1991 Cadillac Deville won't engage in gear

you have to run the car then tow it or else you will fry the transmissions planetary.

have the trans fluid checked for condition , level etc as a first step

You need to find out what is missing when it won't start , spark - ignition system or fuel pressure . No Start Diagnostic Basics EricTheCarGuy

if its manual and you literally cannot pull the gear shifter out of gear, then when the engine is off, take off the parking brake, have someone rock the vehical back and forth gently while you keep the clutch depressed and try to take it out. then it should be towed (still in neutral might i add) to a shop. if this does not work with the clutch depressed, give it a shot with the clutch engaged (pedal all the way up)

you have a leak in the manifold ,probably a crack or air is being drawn in from somehere

Had exact same problem in '91 Cad. Fleetwood FWD, which is same as '91 Deville except for trim fender skirts. My mechanic said it was a broken window regulator. That part is not available new anymore, unless you can find some NOS. I bought a used regulator from Cadillacs Only for $75, and paid my mechanic for 1.5 hours of his time to r&r. Window now works fine. Not happy that I had this problem; regulator not a great design.

I have a 1991 Cadillac Fleetwood 4.9 120000 miles, I have the same problem. replaced plugs,wires,cleaned throttle body, and O2 sensor and have same problem. Taking it to a GM shop on 02/24/10.

try fuel pressure regulator

Here is a diagram tell me which pulley came off asap. http://www.2carpros.com/car_repair_information/tec_info/1991/cadillac/deville/1991_cadillac_deville_drive_belt_routing_diagram.htm

My need to adjust shift cable