Wednesday, June 1, 2011

air flow meter problem on ALFA 156?

Air Flow Meters are a common problem on most European cars at the moment and Alfa Romeo are no exception.

Air Flow Meters measure the amount of air passing into the engine from which the computer can work out how much fuel to add to get the perfect mixture for that specific throttle opening, the computer does this by referring to a map in its memory. The Air Flow Meter works by seeing how much current it takes to keep a wire at a specific temperature. However it is a sensitive instrument and it can be contaminated by oil (often used in aftermarket performance air filters), damaged by excess vibration caused by broken air boxes and we have seen problems with GTV 3.0 litres being fitted with 2.0 litre filters which leaves about an inch gap through which contamination gets in.

The fault will often not be shown on diagnostic system as the meter will still be operating within the limits the computer believes are OK, but putting the correct amount of fuel in for idle when the throttle is fully open will result in a very weak mixture and thus a lack of power.

The V6 and Diesel engines generally first get a flat spot ¾ of the way up the rev range and this gradually this extends up to the rev limit and will rob up to 50 BHP from the V6 engines, we have known them to be lucky to get over 80 mph in really bad cases.

The 4 cylinder engines are less prone to failure than the V6’s and the symptoms are more subtle with again a flat spot at approximately ¾ of maximum revs but just a reluctance to want to rev to its maximum when the problem is acute.

The air flow meters will occasionally fail completely which will bring up a light on the dash and the symptoms described here are only the usual ones as we have known the fault to become apparent in other ways.

The easiest way to check if the Air flow meter is faulty is to get your local friendly Alfa Romeo garage to try one for you as they take very little time to fit, in most cases it is quicker than putting the car on the examiner and it is always nice to see the look on a customers face as they rocket off into the distance. Beware however as Lambda (or Oxygen) sensors in the exhaust can also give very similar symptoms to Air Flow Meters.


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make sure first that the obvious has been checked first (spark plug wires haven't been swapped inadvertently, no air leaks, valve inside of AFM moves freely, etc.)

One way to confirm would be to pull a few spark plugs and see if they're severely carbon-fouled (i.e. black). This indicates that your engine is running super rich, and likely points to the AFM.

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First is to clean the carbon out of the idle motor channels and the throttle body. Then check for excess back pressure in the exhaust system. Take out the O2 sensor and install a fitting to attach a pressure gauge. At 2000 rpm there should be less than 2 psi showing.

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