Compression and leakdown testing

Diagnosing engine condition without disassembly

European Car magazine
February 1992
Photography by the author

You may have heard us say that the internal combustion engine is a type of air pump; it pumps the air (and fuel) in on the intake stroke, and pumps it back out again (as exhaust) at the end of the combustion cycle. The more efficiently the engine performs this pumping action, the more power it will generate. Camshaft timing and profile, compression ratio, ignition timing, and other factors contribute to making the engine more efficient. Friction and pumping losses are among the factors that reduce efficiency.

Another factor that can play an important role in engine efficiency is internal leakage, which is directly related to the condition of the engine’s internal sealing surfaces. One of the first things most people think of when confronted with an engine that may or may not be in good condition is to grab a compression tester and see if all the cylinders pump up. For a quick-and-dirty indication about the goings-on inside an engine, a compression test is not bad. Pull out the spark plugs, disconnect the ignition, open the throttle butterfly, and crank ’er up.

As illuminating as a compression test can be, it is far from comprehensive. It is best used on an engine with which you are familiar, and upon which you have performed compression tests in the past, so you can detect changes in engine condition from reading to reading.

With experience, you can learn more from a compression test if you pay attention to more than the final value. But for even more insight, there is another related test that is more revealing: the leakdown test.

This is not meant to denigrate the compression test. As comprehensive as a leakdown test can be, it is also much more time consuming than a compression test. It does not take too much extra effort to perform a compression test at every major service (usually 15,000 miles, when you have the spark plugs out anyway), while performing a leakdown test that often can be considered above and beyond the call of duty.

So instead of discarding the compression test, let us review its pros, cons, and techniques.

Compression testing

If the battery is okay and the engine spins up to normal cranking speed, a compression test will give you a nice set of numbers that, with any luck, will correspond to the engine condition. But compression tests can still show a weak cylinder as being okay — if the compression leak is slow enough that the engine still generates a certain amount of compression on the compression stroke. Some slow leaks can be spotted with a compression tester (and a little experience in using it), but not all. And if the previous owner slapped in a hot camshaft with a lot of overlap, a compression test may lead you to reject an otherwise good engine for the wrong reason.

Now what if the battery is a little weak and it won’t spin the engine to normal cranking speed? A compression test will show low pressures across the board, even on an engine that is in good shape. Of course, if the engine is not running at all (as it won’t if the battery is dead, or if you are dealing with a long block of unknown quality), a compression test is out of the question.

If a compression test is possible, there are a couple of things you can do to maximize the amount of information you get. For example, after you have readied the engine for the test (plugs out, coil shorted, throttle open) and have inserted the compression gauge into a cylinder, listen to the engine as it spins over. It should spin freely until it gets to the compression stroke for the cylinder into which you have inserted the gauge. If the starter motor does not seem to be encountering much resistance on the compression stroke for any cylinder, suspect a dead cylinder.

It is also extremely helpful to be able to observe the gauge while the engine is cranking over, rather than simply counting seven or eight compression strokes and then checking to see what you got. If you have a friend who can turn the ignition key while you watch, you are set. If not, you should consider purchasing a remote starter button, which will allow you to crank the engine without having to sit behind the wheel. These are widely available, but if you can not locate one at your neighborhood parts emporium, Snap-On sells one under the part number MT-302A.

Finally, you must have a good seal between the compression tester and the spark plug opening. If you are using a compression gauge with a screw-in end, there should not be a problem. If you are using a compression gauge with a forcing cone, listen for the sound of escaping air on the compression stroke. The more air that escapes, the less accurate your reading will be.

Once you get all set, watch how the needle on the compression gauge reacts on each compression stroke, and record the final compression reading. To decode your findings, refer to Table 1. (If you do not have a compression gauge and cannot obtain one locally, Snap-On sells one under part number MT-337A).

Table 1
Gauge reading Probable cause Indicated condition
First pulse 90psi; subsequent pulses build to normal reading Normal Good
First pulse low; subsequent pulses build to normal reading Slow compression leak May start hard, but should run near-normal
First pulse low; subsequent pulses build to low reading Large compression leak May not start. Will run poorly
First pulse medium; subsequent pulses add a little; final reading less than normal Excess camshaft overlap May start hard, but should run okay with dynamic compression
Pulses read low, medium, or high, but fall between strokes Stuck or leaking check-valve on compression tester Clean or replace check valve
No reading, even though engine seems to have compression Broken compression tester Run to store for new gauge

Leakdown testing

Even the most expensive compression gauge will do you little good on a long block, or on an engine that can be turned over only manually. Where compression gauges depend on the starter motor being able to provide air pressure to the cylinder, leakdown gauges work by pressurizing the cylinder (with the engine stopped at TDC) and measuring how much air escapes.

To perform a leakdown test, you need a leakdown gauge (such as the Snap-On SVT 262) and an compressor capable of delivering 90 to 100 psi of pressurized air. Remove all the spark plugs, open the throttle butterfly (as you would for a compression test), and remove the radiator cap, distributor cap, and oil fill cap.

Bring the engine to TDC for any cylinder. If you are checking a car with a manual transmission and have a large, level surface to work on, you may find it easier to put the car in top gear and push it back and forth to turn the motor over. Otherwise the best bet is to use a wrench on the end of the crankshaft. If you have trouble locating TDC, check the position of the distributor rotor. You should be able to get pretty close by lining up the tip of the rotor with the spark plug lead posts on the distributor cap.

Most leakdown testers have some method of zeroing the gauge to compensate for variances in the pressurized air supply. After establishing a baseline, attach the leakdown tester and carefully connect it to the air supply. I stress the word carefully because if the engine is not at TDC the force of the air pushing against the top of the piston is going to rotate the engine — sometimes violently. If you have the car in gear, set the parking brake before connecting the gauge to the air supply. If you are dealing with an uninstalled engine, use a flywheel lock to hold the crankshaft in position.

When you get everything connected up, the gauge will show a certain percentage of leakage. Some leakage past the rings is normal. What you are looking for is excessive leakage.

Table 2
Percent of leakage Engine condition
2 – 10% Okay
10 – 20% Marginal
20% and higher No good

If you find a cylinder with high leakage, the first thing to do is to turn the crankshaft slightly to see if the rings are properly settled. When bringing the motor up to TDC, you should always turn the engine in the direction of rotation, rather than turning past TDC and then backing up a little. When turning the engine while the air supply is connected, be very cautious: the crankshaft may rotate suddenly and try to take the wrench right out of your hands. As you turn the crankshaft, watch the gauge and note the lowest percentage of leakdown you see. This low reading is usually more correct than the higher reading you saw before.

Leakage readings of up to ten percent indicate the cylinder is fine. Readings between ten and 20% indicate trouble may be on the way. Readings over 20% generally mean it is time for the engine to receive some mechanical attention.

If you do get a high reading, leave the gauge connected while you listen at various points in the engine for the sound of escaping air. The location of the escaping air will indicate the area of concern. For example, if the sound is loudest through the intake manifold, the intake valve is bent, burnt, or poorly seated. Air escaping through the exhaust system indicates similar problems with the exhaust valve. Air bubbles in the coolant mean the head gasket is allowing pressure to escape into the water jacket (you did take off the radiator cap, didn’t you?). Air leaking into adjacent cylinders show head gasket problems as well, although not all engine designs share this characteristic. On air-cooled engines, for example, you should listen for air escaping between the cylinder head and the top of the cylinder.

One place where you will always hear the sound of rushing air is at the oil fill or at the crankcase breather. This is due to the normal leakage past the rings. However, by comparing the sound among cylinders, and by comparing the sound of ring leakage to other types of leakage, you will soon get an idea about what is normal and what is not.

Keep in mind that there are a lot of production-line motors out there that will show leakdown percentages of between ten and 15% when new. This is usually due to deficiencies in the strength of the block, which allows the cylinder walls to deform. If your readings are all in the low teens, continue checking the engine periodically (using the same leakdown gauge). If the leakdown percentages are stabilized, it is possible that the motor is doing the best it can under the circumstances. Of course, any change for the worse is a warning to start planning to make repairs.

A cylinder leakdown test will not tell you everything you need to know about an engine, but it is a valuable technique, and for testing an engine before installation it can save you a lot of time by showing problems that would otherwise remain hidden until after you hit the starter the first time.