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Engine trouble? An engine that's down on power or has a dead bore calls for close attention to what the problem is. Is a compression check telling enough? We've long been taught that a good engine tune-up includes spark plugs, ignition parts, carburetor tuning, and the like. We've also been advised to check compression and intake manifold vacuum as a barometer of engine health.
But does this cover all the bases of a good engine check-up?
A compression test along with intake manifold vacuum at idle are good indicators of engine health, but static compression and manifold vacuum don't always say it all especially if numbers are marginal. A cylinder leak-down test is the final word on engine health and it's not hard to accomplish. A cylinder leak-down test is where we apply air pressure to a cylinder with the piston at top dead center with both valves closed to create dynamic compression, which is as close to operating pressures (fire lit) as it gets.  Visit your nearest Harbor Freight store or website, www.harborfreight.com, and order up a Cylinder Leak-Down Tester #94190 for $39.99, which will get you headed in the right direction for good engine troubleshooting. Next, you're going to need an air compressor capable of making a minimum of 100 psi along with all the right quick-disconnect fittings.  Bring each piston to top dead center, engine warm, (one at a time, of course) with both valves closed on compression stroke. You're going to want to rock the crank back and forth in order to settle piston rings in a seated position.
Be very careful about engine kickback from applied air pressure. Air pressure can cause the crank to rotate violently and quickly. Screw Leak-Down tester into the spark plug hole, making sure there's no leakage, and apply 100psi. If your Mustang has a manual transmission, make sure shifter is in neutral for your safety. Also make sure piston is locked at top dead center to prevent abrupt turnover. At 100psi applied pressure, you want no more than a 10-percent loss of pressure. So at 100psi, you'd want to see at least 90psi on a healthy engine.
Ideally, you will see all cylinders within 4-5-percent of each other in a gang cylinder leak-down test.
It is a good idea to listen to each cylinder when you do a leak-down test. Listen for air leakage and where it's coming from. Leaky valves will be heard in the intake or exhaust. Leaky piston rings will be heard in the crankcase.
If you find an unusually low cylinder reading, listen closely to where air is escaping. Make sure both valves are closed.
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