If you suspect that you may have a bad head gasket leaking cylinder pressure into the cooling system — whether it’s because of a bad installation or a damaged gasket — here’s a quick check you can perform. This will only work on older engines with a top-mounted thermostat housing at the front of the engine. Before pulling all the spark plugs to do a compression check, remove the water pump belt and drain enough coolant to remove the upper radiator hose and the thermostat and housing. Leave sufficient coolant inside the manifold cross-over under the thermostat.
Now start the engine, let it idle, and watch for bubbles coming from one side of the engine or the other. The bubbles will indicate the side of the engine that has the bad head gasket. Now you can pull the plugs from that side and identify the offending cylinder(s). This diagnostic test costs nothing except a little time and effort.
Sometimes there will be small bubbles that appear from either side of the engine — these occur naturally as the engine runs, creating hot spots in the combustion chamber, and are not an indication of a bad head gasket. What you are looking for is a continuous stream of bubbles caused by cylinder pressure consistently leaking into the cooling system.
While this diagnosis isn’t the end-all, be-all, and will still require further diagnostics, it can help you cut down your diagnosis time in half. Unless, of course, the stream of bubbles is coming from both sides of the engine. Then you know you have a fun weekend ahead of you.