I work at a Cadillac dealership as a tech. Northstar engines are notorious for blowing head gaskets. Do you have access to a 5 gas analyzer? I know that'* a stretch but if you live in a state where vehicle inspections are done you could go to a service center and just ask them to check your coolant reservoir for engine gasses. If not they do make a Combustion leak tester you can pick up fairly cheap if you don't have access to the 5 gas analyzer. If it is a leaking head gasket it will be hard to find someone to replace it. Those engine have to be helicoiled when you remove the head bolts. I have had luck with the blue devil head gasket repair though. You have to do it step by step but it definitely works. And if it doesn't you always have the money back guarantee! I have had to do it twice on a Elderado once. Still worked after the second repair.

Combustion leak detector - http://amzn.to/2zaJEvD
Blue Devil Sealer - http://amzn.to/2zv8IAI

Carnau, thanks for your input.....but you should check the dates on the last post.....they are several months old, and the original poster has not posted anything since...either he fixed it or gave up.....

His last post seemed to indicate that his heater core was blocked, and if this was the case, a back flush might have solved his problem....

You are definitely correct on the head bolts, but if he was losing coolant in the cylinders, there should have been a drop in reservoir/radiator, which he did not indicate....

By the way, glad to have a Caddy Guy on the site....

Yeah I agree 100%. I don’t know why they are showing up on my list as New posts. The admin already said something to me about it. I feel like I am about to get booted and all I’m trying to do is help. Lol. On my list it shows new posts in dark blue and old posts in light blue. I just click on the dark blue ones at the top and answer. But I will definitely be looking at the dates from now on. It sure if it is the google browser I’m using or what.