Besides all of the moving parts and complicated-looking pieces of metal, an engine really only needs four things to run: fuel, air, compression, and a spark. It sounds easy enough, but once there's a problem with any one of these four things the chances of your engine actually running become pretty slim. The first tool many do-it-yourselfers, and even mechanics, grab when an internal engine problem is suspected is the compression tester. While the compression tester serves its purpose, it'll never be able to pinpoint the actual problem. Sure, it can tell you that cylinder number three has just taken a crap, but it won't tell you why. Is it the piston rings or a valve? How about a cracked cylinder wall or a blown headgasket? It's times like these you'll need a leak-down tester.
Cylinder leak-detection testers work by shoving air into an individual cylinder through the spark plug hole and measuring how much, percentage-wise, leaks out. Where the air actually leaks from can help you diagnose what the problem is. For example, pull the oil cap. Hear a howling sound inside the crankcase? This is a good indication there's a problem with the rings or cylinders - either way, the head's coming off. What about exhaust puffs gently streaming from the tailpipe? You've probably got bent or burnt exhaust valves here. You can also open the throttle body and listen for funny sounds, which can indicate a problem with the intake valves. If all of this checks out but you've still got poor leak-down results, you've probably got a head gasket that needs replacing. If that's the case, air will likely be escaping into the cylinder next to the one you're testing. A strong engine will yield leak-down results of less than 5 percent per cylinder, with readings that vary less than 3 psi from one to the next. Anything over 10 psi should be investigated.
Besides the leak-down tester itself, which ranges anywhere from $75 on up, you'll also need an air compressor capable of at least 100 psi, and the ability to find TDC (top dead center) at the compression stroke. While we can't help you with the whole compressor thing, it's easy to build your own leak-down tester on the cheap. Here's how:
Leak-Down Procedure
1 Bring the engine to operating temperature and turn the ignition off.
2 Bring the number-one cylinder to TDC and remove its spark plug.
3 Connect the leak-down tester to the air compressor and regulate it to 100 psi.
4 Thread the leak-down adapter into the spark plug hole.
5 Connect the leak-down tester's air coupler to the adapter and monitor the results.
6 Replace the spark plug and repeat according to the engine's firing order.
 1. The first thing you'll...  1. The first thing you'll need is a mini air pressure regulator. Look for one capable of handling at least 100 psi and with 1/8-inch or 1/4-inch NPT ports, like those used with air compressors or automotive paint spraying guns. This is the most expensive part you'll need; plan on paying about $20 for this. |  2. You'll need at least two...  2. You'll need at least two fittings to get air from your compressor to your regulator. This regulator has an 1/8-inch NPT inlet but the air line fitting we need is 1/4-inch NPT. Brass reducers like the one shown make otherwise non-compatible parts compatible. |  3. Of course, you'll also...  3. Of course, you'll also need to get air out of the regulator and into your engine's cylinders. You'll need a quick-disconnect coupler - like the one on your compressor - and some way to adapt it to the other end of the regulator. This special brass adapter has an extra port on top for a pressure gauge, which will come in handy later. |
 4. You can always make your...  4. You can always make your own adapter out of a simple pipe tee and a couple of nipples. |  5. The adapter that we special-ordered...  5. The adapter that we special-ordered has a 0.040-inch restrictor inside to reduce the volume of air that's introduced into the cylinders. The restrictor's got to be there. If you can't find a fitting like this, just fill your pipe tee with epoxy, let it dry, and drill a small hole through it. |  6. The assembly should look...  6. The assembly should look like this. Air enters on the right and exits on the left. Once it's hooked up to the compressor, use the regulator's knob to adjust the air pressure accordingly. |
 7. Here's a typical gauge...  7. Here's a typical gauge used for calibrating air pressure. Thread it into the assembly or into the regulator's remaining port, turn the compressor on, and set it to approximately 100 psi. The built-in regulator allows you to keep your air compressor set at, say, 150 psi but not worry about shoving too much air into your cylinders. If you're only going to use one pressure gauge, you'll need to zero it out once the system is set at 100 psi. |  8. Getting the air from the...  8. Getting the air from the tester to the engine's cylinders is the tricky part. You'll need a grease pump hose for this. Most grease pump hoses have 1/8-inch NPT female adapters at each end, so transitioning from that to the correct air fitting and spark plug hole adapter is easy. |  9. A typical leak-down tester...  9. A typical leak-down tester will come with a pre-made hose that's compatible with at least a couple of different spark plug threads on one end and an air line adapter on the other. You'll need to make your own though, which is what the grease pump hose is for. |
 10. You'll need one spark...  10. You'll need one spark plug, a vise, and a big hammer to start. Carefully bust the porcelain off and clean up the remnants. |  11. You'll be left with something...  11. You'll be left with something like this. Grind away the rest of the porcelain and drill and tap the spark plug for 1/8-inch or 1/4-inch NPT threads, depending on how you plan on adapting it to the grease pump hose. |  12. A second pressure gauge...  12. A second pressure gauge means you can use one to monitor the regulator pressure - setting it to 100 psi - and the other for monitoring the cylinder that you're testing. |
 13. Here's an example of the...  13. Here's an example of the ideal DIY leak-down tester: two gauges, a 100-psi regulator, and all of the proper air couplers and adapters to make it work. You could do a whole lot worse for less than $40. | | |