[Jaglavak]

That's the one, the plastic tank thingy. First off, unless your weather is freezing stop pouring antifreeze into it, you're killing the fish. Just use water, distilled or purified if you have it. Try filling up the radiator and the expansion tank to the line. Then warm the engine up while watching the expansion tank. If it pushes out a bunch of water while running, and then after shutdown sucks the tank dry while cooling, that's a strong indication. You may have to drive it around a little to make it happen.

But the compression check is the one. They take out the spark plugs and use a pressure gauge to see how well the cylinders are making pressure. A typical healthy number might be 150 to 180 PSI, but the main thing is they should all be close to the same. If there's a blown head gasket one or two will be way down.

They can also do what's called a leakdown test. Instead of a pressure gauge they screw in an air fitting and blow in air through a flow meter. With both valves closed the cylinder should only leak a little bit. They can listen at the air intake, the exhaust, and the crankcase to see where the air is leaking. Usually if there is a blown head gasket the leakdown test will blow bubbles into the radiator. Or out the exhaust, or into the crankcase. If it's blowing into the crankcase they should be able to tell because it will blow back out the oil filler tube and you can hear it.

Both of these tests are basic wrenching techniques. Any shop should be able to do both for you in an hour. If the engine passes the compression and leakdown tests then the head gasket is probably good and we have a mystery.

The head gasket could also leak water out directly without going through the engine. That's theoretically easy to see coming out the gap between head and block, except that's usually buried under every damn thing.

I solve the car payment problem the standard guy way, by buying an old beater and then spending night after night in the garage replacing every moving part. I kept a trusty old Chevy pickup running for almost 300,000 miles. But finally admitted defeat when I could see botts dots through the floor, the engine was drinking oil, and the rear end went out.