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Leak Down Testing 


Performing a leak down test can be very helpful in determining if piston rings have started to leak, valves are leaking, or if there is a headgasket leak into the cooling system.  I purchased this leak down test kit from Summit Racing.  It includes adapters for most spark plug sizes and works fine with my Toyota MR2.

The leak down tester includes two gauges, a regulator, a port for connecting to an air compressor and a port for connecting into your spark plug hole.  Testing with a leak down tester is similar to a compression test, but the position of the cylinders is critical to getting valid results.  In general the piston should be at top dead center on the compression stroke.  You can also test leak down with the piston at bottom dead center, but most consider top-dead-center to give the most valuable results.

A typical leak down test kit:

The first step is to warm up the engine to normal operating temperature.  This is done so that the rings have expanded and hence will seal better, and should give more meaningful results.  This is not absolutely necessary, however, and the test can certainly be done with the engine cold.  The test results will not vary (in my experience anyway!) all that much.
Next, remove all the spark plugs.

How to position the engine

After the spark plugs have been removed, you can position a piston to top dead center by taking a thin, long screw diver and putting it into the spark plug hole.  If you have a manual transmission, put the car into top gear (5th gear in most cases), and release the brake.  As you move the car forward, or backward you can watch the screw driver rise and fall.  After the screw driver reaches the top, and just begins to fall, put on the parking brake.  Screw in the test tube into the spark plug hole, and run the test.  If you have an automatic transmission, I believe your only option is to turn the engine by hand.  This can be done by turning the crankshaft pulley with a socket wrench.

To run the test connect the regulator end of the tester to an air compressor.  Adjust the air compressor to at least 100psi.  My own compressor goes to 120psi, and that's what I use.  Adjust the regulator so that the left gauge reads 100psi.  If you hear a lot of air coming out, and the right gauge reads very very low (like 40psi or less), chances are that the engine is in the wrong position - that is - it was on the exhaust stroke instead of the compression stroke.  Remove the test tube, and move the car again so that the piston drops and comes back up to top dead center again.  Re-install the test tube, and adjust the regulator to 100psi.  If the right gauge reads 90psi, that means you have a 10% lead-down.  If the right gauge reads 95psi, you have 5% leak down (time to celebrate!).  According to sources, anything greater than 15% indicates excessive leak down, and repair should be done.  If you listen to where the air is coming out of, you can hopefully determine this repair.  Make sure that the leak down test tube is sealed well in the spark plug hole.  I had to get a better O-ring for the kit, and I actually super-glued the end of the test pipe so that if I tuned the rubber part of the tube, the threaded end would turn as well.  This allowed me to get a better seal in the spark plug hole.

The following was taken from the following website (Walt Osborn):

"Listen for leakage at the following places:

* Adjacent cylinders sparkplug hole. Use a piece of small rubber vacuum hose, stick one end near the spark plug hole and the other end in your ear. Leakage, air hiss, heard here may be a blown head gasket between cylinders or it may be leakage heard through an open valve.
* Exhaust pipe. May indicate a burnt or stuck exhaust valve.
* Carburetor or throttle body. May indicate a bent or stuck intake valve.
* Oil filler or dipstick hole. May indicate broken rings or a damaged piston.
* Radiator filler cap. Bubbles here will indicate a leaking head gasket or cracked head.

Repeat the above steps for each cylinder, record the numbers, and check the above locations for air.  If the numbers are suspect, you might want to run the test again with the piston close to bottom dead center.