In this section we are going to explain the process to complete a leak down test the best way we know how to explain it.
A leak down test is a good test to perform after a compression test has been done. A compression test will tell you there is a problem but a leak down test will actually pinpoint the problem and where it is coming from.
A leak down test essentially air pressurizes the enine block to figure out where compression is escaping from the problem cylinder or cylinders. The escaping air most of the time can be heard coming out somewhere in the engine. A leak down test will also tell you the percentage of escaping air coming out and if that escaping air is within an acceptable range or not measured in percentage (%). Generally, you want the cylinders tested to be within the green range of the test (10%).
Ok on to the testing procedure.
Warm up engine until operating temperature is reached. Shut down engine and proceed to step 2.
Remove the spark plug and wire from the cylinder to be tested. Keep all other spark plugs in. You can disable the fuel and spark if you want or just simply remove all spark plug wires from the rest of the plugs. We are trying to eliminate a potential starting hazard when we slowly crank over the crankcase. Start with the cylinder to be tested at TDC (Top Dead Center). This means the piston needs to be at the top of its compression stroke this insures the valves in the cylinder are closed. To do this put a ratchet on the engine crank and a tissue in the cylinder hole where the spark plug was. This allows us to watch and see when the piston is coming up to TDC. The tissue should start to blow out when the piston is coming up to TDC. When the tissue moves stick a pencil in the cylinder hole and crank the case until the pencil is barely in the hole showing us that the piston is at its highest position.
Insert tester probe into cylinder hole. Pressurize other tester end with shop compressor. Read percentages of leak but also listen as to where the leak is coming from if a leak is present.
Problems are pinpointed simply by determining where the air is leaking out of the cylinder. Air leaking out of the exhaust system (you can hear it in the exhaust pipe) indicates a problem with the exhaust valve. Air coming out of the carb or throttle body indicates a bad intake valve or seat. Air going into the crankcase is leaking past the rings and does not indicate a problem if the percentage is low enough. A leak where the air is going into an adjacent cylinder or into the coolant overflow tank indicates a blown head gasket or cracked head.
A brand new engine might measure from 5% to 8% depending on the engine, manufacturer, and degree of break in. A engine that measures 10% to 20% per cylinder, although indicating some wear, if there is consistency between cylinders and if all of the air is leaking past the rings into the crankcase indicates a reasonable engine for daily use that does not need any immediate work. Any readings 30% or higher indicates a severe engine problem.
Using the leak down test, especially in conjunction with a compression test, should allow you to quickly determine the basic condition of any engine. If you are not sure what the levels should be, differences in readings between cylinders is a key indication of a problem. After testing, you will know whether the top or bottom end really needs a rebuild.
A cylinder that has poor compression, but minimal leakage, usually has a valvetrain problem such as a worn cam lobe, broken valve spring, collapsed lifter, bent push rod, etc.
If all the cylinders have low compression, but show minimal leakage, the most likely cause is incorrect valve timing. The timing belt or chain may be off a notch or two. If the boat runs, do a vacuum test, very low steady vacuum would verify the incorrect valve timing.
If compression is good and leakage is minimal, but a cylinder is misfiring or shows up weak in a power balance test, it indicates a fuel delivery (bad injector) or ignition problem (fouled spark plug or bad plug wire).
Guerilla style. If you want to just check the engine for leaks without buying a leak down tester, simply use your compression tester probe and remove the schrader valve from it. This will allow compressed air to enter directly into the engine block. You can then begin listening for escaping air.
By: Travis L. Palmer
Principal Marine Surveyor
Corsica River Marine Surveys