A friend has got a Mk3 GTI with running problems. On idle it is rich, hunts and chugs but doesn't cut out, it drives but feels lumpy, revs okay. The system shows a fault - Lambda sensor upper limit. We have fitted a new sensor as the car has done about 140K and cleaned out the throttle valve and the MAF, they both now look spotless. If with the engine running on idle we dis-connect the wiring to the MAF the the idle seems fine, when it's plugged in again the engine starts to hunt after a few seconds. The question is, does this definitely point to a faulty MAF? Would you risk buying a second hand one from ebay?