While any competent shop should be able to service this, a Cadillac dealer will have the most direct experience.
It is important to maintain the fluid levels of the vehicle. A 1995 Cadillac Deville will take any general power steering fluid.
Linked is the diagram for a 1976 Cadillac DeVille, 1977 should be roughly the same...
The fuses are located under the left side of the steering wheel. There should be a plastic cover covering the fused, once you pull it, you will see the fuses.
It is probably your egr valve that needs replaced!
You should check out the Cadillac Deville suspension conversion kits at strutmasters.com If you can do brakes you can install their conversion kits yourself. http://www.strutmasters.com/cadillac-deville-suspension-s/42.htm
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your local auto parts store should have a guide to replacing. or look in your manual
Yes, it should but watch out for different features in each door like heated rear seats etc.
A 1990 Cadillac Deville air conditioner should hold 46 oz. R12 Freon. This should be verified through the owners vehicle manual.
The "On Board Diagnostics" (OBD II) has detected a problem with something in the system. It should be looked at soon by a trusted mechanic.
5x115 Center bore should be 70.3
Should be two front and rear of engine