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A cracked block or head gasket failure can total an engine. Use this easy test to rule out a severe engine problem!

When it comes to checking your vehicle for ailments under the hood, very few repairs hurt a wallet more than a cracked engine block or a blown head gasket. When your car is suffering from a cracked block or head gasket, it will be running extremely rough, making you automatically assume the worst possible outcome for your engine, but before you open your checkbook and hand over a few thousand dollars for repairs, there is one test that you can do easily in your driveway to test for a cracked block or blown head gasket before you start panicking.

This is all about block testers: what they test for, how they work, and how to get one yourself!

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A crack in an engine block is a simple but costly ailment to an engine. A cracked block is exactly what the name implies, due to overexertion or a mechanical fault or failure, your engine block can crack, which usually means an entirely new engine is needed for your car. A repairable cracked engine is rare, and the labor cost alone can total the engine.

When a block is cracked, you can expect a poor running engine, smoke from under the hood, and of course, a loss of oil or coolant from the crack.

A blown head gasket is much more common. Your engine is separated into two main parts, the block, and the head, or heads depending on the layout of the cylinders. In between the head and the block is a gasket, helping everything move through the engine without leaking. Over time, these gaskets get old and fail, leading to all of your engine’s fluids mixing in the combustion chamber, causing excessive smoke out of your tailpipe, low compression in the cylinders, a loss of coolant, and milky oil.

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When an engine block cracks or a head gasket fails, there is usually one common invisible side-effect of these ailments; an exhaust leak into the radiator. As a crack in the block or failed gasket can no longer route exhaust gasses to the proper manifold, some exhaust gasses can escape to the radiator, creating an obvious bubbling noise.

By using a fluid that turns color when subjected to exhaust gasses, block testers can detect exhaust in the radiator easily, with very little chance of error.

To help the block tester suck in possible exhaust gasses, a bulb attached to the tester is squeezed to force gasses into the testing fluid.

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Before starting your test, remember, radiator caps are pressurized, and when these caps are removed when the radiator is hot, hot coolant may shoot out and burn you. Make sure your vehicle is well-cooled off before performing your test.

To start your test, remove the radiator cap or overflow tank cap and ensure that the tester fits snug over the radiator opening, without touching the coolant below. If the tester cannot be inserted without touching the coolant in the radiator, siphon a small amount out of the radiator so it doesn't get sucked into the tester.

Fill the tester up with testing fluid to the line on the reservoir, and attach the rubber bulb to the top.

After everything is ready, start your car and let your engine heat up. Once your thermostat has opened and coolant is cycling through your engine, insert the tester into the radiator cap opening and start squeezing the bulb to suck air into the testing fluid.

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As you squeeze the test bulb, watch for the fluid in the tester to change color from blue to yellow. If the liquid turns yellow, you have exhaust gasses in your engine that is escaping through your radiator, indicating a cracked block or head gasket.

If after a few minutes of squeezing the testing bulb proves to not alter the color of the combustion fluid, then you can safely assume you do not have a cracked block or blown head gasket!

While there is a small chance of a false negative result, these chances are very low, with most cracked blocks and blown head gaskets sending some exhaust gasses into the radiator.

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A block tester is a tool that any automotive enthusiast should have in their toolbox, and with the low price point of $30 to $50 for the fluid and test kit, it is a tool that anyone can buy and keep with them whenever they have a possible head gasket issue to help them further diagnose their engine’s ailments.

If you are a backyard mechanic who doesn’t need a block tester for the long-haul, you can always rent one for free from any major parts store in America, where for a small refundable deposit, you can rent a block tester for a few days; a perfect option for someone that wants the peace of mind that comes with ruling out a severe engine issue, while not wanting to invest in a tool that they will only use once.

So before you estimate a costly repair bill on your vehicle, pick up a block tester and put your mind at ease!

Steven Teleky is an educational professional in the San Francisco Bay Area, and is formerly the Educational Specialist for the National Corvette Museum in Bowling Green, Kentucky. Working on American cars his entire life, Steven has extensive history with all GM, Ford, and Mopar vehicles, as well as experience with German and Japanese imports as well. Currently, Steven races in California Rallycross with a 1987 Pontiac Fiero, and is always scouring dealer lots and junkyards for a forgotten classic or neglected daily to bring back to life.