Do you see steam out the exhaust when first started? Are there bubbles in the recovery bottle after only running a minute or two? Is the engine oil milky color? If any of the above ring true then it is a blown head gasket or a cracked head. If none of these are present then check the DTC for codes and repair as needed and I would also replace the thermostat if it has never been done, make sure all of the air is out of the system and recheck.
Search Cadillac DeVille Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Cadillac DeVille Experts