Many gas powered Volvos have a simple, thermostatically controlled shutter valve in the air filter box. This thermostat regulates the flow of fresh and pre-heated air in to the intake system. (Pre-heated air comes to the air filter box through the aluminum pre-heat hose that is connected between the air filter box and the exhaust manifold or downpipe.
It is common for the thermostatic valve to fail in the pre-heated air position. This routes super heated air through the Mass Air Flow sensor (air mass meter.) Left upchecked, the MAF will soon fail. It will also cause the engine to run very inefficiently as the engine management will excessively lean out the fuel mixture in response to the hot air. Engine management systems were never designed to run with ambient air in excess of 400°F.
Check your system on a day when the ambient temperature is above 50°F. The valve should be fully open to fresh air (not pre-heated.) If it is open to the pre-heat side, then you should replace the thermostat. Also, if the valve fails to reach full extension at hot and cold extreme, replace it.
This thermostat is part of an emission control which means we cannot give you advice on defeating this system.
If the thermostatic valve is stuck in the hot position then super heated air will be routed in to your engine. This will cause a loss in economy/efficiency and in many cases will cause the MAF sensor to fail.
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