On my 1975 auto 240D W115 , the car starts right up hot or cold. Here is my problem. After driving and warmed up I put in netural or park and the idle will increase and have grayish smoke coming out of the tail pipe, I put in gear and the smoke goes away. I can't get the idle down after it warms up. I adjusted the valves and no change as far as the smoke goes. Checked all of the rods and adjusted according to the specs. I sure could use some help from you diesel pros

Thanks