1984 Camaro Z28 Leakdown Test - Pressure Points
Running A Leakdown Test To Determine Engine Condition
From the May, 2008 issue of Chevy High Performance
By John Nelson
Photography by John Nelson
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.
A leakdown test on an '84 Z28
We determined how much this engine is leaking, and where it's leaking from.
Our leakdown tool for this...
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...
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.
After connecting the air source,...
After connecting the air source, we turned the unlocked regulator knob clockwise until the needle on the right-hand "Cylinder Leakage" gauge was positioned in the middle of the area marked "Set." We pushed the regulator knob down to lock it into place.
With the leakdown tester ready...
With the leakdown tester ready and waiting, we ran the engine until it was good and warm. Then we removed our Z28's air cleaner, oil filler cap, dipstick, radiator cap, and all eight spark plugs. The last two are tough when working with a hot engine, but unlike a compression test, it's critical that the engine be warm for a leakdown test to be accurate. We hand-tightened the adapter hose into the cylinder to be tested, which is quite a feat on a third-gen Camaro with headers-this is one of the few cylinders we could actually see to photograph.
Once the adapter hose was...
Once the adapter hose was in place in cylinder No. 1, our starting point, we rotated the engine until this cylinder was at top dead center (TDC) on the compression stroke. In our experience, most accurate results were achieved by rotating the engine to this point by hand rather than bumping it with the starter. We've also heard of removing the rocker arms to ensure that both the intake and exhaust valves are closed. Once TDC was acquired, we inserted the free end of the adapter hose into the quick coupler on the test unit
The tester told the tale for...
The tester told the tale for this cylinder: 45 percent leakdown, which the Proform unit classifies as moderate. It wasn't as good as we hoped for, or as bad as we feared, but while testing this cylinder, we could actually hear the air escaping through the crankcase breather tube, which meant it was getting past the rings. Given that this engine has new heads but its original lower end, it makes sense.
For the most part, our test...
For the most part, our test subjects came in somewhere between 30 and 45 percent leakdown. It's not great but it is mostly consistent from cylinder to cylinder, which is what you want to see. Cylinder 7, on the other hand, registered a very low 20 percent
For comparison's sake, we...
For comparison's sake, we also ran a standard cold-cranking compression test on our 305, registering 140-150 psi on all but cylinder 7, which came in at only 130 psi. Well within 10 percent of the other cylinders, it's not a bad number. We mention it to highlight the fact that the two measure different things-this cylinder doesn't create as much pressure as the others, but it also leaks less than the others. Both pieces of info are valuable in determining an engine's condition
As we mentioned, a leakdown...
As we mentioned, a leakdown tester can help pinpoint problem areas in an engine. The key is to identify the exit path the pressurized air is taking. A few possible escape routes are shown here (arrows). Air passing through the oil fill hole or crankcase breather tube points toward worn rings as the culprit. If you hear it through the carburetor, there's probably a less-than-perfect seal at the intake valve.
If pressurized air is making...
If pressurized air is making it past the exhaust valve, it will make its presence known in the exhaust system. You have to listen for it, but we've heard of cases in which the air could actually be felt exiting the tailpipe.
A leakdown tester can be especially...
A leakdown tester can be especially valuable in tracking down a blown head gasket. If air is getting into the cooling system during a leakdown test, you may see air bubbles in the radiator. During a test on another car, we heard a gurgling noise in the coolant passages, and pulling that head revealed a blown gasket.
Spark plug logistics on a...
Spark plug logistics on a third-gen Camaro require that cylinders 6 and 8 be accessed from beneath the car. Yes, it was a pain, but we learned something from this close-quarters experience; we could hear the sound of pressurized air in the oil pan, indicating that air was getting past the rings. If we'd drained the oil after warming up the car, we might have even felt air exiting from the drain plug hole. It's something else to think about as you play leakdown detective with your own ride.
Possible Causes for High-to-Moderate Cylinder Leakage
The cause of low pressure can be determined by listening for escaping air.* Air escaping from crankcase breather, dipstick tube, or sump plug hole: Defective rings or worn cylinder walls
* Air escaping from carburetor: Defective intake valve
* Air escaping from exhaust system: Defective exhaust valve
* Air bubbles in radiator or air escaping from adjacent spark plug hole: Leaking head gasket or crack in block or head Courtesy of Proform