K-Series poor warm idle, stumbling, noisy inlet (EU3)

[garybee]

Had a very strange fault with my 2004 S3 recently.

When starting it would crank and fire as normal then initially settle to a steady fast idle for its warm-up phase.

After about 30sec to a minute (ish, didn't measure it) the revs started to drop and it began to misfire.  As this was happening a there was a gradually increasing noise from the inlet as the IACV started to close.  It would bog down/stutter badly under light loads/small throttle openings when driven at this point.  This would continue until it was fully warmed up and it would completely go away after a few minutes of 'Itialian tune-up'

TBC - started to write this up but too many disruptions and off to work now.  Back shortly.

[revilla]

Running fine for 30 seconds then suddenly struggling sounds like a faulty lambda sensor; it takes about 30 seconds to heat up and until then the ECU ignores it. For diagnosis, unplug it and see if the fault goes away. If so, safe to drive without for a while, replace - for EU3 will be Rover MHK100840.

[SM25T]

Second that ! You can find lambda sensors at reasonable prices on eBay.

[garybee]

I considered that at first but couldn't work out why the IACV would be gradually closing as the idle worsened.  Anyway...

As the RPMs dropped and the idle became more and more lumpy the lambda reading was gradually dropping from 0.8/0.9v to about 0.4v.  At the same time the short term fuel trim was solid at -30% (I presume this was just pegged at the max reduction).  This would fit with a poor Lambda probe but doesn't account for the IACV.  A gentle application of throttle would get the engine to 'pick up' again but after a few seconds it would return to a worsening idle.

When fully hot it would sustain a proper idle and the lambda would output a voltage more in line with what you would expect (bouncing between rich and lean voltages).  

MAP reading was as expected and inlet temp also looked within range if a little high (49 Deg C but probably suffering a lot from heat soak at this point).

I tried the TPS/idle speed reset in case the IACV was closing as a result of a lost figure for this in the ECU but didn't really expect anything from this (as the absolute throttle was showing as 3.9% at rest and swept smoothly to just under 90% when tested).

Also tried unplugging the IACV whilst the idle was stable.  This obviously got rid of the 'closing IACV' issue but the Lambda behaviour remained the same and the idle quality gradually dropped as before but without the engine speed reduction and noise from the inlet.

So...maybe I have two faults (Lambda and something causing the IACV to close).  Swap the Lambda probe for a known good one and no change, behavior exactly as before.

At this point I fired the parts cannon at it.  IACV, MAP/IAT sensor, coolant sensor, TPS all with no effect.

Changed the ECU (luckily had a spare ECU/5AS/fob) and fault gone.  All the original sensors refitted and no return of the fault.  So it appears that there is a failure mode of the (EU3) ECU that causes this strange behavior in the warm (but not hot) range.  I haven't taken the ECU apart yet to attempt to work out what went wrong but would like to get to the bottom of it.

[Richard Price]

Hi Gary,

Whilst I have no help to offer, I'd like to know how you have read the ECU data. I've not had any joy with the OBDII to USB things I've tried :-(

[garybee]

Hi Richard,

I've had a few bits of kit for fault code reading from laptop based software to 'phone apps .  Up until about a year ago I used an Android app called Torque and a bluetooth adaptor that plugged in to the port.  I found that to be very reliable and would recommend it to anyone who just wants to read engine codes (total cost about £12).  If you want something to do a little more than that I now have one of these:-

http://www.ebay.co.uk/itm/Original-LAUNCH-Creader-Professional-CRP123-LAU-301050116-Diagnostic-Scanner-UK-/272171360851?epid=1161782639&hash=item3f5ead6253:g:PJIAAOSwAuNW59Nm

I think I paid about £90 for it and it does a lot more.  You can read and reset fault codes for all systems on almost any car.  It will do ABS, gearbox, airbag etc.  The next step up is something that will do actuator tests but they seem to start at about £500 so a little harder to justify.

[revilla]

@Richard Price: I've found the EU3 MEMS3 ECU can be very fussy about the sequence of connecting and powering things up when trying to connect OBDII connectors. It's as though the ECU looks to see if there is anything to talk to on the diagnostics port when it first fires up and if not, continues to ignore it.

This always works for me:

• Make sure the ECU is fully powered down. It remains on for a while after you turn the ignition off. Sometimes you can hear a whine from the IACV until it powers down. You can either wait until that stops, wait a good long time, or pull the ECU fuse for a second or two / switch off and then on a master switch etc.
• Connect the OBDII scanner and turn it on (with the ECU still OFF).
• Turn the ignition on to the "Run" position.
• Try to connect to the ECU from the scanner.