1997 Cadillac DeVille Base 8 Cyl 4.6L
when slowing down or idling the engine vibrates the car. already replaced spark plugs and wires. also had fuel injector cleaned. only has 120,023 miles. just started yesterday . mpg went from 25 to 18. check engine light is on, but codes were for new spark plugs and wires. we cleared the reader and it still says the same thing.
First step - have the check engine light or stored fault codes scanned - for free - at any local auto parts store; Advance Auto, Auto Zone, NAPA. Then, post a reply as 'add answer' with the specific fault code for more assistance.