Posted in another forum and not enough info. Try it here.

So here is my situation. 334K on the clock with no issues before.

The problem is when the car is at idle. It will be real smooth and then you get a sort of roll in the engine…the whole car does a shudder. It can be fine for a few minute or more before it shudders. Sometimes it will do this just once and other times it will sort of repeat itself like a stutter.

I did this test. When the car is cold in the morning it will start at the first spin and idle smooth. I left the car idling for 5 minutes and then it would shudder/stutter. Smoke would be coming out from the vent hole at the back of the valve cover. I stepped on the pedal and then it will smooth out. The shudder can be slight or cause the whole car to shake. It is very annoying when it is at the stop light. After driven the car for a while then it would not happen again, not I am aware of anyway.

Car has a lot of power and get good gas mileage. All the filters have been changed, oil changed and compression in the 320+ range across all cylinders.

Any idea what could be wrong?

 

What year is the SDL?
2 things to try.
But not on a full Fuel Tank, after it starts shaking try removing the Fuel Fill Cap. If that improves things it could be your Fuel Tank Vent is restricted.

When it starts shaking remove the Oil Fill Cap. If the Shaking goes away you have a restricted Crankcase Vent System. If cleaning the Crancase Vent System did not help but it runs better with the Oil Fill Cap off you have some issue that is creating too much Crancase Pressure.

My other though is you have an Air leak somwhere in your Fuel Supply System. I am not sure if your year and model use regular Fuel Hose with Clamps or uses the Fuel Hoses with seals (O-rings?) on them.

Less likely is a problem with your EGR getting Vacuum when it is not supposed to. Disconnect the Vacuum at the EGR and Plug the Vacuum Hose. See it that improve it.