carbon deposits in engine

On October 23, 2017, the car suddenly started vibrating like driving on a washboard road and the engine started running very rough. The DIC showed a "Service Stabilitrak" message and the symbols for the engine and traction control. I had to stop for a red light and the engine stalled. I was able to restart the engine and pulled into a repair shop about 50 feet away with which I had done business before. I was told that some sensors may be bad but that they would have to do some investigation like a compression test etc. to know for sure what was wrong.

I left the car there and I was called later that day and was told that two cylinders had zero compression and that the intakes had very bad carbon buildup. They said they would try to remove the carbon they could see by a cleaning process and then would connect a machine that would circulate a cleaning solution through the engine.

The car has now been in their shop for 4 days. They have been trying to clean the carbon deposits by applying a cleaning solution to the deposits and letting it soak, usually overnight, and then brushing or scraping the deposits. They have repeated this process several times and say that the two problem cylinders now have some compression but not like they should.

The car has less than 58,000 miles on it. It is driven very little on city streets and mostly on roads at speeds of between 45-70 MPH and for average one-way distances of 10-25 miles. It is inconceivable to me that the carbon buildup should be this bad. The repair shop owner said that if the deposits can't be cleaned away, the next step could cost several thousand dollars. I am very disappointed in Cadillac's quality to say the least.