FAILURE OF THIS PART IS THE NUMBER ONE REASON FOR AIR MASS SENSOR FAILURE
Many gas powered Volvos have a simple, thermostatically controlled shutter valve in the air filter box. Generally, the thermostatic valve will fail in the pre-heated air position. This routes super heated air to the air mass meter, resulting in an incorrect lean fuel mixture which can cause stalling, hesitation and in severe cases bucking at cruising speeds as if the fuel were momentarily shut off. Left unchecked, the air mass meter will soon fail (a very expensive part)! Check your system on a day when the temperature is above 60 degrees F ambient. The valve should be fully open, directing fresh air flow into the box (not from the pre-heat side). If it's not, then you should replace it. Also, if the valve fails to reach full extension at hot and cold extreme, replace it.
