1998 Cadillac Deville

Here is my thoughts - Is there any liquid present on the floor or under the carpet on the passenger side front area? Does it smell like sweet candy apples whne the car is running hot and the heater is on ? Have you seen any white smoke coming out of the exhaust pipe when the car is hot running down the road ? Why do I ask - wetness on the floor is a leak in the heater core, that smell of something sweet is the antifreeze being burnt ai it gets near something hot (exhaust manifold, intake manifold, heater core leak) and white smoke is an indicator you have warped the heads and the antifreeze is leaking into the cylinders and getting burned off thru the exhaust. Here is my final thought and you take it for what you want - you replaced the engine, then had to go back and have the heads done again, and now they say it "might be" the heater core ? Will it be the windshield washer bottle on saturday (this is friday morning) or what next.

There was nothing but problems with those engines.

The Northstar V8 is notorious for having blown head gaskets, even though there is no leakage on the outside there still could be leaks on the inside. My guess is that when they put the new engine in they probably cracked the block from the inside or cracked the head.

Mine had a bad heater core and then the sensor went out. Its just a coolant sensor. Mine still says check coolant level. Its all good though.