David Trimble

Here's a easy question- How do you perform a leak-down test on a engine? I figure I might was well do that on my 305 since I've still got it apart from doing a compression test. I've noticed some oily crud accumulating around the PCV valve area (I can clean the area and it shows up again a month later) and I'm wondering if I'm getting blow-by and the PCV system can't handle it....

Thanks!

ede

fill the cylinder with air, valve the air off, take reading on the gauge and come back a preset time and reread gauge and figure out %of air leakage, using 100 psi of air makes the math easy. compression test is quicker and easier i believe, but i do both depending on my needs

David Trimble

So a compression test is effectively the same as a leak-down test? And what sort of percentage value would indicate excessive leakage?

Vader

Or...

You can rotate the cylinder to be tested to TDC with both valves closed. Fill the cylinder with compressed air through a controlled/calibrated oriface (1.0 mm/0.3937"), and compare the line pressure to the pressure that the cylinder will maintain through the oriface. The difference in pressure can be calculated as a percentage. Using 100 PSIG inlet air pressure from the compressed air line makes the calculations direct. A cylinder that will maintain 90 PSIG has 10% leakage, and is in relatively good operating condition. One that will maintain only 60 PSIG has 40% leakage, and is in pretty bad condition. Fresh rebuilds that have been broken in often have only 3-5% leakage. 10-15% is fairly common in "good" engines, and as the leakage increases the wear and condition of the engine changes accorrdingly.

You can build a leakage tester for about $40 in parts, or buy one for $75-150 depending on where/how you shop. To make the orface, a .040" drill is perfect:

David Trimble

Thanks, Vader! I'll see if I can cobble up a tester- doesn't sound too hard...

TomP

Vader, the controlled oriface is that easy? How long should the oriface be? Why is the air port so "large" under the "regulated air" gauge, but smaller under the "cylinder pressure" gauge? Is that whole run, underneath the cyl pressure gauge, a 0.040" hole?

I figured I'd link two gauges together, each with a "T" fitting, and drill a solid piece of tubing with an 0.040" drill, thread the tubing, and connect the two T fittings together. Sound good, or not? Thanks! (Best part is, I already have an air regulator with quick disconnect fittings and gauge (Amflo), I'd just need a t-fitting and another gauge).