your 91 is a 124 body and it has two fresh air flaps above the fan housing.it is common for the diaphrams that operate these doors to break and they are an almost impossible job to replace without removing dash.if you look in front of the windshield you will see the vents thru which the air is pulled.try closing these vents with duct tape[black so it won't show] it is also poss that another diaphram is leaking badly so there is no vacuum for the recirc doors to operate[eg.defrost]if you want to try to repair call me and i will talk you thru it.david poole dallas tx 214-9567007