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

ad (low pressure) regulator on fuel supply system.
CRUISE EMISSIONS. At cruise, the engine is lightly loaded and running at high rpm.
Under these conditions, HC and CO should be low if the oxygen sensor and feed back
control system are working properly, and the catalytic converter is in good condition.
High CO readings during cruise indicate a rich fuel condition. Causes here may
include:
● Defective O2 sensor;
● Exhaust leaks upstream of the O2 sensor (check manifold gaskets and air
plumbing
connections);
● Defective AMM sensor;
High HC during cruise would indicate a steady misfire or loss of compression (leaky
exhaust valve).
DECEL EMISSIONS. When decelerating, the engine will typically either lean out the
fuel mixture or shut the fuel off completely (some fuel-injected engines). The computer
typically uses inputs from the Vehicle Speed Sensor, TPS, Airflow sensor, and engine
rpm to determine when this occurs. When the throttle closes and manifold vacuum
shoots up, the computer cuts back on the fuel. Normally, HC, CO and NOX emissions
drop during deceleration because the engine is no longer under load and is receiving
little or no fuel.
If CO emissions remain high during deceleration, the engine is receiving too much
fuel.
Causes may include:
● Leaky fuel injectors; and
● Faulty VSS, TPS, or airflow sensor.
Volvo Maintenance FAQ for 7xx/9xx/90
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