4tec Leakdown Test

[IDoSeaDoo]

Hey, to diagnoise the health of the rings and valves on a 4tec, I see they do something called a leak-down test. How do they calculate the "leakage %"? Can someone please explain to me how this works and what they're calculating?

 

[ski-d00]

This is common in the 4 stroke and automotive world. Basically, with shop air pressure you pressurize a cylinder at TDC and then the gauge(or twin gauge system as most are) measures the percentage of air leak. A much more comprehensive test than just a compression test. It will tell you where it is leaking from, like a valve, or a head gasket. It also tells you the condition of the cylinder rings as long as your valves and head gasket are sealing. The leaks can easily be heard and seen with soapy water or bubbles in the coolant.

It takes a little time to perform because each cylinder has to be held in postion(the air pressure will push the piston down) which is done by holding the driveshaft and the valve cover should come off to make sure the valves aren't leaking. Normal leakdown percentages for 4tecs are in the 15-20% range where all the leakage is from the rings.

 

[IDoSeaDoo]

The thing I don't get is where are we measuring the leak % between. I understand one pressure is at the spark plug hole, what about the other?

 

[ski-d00]

It's not really between anything, it's just the cylinder chamber that leaks. The spark plug hole is where the gauge is attached and doesn't leak there. The cylinder chamber is what is being pressurized with shop air through the gauge. So, lets say that we had a theoretically perfect cylinder where it had no leaks(not true because all rings leak) and we then would add 100psi of shop air pressure to the cylinder. The leakdown percentage would be 0% because it would hold and keep all of that air pressure. Now lets say we have a big hole in our piston so that it wouldn't hold any air pressure, the leak % would be 100%. Now, a real case would be that we add the 100psi shop pressure to the cylinder chamber through the tester and so one gauge reads 100psi, and the other gauge will read 20% and you will hear hissing from inside the motor. This would be a good engine. Make sense? You can google this and find much more thorough explanations. All automotive guys do this test.

 

[IDoSeaDoo]

Not sure where the other gauge is measuring and what it's measuring (20% of what?) This sounds like a youtube explanation, but that's blocked while I'm at work

 

[ski-d00]

The two gauges are right next to each other on the tester. One reads psi, the other reads percentage of leak(there are other types but this is the most common and simplest). It basically is measuring how much air is being supplied and how much is being lost. The 20% of the air being supplied is leaking out. It will make more sense if you see it on YouTube.