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

Successful repair required replacing the air mass meter (AMM), intake bellows
between AMM and throttle body, and ECU. Parts swapping confirmed the need for all
three. One important finding - the air bellows can leak at the end connections, even
though there are no cracks or holes in the bellows and the clamps are tight.
Apparently they get to be so inflexible that a good seal is not possible. We should
now be ready for another 100K.
Loss of Power; Rough Running; Knocking: Bad Engine Knock Sensor. A failed
knock sensor may be felt as a general weakness throughout the powerband and a
different tone to the motor. When no knock sensor signal is detected the EZK ignition
system defaults to full spark retard. This can also result in the motor running a little on
the rich side, as a kind of side-effect (More ignition advance usually = leaner burn,
more NOx emissions; less advance usually = richer burn, more HC and CO
emissions.) See Engine Sensors for more information.
Symptoms. Higher idle, no power; running rough at RPM higher than idle. It jerks
every few seconds and there is no pattern to it. It idles and starts fine. [Rafael Riverol]
Other symptoms include hesitation, poor throttle response, lack of acceleration,
especially when cold. [Marc] The car starts fine and will run for 2 miles and then lose
power. It never stalls just won't move. If I shut it off and immediately restart it, it will run
fine again for another 2 miles and so on and so on. When it runs fine, with a timing
light hooked up on acceleration the timing will retard 1 or 2 degrees before it
advances. If I check it when the problem is occuring it will retard 6 or 8 degrees before
it tries to advance. Solution: new knock sensor.
Diagnosis. Check the knock sensor, it senses knock (no kidding) and retards the
timing. When this gets faulty, it will make the engine have very poor power, lousy and
jerky acceleration, but will start and idle fine. It's a small black plastic covered unit
bolted to the block (ten mil. bolt) under the intake man. with a plug attached to it. One
minute to remove. It "listens" to the engine. Mine had a cracked plastic cover and was
covered in oil. I put a used one in (new they're ~$40) and it's like I suddenly had a new
car.
Mid-Range Engine Knock. Anecdotal evidence notes that if you experience mid-range
engine knock when the engine is warm, inspect the airbox thermostat for proper
operation. If it sticks in the "warm" position, it allows preheated air into the intake
system. This will also, over time, ruin your air mass meter.