Hi

I've recently bought a Beetle (2.0 AZJ) which was sold as spares or repair. The accelerator pedal was not working. I diagnosed that the post cat lambda sensor had shorted from the heater circuit to the sensor circuit and fried the ECU. I replaced the post cat lambda sensor, got the pin from the ECU with VAG Commander and replaced the ECU with one with the same part number. The car runs now.

I took the car for its MOT and it failed on emissions, high CO. Lambda and HC were fine. Checked the cat and the insides rattled so I have just replaced that. The insides of it were all broken up and fell out through the pipe. I've just put it in for a retest so fingers crossed it will pass now.

One thing I have noticed with it though is that the idle speed seems to be perfectly stable at 760 rpm apart from some small random peaks where it spikes to 800 rpm. There is also a slight hesitation/stumble at 1000 - 1200 rpm. There's no fault codes showing.

I've checked the plugs and there's not one that looks any wetter or sootier than the others and the coil pack looks new. I've removed leads from spark plugs one at time and all give the same rpm response. One thing I noticed when I removed lead 3 from the coil pack was that it was arching back onto itself. I've done a visual check for vacuum leaks.

Does anybody have any ideas what this could be?

Thanks