An exhaust gas heat exchanger system of a combustion engine has a bypass
duct and a heat exchanger duct with a heat exchanger. A valve is
controlled by determining when heat is to be transferred from exhaust gas
flowing through the exhaust system to the heat exchanger. When heat is to
be transferred, the valve is switched into a first position in which all
of the exhaust gas flows through the heat exchanger duct. A pressure drop
across the heat exchanger system is monitored either directly or
indirectly. If the pressure drop reaches a predefined limit, the valve is
switched into an intermediate position, resulting in a smaller portion of
the exhaust gas flowing through the heat exchanger duct than when the
valve is in the first position and a remaining portion flowing through
the bypass duct, thereby reducing the pressure drop across the heat
exchanger system. If the pressure drop again reaches a predefined limit,
the valve is switched into a second position, resulting in an even
smaller portion of the exhaust gas flowing through the heat exchanger
duct, thereby further reducing the pressure drop across the heat
exchanger system.