Golf GT BMN Engine rough idle

[aurumdeus]

Hi,

I've got a Golf GT engine code BMN. The idle is really rough, and the EGR seems to keep bouncing up and down. When the revs are held higher then the engine smooths out and behaves more normally. The EGR spec and actual values move around significantly while the engine idles. When the EGR is forced on or off via vcds output tests then the idle smooths. My first thought was clogged EGR valve, however it seems like the ECU is ordering the EGR to keep fluctuating like that. There are no error codes. The engine also keeps forcing the air conditioning compressor off via CANBUS while this is happening (possibly due to low RPM? however the RPM's don't go particularly low when fluctuating), but lets it run normally at higher engine speeds.

The engine speed also climbs or drops in neutral under constant accelerator position, but quite a long way. Boost pressure at least tries to follow the spec value. Sometimes it manages it, sometimes it doesn't. I'm still thinking the most likely cause is heavy carbon build up in the turbo vanes, EGR valve etc, but this varying RPM under constant throttle doesn't seem to match that. No output tests are producing any error codes. The EGR test which cycles the valve open and closed with the engine running is doing it too quickly to get a good idea of what it's doing, although the values do seem to go up and down by at least 100 (the amount specified in the test).

Below are some graphs. First is showing engine idle speed (red) vs throttle position (green):

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Next is boost pressure spec (green) vs actual (red) (I think it's that way round)

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Lastly is engine RPM (red), EGR spec (green), and EGR actual (yellow). The graph starts with the ignition on, then engine starts and the idle issue begins.

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Does anyone have any pointers for this? I will probably start by removing and cleaning the EGR, but I gather that it's a bit of a pain to get at on this engine so seemed best to ask first in case anyone has dealt with something similar. The car is on approx 240k km.

Thanks!

[Crasher]

The BMN is well known for its idle issues, usually down to worn injectors. Has the DPF been removed and has it been remapped? At £1531.20 a set plus fitting it is beyond the value of the car to replace them and if the DPF is missing as well, not worth it.

[aurumdeus]

Hi,

Thank you for the reply. Good to know re the injectors. As far as I know the DPF is still there, and the engine hasn't been remapped. I'm not certain about the remapping though. I'll double check on the DPF in the next day or so.
Would injector wear lead to the EGR pulsing like that, plus the idle smoothing out when the EGR is locked in position via a laptop? The exhaust smells bad as well, something else I've seen mentioned in relation to this engine.
The injection amount correction values seem to jump around a lot for each cylinder as well while idling.

[Crasher]

Stick your finger up the (cold) tailpipe, if it comes out black the DPF is empty or faulty, the eye watering stench is a characteristic of a de DPF’d BMN.

[aurumdeus]

No sign of soot in the exhaust pipe, so the dpf must still be there and working. I did see some significant correction (up to around 2mg I think, I don't know what a normal acceptable amount is with this engine but that seems a lot) to injection quantity that pulses in time with the idle RPM. Cylinders 2 and 4 got a positive correction, while 1 and 3 got negative.

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The engine idles fine when first warming up, then suddenly something switches and the idle roughness begins. Is the EGR locked in a certain position to speed up engine warming?
If the RPMs are increased slightly then the corrections drop significantly.

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At one point the idle roughness just stopped instantly, for no apparent reason. The engine was warm, nothing else was pressed or activated that I know of.

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The issue also disappears if the EGR is forced open or closed via output tests.

The injector wear makes sense for idle issues in general, but this seems to heavily tied to the EGR that I think I'mm have to start by stripping and cleaning it, and seeing if that makes a difference.

[Crasher]

I have had many completely standard BMN’s in all four VAG brands where it has been impossible to cure poor running and exhaust stench problems without replacing all four injectors and at this point in the life of a BMN that means the scrap yard. All non EU6 diesels should turn into the breakers ASAP.