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98 Deville high idle and missing

My 98 Caddy Deville has 2 problems I think are related. When I start it the idle is high and slowly returns to normal after about 10 seconds. If I drive it before idle returns to normal idle will steadily increase (up to 2500 rpms) until I stop and turn car off an on several times until idle reduces. Secondly, there is a slight miss that is amplified when A/C is on and when going between 40-45 and step on the gas the car will buck and sometimes backfire. I have replaced plugs, wires, MAP sensor and Throttle position sensor. a friend says it may be a computer but all “check engine” codes say TPS sensor. Any help would be greatly appreciated as I have been dealing with this for over a year and noone, including dealer, can figure it out.

" I have been dealing with this for over a year and noone, including dealer, can figure it out."

This is what leads a company into bankruptcy…I wish I had a better answer for you.

You might try this…Remove and plug ALL the vacuum lines that lead away from the engine. See how it behaves now. If it starts and idles normally, replace the hoses one at a time until you find the problem child. You might get lucky…Check the EGR function…