Replaced Water Pump,
Join Date: Aug 2009
Location: New Zealand
Really the only way to know for sure is to read the error codes. The Bosch Motronic system relies on the values presented by an array of sensors (MAF, Throttle Postion Sensor, Crankshaft Position Sensor, camshaft position sensor, lambda(s), etc) to not only determine mixture, but also injection point and ignition timing; any of which if incorrect will cause the engine to run ratsh*t. The codes will point you in the right direction otherwise your in the expensive realm of replacing things until you fix finally the fault.
The intake plumbing has to be leakfree to ensure that the MAF (if it is indeed working properly) gives a correct signal value. It is quite possible for a dud MAF to give a false value that is within the parameters acceptible to the ECU, so no 'injector light' but an incorrect MAF signal is likely to show up as a fuel trim error code. Cracks in the concertina are often cause problems and with all respect, 'bodging' may create similar problems.
Breather hoses also have to be connected and leak free. The V6 as with most engines uses a PCV (positive crankcase ventilation) system that relies on manifold vacuum to operate. Removing a hose effectively creates an air leak that invalidates the MAF signal - engine runs lean.
Given that disconnecting the MAF hasn't given an improvement it is likely (assuming the other sensors are OK) that the lamdba is dead (and the MAF could be too). Faulty MAFs and lambda often give similar symptoms as both sensor are involved with adjusting fuel trim in closed loop (normal) mode. When you get your cable, connect up using FiatECUScan, KWP2000 (Sanders Alfa version) or Alfadiag and read the fault codes. A dead lambda is easy to spot if you graph its output using this function on the above software, the voltage should ocillate between almost zero volts and 1 volt. If it occillates very slowly or gives an irregular pattern, or is a flatline, it is stuffed and requires replacement. A disconnected MAF sends the ECU into open loop and the ECU runs off the injection/ignition map programmed into the ECU using the TPS and lambda to calculate/adjust the the fuel mix and spark timing.
The MAF in my twinnie died on Friday -injector light on, stalling at the lights, etc- after playing up fo the last month or so. I connected up and saw the error codes, which once cleared reappeared as soon as the engine was started. Checked the MAF output which was constant regardless of rpm. So shes f00ked. I reset the ECU using KWP2000 and this fixed the idle problem and the car is actually now running better than previously as I wait for my new MAF to turn up from EB Spares. The reset cleared the fuel trim settings that were trying to compensate for the dodgy MAF signals. It is running in open loop, the lambda which is 6 months old providing correction for the mix (although it will be a bit richer than when in closed loop). If the lambda was poked I'd expect the sort of symptoms you describe.
Darryl in New Zealand