Vehicle: 2007 Buick Lucerne, V8-4.6L, Automatic Transmission/Transaxle

Mileage: 138,667

Problem: The vehicle came in because the engine kept overheating.

Case Details: The technician pressure-checked the cooling system but found no leaks and the cooling system was full of coolant. He checked the fans. They were working properly. He even tried replacing the thermostat with two different brands, but the engine still overheated.

An ALLDATA Tech-Assist consultant advised that it may be possible to have an air pocket in the cooling system. He recommended raising the front of the car so that the overflow reservoir was at the highest point and then installing a funnel (when the engine was cool) and topping the coolant if necessary. Then, hold the engine speed steady at around 1,500 rpm with the heater temperature set to “full hot” and the blower fan on high speed.

He said to run the engine like that for 10-15 minutes but keep an eye on the temperature. Once the engine reaches operating temperature, look for bubbles in the overflow bottle. Next, use a block tester to pull in any fumes from the funnel to see if there were any combustion gases in the cooling system, which would indicate a bad head gasket, cracked head or block.

The technician performed a block test on the engine, and it indicated that it did have combustion gases entering the cooling system.

Confirmed Repair: The technician removed both heads and found that one head gasket was leaking between the #3 and #5 cylinders. He installed new head gaskets and test-drove the car. No more overheating. Fixed!