hi, hope i have picked this up correctly if your truck is 1997 then it may be vacuum controlled so first check for vacuum leaks to the imcr units. it is possibly to remove the pipes and check by adding a pipe to the units and simply sucking and hopefully you will be able to feel / hear the flaps opening and shutting. only with the engine off. yes they do get choked up and stick on the shaft bearings. bokker.