As we mentioned a couple of months back, a compression test is a relatively easy way to gauge the overall health of your engine’s cylinders. Each cylinder needs to produce enough compression to extract the maximum amount of power from the fuel it sucks in, and a simple gauge will show whether or not that’s happening.
Begin by opening up all of the engine’s orifices, so that it will be easy to track down any escaping air. This means removing the air cleaner and securing the throttle in its wide-open position, as well as removing the oil dipstick and the radiator cap.
And yet, the compression test has its limitations. Yes, it can point out poor compression in one or more cylinders, but it can’t identify the cause. And there are a number of things that can cause low compression: Worn rings or cylinder walls, burned valves and blown head gaskets among them.
This is where the cylinder leak-down tester comes in. This is a device that pressurizes each cylinder with compressed air, and then measures how much of the compressed air is being lost. More than that, it allows you to track down the source of the pressure loss by listening for the sound of rushing air. If an intake valve isn’t sealing properly, the air will hiss through the intake manifold. If it’s an exhaust valve, you’ll hear the rush of air out the end of the exhaust pipe. If the rings are worn, the air will enter the crankcase, where you’ll be able to hear and feel it at the dipstick tube. And if the head gasket has failed, you’ll see bubbles in the coolant at the radiator filler neck, or hear air escaping into an adjacent cylinder.
Mark the spark plug leads so you’ll remember where they go, and remove all of the spark plugs; this makes the engine easier to turn over. Use compressed air to blow away any debris from around the plugs first, so that it doesn’t get into the cylinders.
We paid $40 for our leak-down tester; prices range up to $300 or so. If you call your local auto parts supplier, you’ll want to be specific about what you’re looking for so that you don’t waste a trip–some counter people thought we wanted a compression gauge, while one was prepared to sell us a leak test kit for an air-conditioning system.
Each cylinder must be tested at top dead center, or TDC, of its compression stroke. We turned the engine over by hand until the timing mark on the crankshaft pulley lined up with the ”0” on the scale. (The silver mark is at 10 degrees BTDC, the usual timing mark.)
You’ll also need a supply of compressed air. The instructions that came with our tester said that it would work with air pressures from 7 PSI to 100 PSI; we split the difference, and set the regulator on our compressor at 60 PSI. The engine should be warmed to normal operating temperature.
The test took about two hours, and that included learning how to attach and operate the gauge. In fact, we were unable to get any meaningful results until a web search revealed that two steps in the instructions were reversed. Once we followed the procedure the right way, the test was straightforward. If you can correctly adjust your valves, you can do a leak-down test.
Removing the rocker cover makes it easy to find out if a cylinder is in its compression stroke; with both valves closed, the pushrods should be loose. If not, turn the crankshaft over 360 degrees in its usual direction of rotation, and check again.
Now it’s time to connect the tester to the source of compressed air, in our case set at 60 PSI. There’s also a short length of hose that gets connected to the spark-plug hole with one of two supplied adapters; that doesn’t get connected to the tester yet, though.
The gauge on the right is the one that reads cylinder leakage; the black cylinder is the locking knob for the pressure regulator. Starting with the valve turned completely counter-clockwise, turn it to the right until the leakage gauge reads zero, and lock it in place.
When the gauge is connected to the cylinder with a quick-release fitting, the leakage gauge will show what percentage of the air is leaking out. Even a new engine will have some leakage, but anything above 15 percent is cause for concern. Cylinder number 1, here, is fine…
This is cylinder number 3, the one that read below normal during our compression test. Sixty-five percent leakage indicates a major problem. An ear to the tailpipe confirmed that air was rushing out past the exhaust valve. Stuck? Tapping with a plastic hammer didn’t help.
A mechanic’s stethoscope made a simple job out of tracking down the sounds of escaping air, but we could have gotten along without it. We did discover that the 10 percent leakage in cylinder 1 was escaping past the rings; we could hear it at the dipstick tube.
