BMW Repair: Fuel Consumption - MY 1998 523i, mass airflow meter, mass air flow sensor

Question
QUESTION: Hi Evan,
I have recently purchased an E39 523i. (has 2494cc motor). Many reviews I read pre-purchase suggested a city fuel consumption of around 11.1 litres/100km. I am getting no better than 16.2 litres/100 km, with careful driving , averaging about 37km/hr, with no air conditioning needed at this time of year. The car has been very well maintained, full service history, has done under 150,000 km. can you help?
Thanks
Simon Rushworth - Australia

ANSWER: I would replace tune up items and parts having an effect on fuel and air delivery. Spark plugs, air filter, fuel filter, check for vacuum leaks from the engine. Run some Techron fuel system cleaner (distributed by chevron don't know if aussies have that) in two consecutive tanks of gasoline. Replace Front Oxygen sensors.
Start with these. By my calculation you are getting 11.23 miles per gallon, which is terrible. You should get 18-28mpg. You could also clean the throttle body and idle valve. If this doesn't help try swapping in a Mass airflow meter.
-Evan

---------- FOLLOW-UP ----------

QUESTION: Hi Evan, I'm not sure what you mean by a Mass airflow meter. Could you explain this? Also the Vanos hoses were checked for leaks but were considered not leaking by a BMW mechanic. Are there any other problems with Vanos hoses, such as blockages?

Answer
Mass air flow sensor/meter is round plastic sensor in the intake duct just behind the air filter box. Should be flat four or five wire connector on the top of it. It measures the amount of air entering the engine and helps it to calculate how much fuel to give the engine through how long the engine management computer (DME, ECU, what ever you want to call it) holds the fuel injectors open. Vanos controls the Variable Valve timing (also controlled by DME). To my knowledge there is only one external Vanos line (oil pressure feed line). It goes from the oil filter housing to the Vanos adjustment unit bolted on the front of the cylinder head. If can leak or seep oil through the rubber line over time. Also oil filter housing seals are common to leak as well at the oil pressure switch near the bottom of the line. Never seen a vanos line clogged. If you have a lot of engine sludge in the engine I suppose it is possible, but I would think the line would burst due to the increased pressure from the restriction. A problem in the vanos system would cause various driveability issues. Run a compression test on all the cylinders and maybe even a leak down test if you find a bad cylinder.
-Evan