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How to perform a

Cylinder Leakdown TesterMost of us are familiar with compression testing which helps, but does not give you all of the information you need. In this process, the test gauge is connected to the spark plug hole and the engine is turned over with the starter. The gauge reads in pounds per square inch (PSI) and the reading is compared with the specifications for that engine, previous test results, and the readings from the other cylinders in order to determine the health of the particular cylinder or engine. The gauge measures the engine's ability to pump air at the starter RPM and changes in RPM can mean changes in recorded pressure. It is not uncommon to have lower pressures for each successive cylinder just because the battery is running down. If there is a low reading, it is difficult to tell whether the problem is in the rings, valves, head gasket, etc. (although the trick of squirting oil into the cylinder to see if the problem is in the rings sometimes works). Some performance modifications result in lower compression readings, at least partly because the modifications are designed to improve the engine's performance at high RPM, not at cranking speed. A worn cam may give a lower reading, but so will changing to a high performance camshaft with longer duration and significant overlap. Thus a racing engine in good condition may give a lower compression test reading than a stock engine in average condition.

Doing a leak down test, particularly in conjunction with a compression test, will give you a great deal of additional information and allows problems to be pinpointed. The condition of the engine is determined by measuring the degree to which a cylinder with valves closed leaks air. In simplified terms, if air is pumped into the cylinder at 100 PSI and the gauge reads 97 PSI, then the leak down percentage is 3%. Doing a leak down test is a fairly simple matter. The tester is connected to an air source and is attached to the engine through the spark plug hole. There are two types of instruments, those with single and double gauges. The cost of the tester is usually between $50 and $250 depending on whether it is the single or double gauge and where it is purchased. The double gauge constantly measures the input pressure and the cylinder pressure. The single gauge instrument relies on checking the input pressure and then switching to measure the cylinder pressure. Each requires some adjustment to be accurate. The engine should be rotated so that the valves in the cylinder to be tested are closed and the piston is at top dead center. Tests should be conducted when the engine is warm.

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 indicates a blown head gasket or cracked head.

A brand new street engine might measure from 5% to 8% depending on the engine, manufacturer, and degree of break in. A street 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 street engine for daily driving 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. These are tests that can and should be done by any competent garage. 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 vehicle 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).



The tester requires an outside source of air pressure (70 to 200 psi) to be connected to the "air inlet" connection. Before connecting this line to the air inlet connection, turn the CLT REGULATOR KNOB to the left a couple of turns. Then connect the airline and turn the CLT REGULATOR KNOB to the right in small steps until the gauge reads "0". Momentarily depress the valve in the connection end of the "outlet to engine" hose. The gauge should rise, then return to "0". If it does not return to zero, turn the CLT REGULATOR KNOB to the left again and repeat. When the Cylinder Leakage Tester reads on "0" it is ready to use.

With the engine at normal operating temperature, this air pressure will be applied to a cylinder when the piston is at TDC on the compression stroke. At this time both intake and exhaust valves are closed and except for a small amount of air leakage past the piston ring gaps no air should escape from a good cylinder. The amount that does leak out is measured on the % CYLINDER LEAKAGE gauge.

The Cylinder Leakage Tester uses the % Cylinder Leakage gauge, the CLT Regulator, a tester hose and one of several adapter hoses for various types of engines.


Make this test to determine the location of compression losses.

  1. Connect the remote starter switch to the starter solenoid. Turn ignition key ON.
  2. Ground the coil’s high-tension lead.
  3. Remove radiator and crankcase filler caps.
  4. Screw the proper adapter hose, finger tight, into the cylinder to be tested and put the whistle on the end of the hose.
  5. Crank engine over slowly until whistle just stops sounding. Remove whistle.
  6. Connect the tester hose to the adapter hose.
    Note: Watch the engine fan; if it turns, stop. Remove the tester hose, reposition whistle, and repeat step 5. Do not attempt to crank engine with the tester hose connected.
  7. Listen for air leakage at exhaust pipe, radiator and carburetor and from the adjacent cylinders. If the engine has a PCV valve, be sure to pinch off PCV hose when listening at carburetor.
  8. Look at gauge, record reading.
    GOOD No "Air Leaking" heard in exhaust pipe, radiator, carburetor, adjacent cylinders and leakage less than 20%.
    BAD "Air Leaking" heard at exhaust pipe, radiator, carburetor, adjacent cylinders or more than 20% leakage on a properly broken in engine, indicates a need for service.
    • Air escaping at carburetor indicates a leak at the intake valve.
    • Air escaping at exhaust pipe indicates a leak at the exhaust valve.
    • Air escaping at radiator filler cap indicates a blown head gasket or cracked block.
    • Air escaping from adjacent cylinders indicates a blown head gasket or cracked block.
    • No "air leaking" heard in exhaust pipe, radiator, or carburetor, but gauge reads over 20% indicates worn or broken rings or scored cylinder walls.

    If additional cylinders are to be tested:
  9. Repeat steps 4 to 6 to bring each cylinder in firing order to TDC test position.