High fuel consumption Computer diagnostics may not help
I have only just recently found a fault with a Range Rover 1997, that has taken me four years to find. Every now and then I would use 30 litres of fuel to every 100km. The thing was the computer diagnostic system did not pick up any fault codes that seemed to relate to this excessive fuel consumption. I was getting a warning that it was taking a long time for warm up on coolant temp and going into open loop mode.
I cant recall the fault code but it was strange. Coolant temp sensor was fine, air temp sensor was OK when checked on its own but seemed to be indicating say 4 deg C when air temp was 20 deg C. MAF sensor seemed to have correct signal at idle fuel pressure was OK. I checked all the sensors by themselves (made up my own jigs to do this). It was driving me up the wall. In many ways experts did not help , I am more of an electronics person than a mechanic. My background told me something was wrong when I checked the resistance of the sensor common line to the chassis I was getting a high resistance reading >1M ohm. The trouble is this was intermittent. I was told that this line should float and not go to ground. This to me did not make sense surely it has to? Well after looking closely at circuit diagrams it turned out this sensor common line had to go to a "splice point" on the chassis. It was right behind the ECU and when I dismantled the "point" the terminals where corroded. Cleaned up terminal got correct reading of resistance. Now no fault codes fuel consumption 12 litres/ 100km which for a 4.6 V8 is pretty good. I know its not an Alfa but my experience with computer diagnostics systems has not been that good they more like a cryptic crossword
Sorry about this being a bit long but its a warning that computer diagnostic systems can mislead, it has taken me four years to track this fault down.