You must have cylinder pressure to make horsepower and torque. If a cylinder-or cylinders-isn't holding pressure when the spark plug fires, there's gonna be less power in the power stroke. Performance suffers, and more serious engine damage could be on the horizon. A traditional compression tester can tell you how much pressure is being created in each cylinder; comparing the numbers reveals much about an engine's condition. A leakdown tester, on the other hand, measures how much cylinder pressure is being lost-better yet, it reveals where this pressure is going, for a more detailed diagnosis.
While a compression tester measures the cylinder pressure created by the pumping action of the piston, a leakdown tester utilizes an outside air source to pressurize the cylinder. The less pressure being lost the better that cylinder is sealing. An engine in good shape will see a minimal drop-if it's seen some wear or has a serious problem, the drop will be greater. All cylinders will show some leakdown due to standard engine clearances and normal wear; the Proform unit shows up to 40 percent leakdown as within the "normal" range. It's more important that the reading be consistent-if one cylinder is out of line to the bad side, you know there's a problem.
And in any case, but especially when the readings fall into the "moderate" or "severe" range, it's vital to know where this pressure is going, and that's where a leakdown tester really shows its worth. The air being introduced into the cylinder escapes by any path available to it. By listening for this exiting air and noting its route, you can pinpoint problem areas in the engine.
That's what we did here. We ruled out the head gasket failure we feared but found moderate leakage past the rings. We now have a much better picture of this engine's condition-and can proceed accordingly.
Quick Notes
The job
A leakdown test on an '84 Z28
Bottom Line
We determined how much this engine is leaking, and where it's leaking from.
Price (APPROX)
$75
 | 
Our leakdown tool for this project was Proform's Dual-Gauge Leakdown Tester. One gauge regulates the amount of air being pumped into the cylinder, and the second measures the amount of pressure being lost; i.e., the rate of leakdown. It comes with both 14- and 18mm thread adapters. | 
To prepare the tester, we pulled up on the regulator knob to unlock it then turned it fully counterclockwise. The tester was then connected to a shop supply of air capable of producing 50-150 psi. (We made the connection with the compressor turned off.) Remember, the regulator will actually control how much of this pressure enters the cylinder. |