Catalyzes endohydrolysis of 1,4-beta-D-glucosidic linkages in xyloglucan with retention of the beta-configuration of the glycosyl residues. Specific for xyloglucan and does not hydrolyze other cell wall components. xyloglucan + H2O = xyloglucan oligosaccharides. Optimum pH is 3.4. Stability declines sharply below pH 2.8 and above pH 5.0. Optimum temperature is below 30 degrees Celsius. Is very stable below 35 degrees Celsius, but at 50 degrees Celsius, it loses 80 percent of its activity within 2 h. Belongs to the glycosyl hydrolase 12 (cellulase H) family.