Volvo Maintenance Hints for 7xx/9xx - Bill Garland's Nuclear ...

single experience with a bad AMM was that the engine would barely run at all and was
not driveable. I think it reverts to "limp home
mode" if you pull the connector off. But I still
think you should clean the throttle body. Since Air Mass Meter
the purpose of an AMM is to regulate air flow
to the throttle body, it usually sits right on top
of the air cleaner or close by and there is
some sort of connector on it. Remove the connector and see how your engine runs. It
should run like crap, i.e. no power, idle fluctuates wildly, or engine stalls. If this is
similar to the problems you're experiencing now, chances are that the AMM is bad.
Another thing to check is that there aren't any air leaks between your AMM and the
throttle body. [More Symptoms] When I depress the gas, the car hesitates severely,
backfires a few times and if I keep pressing the gas, will stall. If I back off the gas I can
usually accelerate very slowly and once I get up to speed it is ok. When this is
happening the idle goes up to 1300 instead of the normal 750. I get codes 1-2-1
(Faulty signal to/from Air Mass Meter), 3-2-2 (Air Mass Meter wire burn-off signal
absent or faulty) and 2-3-1 (Fuel trim (lambda control) too lean or too rich at part load).
Pulling fuse 1 for fifteen minutes to reset the ECU would solve the problem
temporarily. The solution was to replace the AMM.
Connector Problems:
More than one inexperienced technician has failed to cure an intermittent driveability
problem by replacing the mass airflow sensor. Even if your pinpoint tests suggest a
failing sensor, always inspect the sensor's harness connector for loose terminals first.
Be sure all the terminals are locked securely inside the connector before you condemn
the mass airflow sensor. Some routinely remove a harness connector and then
reinstall it to see if "reseating" the connector solves the car's problem. But reseating
the connector on this Bosch airflow sensor may complicate diagnosis by pushing the
loose terminal outward. So, your quick-fix trick actually makes the car run worse than
before! Also, try cleaning and de-oxidizing the connector before reinstalling it.
Disintegrating Air Box Foam:
[Tip from Chris de Courcy-Bower] Check the air mass detector. This can be defeated
or even damaged by bits of disintegrating foam rubber that break away from the upper
casing of the air filter box. As the foam rubber seems to serve no purpose, it is my
recommendation that owners remove every trace of it before it causes a failure.
Backfire, Poor Acceleration: Failing Radio Suppression Relay.[Tom Rolyak] I've
had two radio suppression relay failures. Both times Both times I noticed abnormal
backfiring when decelerating. Much more pronounced than when a throttle switch