A housingless heat exchanger including a stack of flat tubes with at least two being cooling tubes and at least one being a bypass tube. Collecting tanks are on the tube ends and diffuse gas streaming in the tube flow paths. The flat tubes each comprise a connected pair of plates defining a flow path, and an enclosed space is defined between adjacent flat tubes. Coolant inlet and outlet channels are formed by connected plate openings connect to the enclosed space between the cooling tubes whereby coolant flows through that enclosed space. A switching valve in one collecting tank is movable between a cooling position in which the gas streams through the cooling tubes and a bypass position in which the gas streams through the bypass tube. A closure in the coolant inlet and outlet channels blocks coolant from adjacent the bypass tube. An insulating plate may also, or alternatively, be between the bypass tube and the cooling tube adjacent the bypass tube to block heat exchange between coolant and the bypass tube.