Air Box Filter Housing Flap Valve Thermostat

Failure of this part is the number one reason for MAF sensor failure on Volvos.

Many gas powered Volvos (and some diesel models) have a simple, thermostatically controlled flapper valve in the air filter box. This thermostat regulates the flow of fresh and pre-heated air in to the intake system.  The purpose is to speed up the engine warm-up period in cold weather (primarily for emissions purposes but also for heater function.) 

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. 

When this thermostat fails, it defaults to the pre-heated air position. This causes super heated air from the exhaust manifold to route through the Air Mass Sensor full-time.  Left upchecked, the MAF will soon fail because the electronics overheat.  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.  Keep in mind that the thermostat is really only designed to route hot air when the ambient air is below 40 degrees so this may not even be relevant in your environment.

Some later K-Jetronic (mechanical) injected models and early diesel models also have this thermostat.  While there isn't a MAF sensor to fail on these models, it will cause all sorts of drivability issues since the engine isn't running on ambient air.

Volvo's VIDA parts catalog is a bit unclear on exactly which models were equipped with this thermostat, but we think it's safe to assume:

• 1980-1993 240, 260, 740 & 760 non-turbo models 
• 1985-1987 760 w/ B28F or B280F
• 1985-1986 740/760 Turbo Diesel
• 1985-1990 740/760 Turbo (B230FT)
• 1989-1991 780 Turbo (B230FT)

Additional Keywords: THERMOSTATE