Ancient_Hacker wrote:
You might have carbon buildup on the intake valves. No simple way to
fix that.

take the highway, driving as fast as reasonable for 20 to
40 miles. That might do the trick.

GM sells this stuff called "Top Engine Cleaner." I've used it before, a
few years back, on this car. In August, the Cadillac techs felt it
would be a waste of money, but after I flunked the test again on the
4th, one of them suggested pouring in a can. I bought a can and poured
it in, following the instructions on the can religiously. Huge clouds
of smoke came out with the Top Engine Cleaner in the combustion
chambers, presumablyfrom the carbon being burned off.

The tech also said to take it out and "drive it hard." I've had some
some other non-automotive chores that had to be done, so I've haven't
got around to the brisk drive yet. I will do that.