leak down test problem

[CPHunter]

i did a leak down test and i put the hose in and it hits 100 and it drop to like 10 in less then 7sec,but thats only on cyl 1&4. when i get around to doing 2&3 when i put the hose on and it hits 100 it drops to 20 while the timing belt starts moving around with all sprockets. then it shoots back to 70-80. anybody have an idea of what this is?

 

[trevor_shiggs]

Are you sure you have the cylinders at Compression TDC while injecting air?
Where is the air leaking out of?
Rad? Exhaust? Intake?

 

[Big Woo]

Well to do a leak down test you need to ensure that the cylinder under test is at TDC of the compression stroke for that cylinder. When you have completed the test for that cylinder you will need to rotate the assembly to bring the next cylinder to be tested to TDC of the compression stroke. You will need to do this each time you change the cylinder tested.

If you are using 100 psi and your engine is std bore you will have about 525 pounds of force pushing down on the piston being tested. This is can be found from the area of the piston multplied by the pressure applied. With that said it is best to make something to hold the crank in position before applying pressure to the cylinder. Or if your leak down tester has the capibility, prehaps you could use a lower pressure. If you do not keep the engine from moving then when pressure is applied the engine will have a tendency to rotate from the applied force.

If the engine rotates clockwise (as viewed from the T-belt end) far enough then the exhaust valve will open, since it opens prior to BDC of the power stroke. If it rotates counter-clockwise far enough then the intake valve will open, since it closes ABDC of the intake stroke.