Diagnosing 2.0 TDI AZV (BKD?) EGR problem

[VAG-Abound]

This is actually for a 2.0 TDI AZV engine in a Touran, but there seems to be more activity over here and it is the same mechanics as the Golf BKD so putting it here may help others in the future . The car is a 2003 model that has covered 140k miles with no significant parts replaced

For the past 18 months or so I've been getting and occasionally clearing the 'Emissions Workshop' dash warning. Each time it pops a scan with VCDS gives (sometimes with and sometimes without the " - Intermittent"):

16785 - EGR System
P0401 - 000 - Insufficient Flow - Intermittent
Freeze Frame:
RPM: 1449 /min
Torque: 76.0 Nm
Speed: 46.0 km/h
Load: 19.2 %
Duty Cycle: 14.7 %
Bin. Bits: 00001000
Mass Air / Rev.: 315.0 mg/str
Mass Air / Rev.: 510.0 mg/str

I'd assumed it was a crudded up EGR valve, but when I finally took it out at the weekend it was remarkably clean compared with some I've seen pictures of . But I scrubbed it up some more, checked that there was movement on the valve (it looked there was already due to a shiny bit at the base of the valve stem) and put it back. And unsurprisingly the fault came back .

To dig a bit deeper I logged the EGR specified / actual blocks for a 40 min drive and they look like the graph below. I know it came out small, so: Grey is actual, orange is specified, and little yellow line lower down is duty cycle [see below on this!]. Time axis in 5-minute intervals along the bottom, vertical axis at 200mg intervals - so the base line for 'Specified' near the start of the graph is ~275mg.



So it looks like everything is moving around which to me suggests the ECU is getting a variable measurement from whatever senses the flow, and managing to move the EGR to adjust the flow. But there seems to be a fairly constant difference between specified and actual. And the occasional crazy spike.

I've seen various suggestions for replacing EGR valve, vacuum solenoids (which look to be in a very awkward place!), sensors, hoses, etc. Are there any things I can / should do next to identify which part is at fault? For example, I've thought about disconnecting the EGR vacuum hose to see if that gives a flat sensor reading (I assume it should) which would indicate that the sensor is OK. Or using a large syringe to draw the EGR valve to a few positions and again check that the sensor measures a number of different, level readings.

BTW, the reason that the yellow duty cycle line goes permanently high about half-way across may be because we seem to have a turbo overboost problem (actually not frequent enough to be a real problem) and during the logging above I tripped the car into slow-mode which I assume disables the EGR management.

As ever thanks in advance for any insight and advice.

[browser1]

I had same problem with my BKC engine, cleaned egr valve though it wasn't too mucky stopped the eml light for about 100 miles then back on. I replaced egr valve and job done. Not difficult to replace.


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