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Hi there,
This is my first TDI. I absolutely love diesels, so naturally I gravitated to the TDI (5-speed, of course!) as my choice for a fuel-efficient car. It now has 224k miles on it, but it still runs like a champ. That being said, a year ago, I noticed that the idling was a little rougher than normal, but it wasn't really noticeable, and if I put the A/C on, it idled like it was brand new. I just ignored it. About a month ago, it threw a (p0401, I think) OBDII code for the EGR sensor and really started chugging at an idle. I took the EGR valve off and cleaned it (there was a bit of buildup, but less than I expected for the mileage) and also attempted to clean the MAF sensor. Just for maintenance, I replaced all the vacuum lines since a few of them were showing signs of wear. After resetting the codes, it still idles a little rough (roughness goes away under any sort of load again) and the P0401 code is back. (I am assuming that, since the light popped right back on after about 20 miles)

As an interesting side-note, it idles perfectly until the engine is *almost* fully warmed up, and then it starts idling rough. For example, if I drive to town, (a full warm up) go into a store for about an hour, come back out, start my engine up and then start fiddling around with my phone or something, the engine will just start chugging at some point (~5 minutes) like a switch was flipped. So my theory is that it has to be something that is electronically regulated.

So I have a few related questions.

  1. What sensor engages at almost full temperature, but the [possibly erroneous] readings are ignored before that point?
  2. I have heard that the idling problem is sometimes related to the MAF sensor. Should I replace that just for routine maintenance at this mileage even though it is not throwing a code? Do you recommend spending the extra and getting OEM, or is something like this ok: http://amzn.to/ensl8n ?
  3. Is it possible that the EGR Sensor/Valve is bad or failed? Is there a good way to test that, and if it has failed, would it even cause the rough idle?
  4. Are there other possible causes that are not showing up through OBDII?
Thanks so much for your time!

Ryan Hoshor
Woodleaf, North Carolina
 

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Welcome to the forum Ryan. welcometomyturbodies

All that I can tell you is

VAG Error Code: 16785

EOBD II Error Code: P0401

Fault Location:
Exhaust gas recirculation (EGR) system - insufficient flow detected

Possible Cause:
Hose leak/blockage.
Basic setting not carried out.
Exhaust gas recirculation (EGR) valve/solenoid.

As taken from my Free EOBD II Error Codes software ;)
 

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Here is a recent related post: http://www.myturbodiesel.com/forum/f5/hard-start-hot-easy-start-cold-4600/

Based off the code, there's EGR insufficient flow. Check the wiring, solenoid, and the valve. It's possible that it's caused by a bad MAF sensor as well.

1. It shouldn't. The car should always run better warm.
2. Possible. I would not recommend replacing it before testing it or else you're throwing money at the problem. Datalog the MAF values and replace it if it's bad. I would get an OEM equivalent. Read the MAF FAQ on how to test and some links to parts, it should be a TDI MAF. http://www.myturbodiesel.com/1000q/a4/MAF-sensor-testing-VW-cleaner.htm
3. Possible. Vacuum test it and see if it holds. You could try swapping the N75 solenoid and EGR solenoid (refer to your vac diagram) as a test to see if that does anything.
4. Bad maf's normally don't trip codes, egr normally does.

The injection pump timing should be checked as well. It's possible it's within range when cold and out of range when warm. Read the ALH engine timing belt article part 2 for how to do this. And when was the TB last changed? Did someone try to turn down the EGR using VCDS? This is mentioned on other sites - if so, undo that mod because the ECU will adapt and you'll get lower mpg. I don't know if rough running is also a side effect. Also check the injection quantity IQ (look in the FAQ)

Also, post moved to mk4 technical.
welcometomyturbodies
 

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Hello, I m really curious how you solved your problem.
I have an 2003 Golf, engine code is AXR.
After the engine warms up, the idle is really rough, but I also saw what is causing this : there is a valve directly under the electrovalves block that goes ON OFF ON OFF like crazy and this is causing the engine to shudder and the rpm to go up and down like 50 rpm or more ...
I scanned the car but there is no EGR error.
A guy who knows these engines very well inspected the electrovalves, pulled off some of the vacuum lines and of course that the shuddering stopped, but this is caused by electronic command from the ECU.
Now my question is, why is the ECU opening and closing that valve like crazy ?
What is that vaccum controlled valved used for ? Is it another EGR valve ?
My friend told me that AXR engines have a two way EGR so 2 valves ?
 

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Actually, my issue was resolved by following the instructions here: under the "Solving the "Slow-down Shudder" and Other Shudder Issues - Recalibrating the Fuel Quantity at Idle" heading. After that, it idles like a dream.

Sorry I can't help you more!

Thanks,
Ryan
 
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