Blue Tester

FAQ's, Tips & Tricks

Q. WHAT IS A BLOCK TESTER® (and why is it better than other combustion leak testers)?

This is a Block Tester®, the superior combustion leak tester:

Block Tester

These are the "other" combustion leak testers:

combustion leak tester 1combustion leak tester 2

Seriously, we have more experience making combustion leak testers than anyone else out there and feel we've perfected the form and function of our Block Tester® when we introduced it over 40 years ago. Combustion leak testers in general are tools for detecting the presence of exhaust gases in the cooling system (uh-oh) of an internal combustion engine. While the competing tool on the left uses a dual-chamber design to attempt to eliminate user error (getting coolant up into the tester), it introduces process error in that we've seen results that showed positive in one chamber and negative in the other. Which are you to believe? The instructions say to believe the top chamber, but our exhaust gas analyzer tells us that the bottom chamber (positive) actually had the correct result. The combustion leak tester on the right saves the manufacturer the cost of the aspirator bulb, but requires you to do more work by having to tap into a vacuum source to use the tool. Also, you typically can't control the amount of vacuum the engine is making - too much, as often is the case, and you suck coolant into the tester.

That's why it's important to look for the unique shape that separates the Block Tester® from its competition!


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" gases from the air pocket created in the cooling system through our proprietary chemical fluid. If exhaust gases are present in that air pocket, they will react with our fluid and change its color from blue to green to yellow as more gases are passed through the fluid.


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.


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.


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.


"Block Tester" and the Block Tester shape are trademarks of Test Tools, inc.
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