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How does the Block TesterĀ® work?

When an engine is suffering from a combustion leak, it means exhaust gases are finding their way into the cooling system, most often caused by a blown head gasket. Those exhaust gases will first be absorbed and dissolved by the coolant until it becomes saturated. Once the coolant is saturated (usually about a 15 minute drive after a fresh coolant change), additional exhaust gases entering the cooling system will begin to accumulate at the top of the cooling system. The Block Tester is used to “sample” these through our proprietary chemical fluid. If exhaust gases are present in that air pocket at the highest point in the cooling system, they will react with our fluid and change its color from blue to green to yellow as more gases are passed through the fluid.

What color change am I looking for?

As more exhaust gases are passed through the test fluid, the color will progressively change from blue to green to yellow. Sometimes it’s hard to show the exact colors in a picture, but here’s an easy way you can see for yourself. Take some fresh fluid and place it in your Block Tester. Now use the tester to “sniff” the air about a foot or two away from the tailpipe of the running engine. As you squeeze the bulb, you will see the color change progression. When you perform a combustion leak test, you’re just looking for those same exhaust gases in the cooling system rather than in the ambient air around the tailpipe. This is also a good way to tell if your test fluid is still good if you question whether it’s expired.

What if I get a green test result on a gasoline engine?

A green result while testing a gasoline engine means that either a very minor combustion leak is occurring during the test, or the test is detecting residual exhaust gases from an intermittently occurring leak. This is PROVIDED that no coolant has gotten up into the tester during the test. Coolant entering the tester will usually pull the result back toward BLUE (negative), but you can find out how your exact coolant will interact by forcing a positive test result as described above with the “tailpipe” procedure and then placing a drop of coolant in the tester.
Keep in mind that combustion pressures are much lower at idle compared to when the engine is under load. So, it is possible to have an intermittent leak whose symptoms only occur under certain load conditions while only seeing a minor positive (green) result when testing at idle.

What if my car doesn’t have a cap on the radiator – where do I put the tester?

Many modern cars do not have a cap on the radiator itself. If you don’t have a radiator cap, your car likely has a remote coolant expansion tank, and your pressure cap will be found there. This is different than the old recovery or burp tanks found on older vehicles. The older recovery/burp tanks are always vented to the outside while a remote expansion tank is part of the sealed cooling system.

Once you’ve verified that your remote tank is sealed to the outside air, this is the appropriate place to perform the combustion leak test if your car does not have a cap on the radiator itself.

How does this compare to pressure-testing the cooling system?

It’s not necessarily better, but a combustion leak test is different from a cooling system pressure test. The easiest way to think about it is the cooling system pressure test tests for coolant LEAVING the cooling system while the combustion leak test tests for exhaust gases ENTERING the cooling system. The cooling system pressure test is performed at about 15 psi of pressure, and as stated above, that is much less than thousands of psi that can be observed during the combustion process. As such, it is highly unlikely that a cooling system pressure test will exert enough pressure (not to mention it’s in the OPPOSITE DIRECTION) on a combustion leak to detect all but the worst combustion leaks. The cooling system pressure test is only suitable for finding coolant leaking from water pumps, radiators, hoses, etc. The combustion leak test is really the only way for the casual mechanic to pick up the telltales of exhaust gases entering the cooling system through a blown head gasket, cracked head or block, etc.

Is there a shelf-life to the test fluid?

Yes, it’s about a year for an unopened bottle when stored in a cool, dark place. Once the bottle is opened, or if it’s exposed to direct sunlight or extreme temperatures, its remaining usable life will shorten to a few months. If the fluid is still blue, it’s usable. If it has turned green, then it shouldn’t be used.