Leak Down test without the tester

[pamcopete]

OK. Here is a technique to do a leak down test without a leak down tester...

1. You will need a compressor with a built in adjustable regulator.

2. Allow the compressor to run until it has reached its maximum pressure, typically 100 PSI.
3. Adjust the regulator for a pressure of 50 PSI.
4. Get an adapter hose that will fit into a spark plug hole on one end and your compressor hose coupler on the other.

5. Warm up the engine, shut it off and remove a spark plug.
6. Rotate the engine to TDC on the compression stroke.
7. Screw the adapter hose into the spark plug hole.
8. With the compressor adjusted regulator adjusted for 50 PSI, quickly connect the adapter hose to the hose coupler from the compressor.
9. Note the pressure reading on the regulator gage.

Note: You must record the regulator pressure before the air tank pressure goes below 50 PSI.
In the example shown, the regulator pressure is 35 PSI, so the percent leak down is 50-35 = 15 / 50 = 30%. Generally, a leak down percentage up to 40% is acceptable. I did this test again with a warmed up engine and got a regulator pressure of 40 PSI for a leak down % of 20%, so warming up the engine is important. However, doing a cold leak down followed by a warm leak down can provide a better picture of the cylinders condition.

 

[jmink427]

I guess I have been doing wrong then...... But if 30% good my 65 Impala should be good for a while longer since it only has 5 to 6% across 8 cylinders and that is using a .032" orifice instead of a .040" which is the FAA standard... ...

 

[pamcopete]

jmink427,

There is no orifice per se in the test that I outlined above. Instead, the test relies on the orifice in the regulator itself.

 

[jmink427]

You got me wondering about my engine so I was gonna do a leak down just for the heck of it.... I can't seem to get mine on TDC every time I put air to it the it forces the piston down which in turns opens one of the valves a bit..... On a Big Block Chevy I have never had this issue but it has a lot more parts it would have to move.... Are you guys using something to keep the crank from turning?

 

[pamcopete]

jmink427,

Try putting it in gear with something jammed in the rear spokes. Also, the 40% OK number is just the percentage of the original regulated pressure that was set up with no leak before connecting the compressor to the cylinder. I think your FAA number is a calibrated figure showing % loss of air from the cylinder.