system was operating as designed. At this point I needed to know WHAT the ECU looks at to see if the EGR system is operating. You guessed it, the temp sensor. Happy motoring!!
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Old 06-19-2012, 03:43 PM
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