Engine Compression Testing
By Larry Carley c2006

HOW TO CHECK COMPRESSION

Compression can be checked two ways: manually with a compression gauge, or electronically with an engine analyzer the measures cranking compression. With electronic testing, a computer analyzer estimates compression in each of the engine's cylinders by measuring slight variations in engine cranking speed.

The results correlate well with actual gauge readings, and can be completed in a matter of minutes without having to remove any spark plugs. What's more, the analyzer prints out the results of the compression test making it easy to see and compare the actual numbers.

To check compression manually with a gauge, all the spark plugs must be removed. The ignition coil must then disabled or the high tension lead grounded. If the engine has a distributorless ignition, the ignition coils must be disabled to prevent them from firing. The throttle must also be held open.

The engine is then cranked for a few seconds using a remote starter switch or a helper while a compression gauge is held in a spark plug hole.

The maximum compression reading is noted, then the process is repeated for each of the remaining cylinders.

The individual cylinder readings are then compared to see if the results are within specifications (always refer to a manual for the exact compression figures for your engine because they do vary)

IS IT THE RINGS OR THE VALVES?

If compression is low in one or more cylinders, you can isolate the problem to the valves or rings by squirting a little 30 weight motor oil into the cylinder through the spark plug hole and repeating the compression test. The oil temporarily seals the rings.

If the compression readings are higher the second time around, it means the rings and/or cylinder is worn. No change in the compression readings would tell you the cylinder has a bad valve.

Compression testers can be purchased at most auto parts and hardware stores.

A leakdown or cylinder leakage test is similar to a compression test in that it tells you how well your engine's cylinders are sealing. But instead of measuring pressure, it measures pressure loss.

A leak down test requires the removal of all the spark plugs. The crankshaft is then turned so that each piston is at top dead center (both valves closed) when each cylinder is tested. Most people start with cylinder number one and follow the engine's firing order.

A threaded coupling attached to a leakage gauge is screwed into a spark plug hole. Compressed air (80 to 90 psi) is then fed into the cylinder.

An engine is great condition should generally show only 5 to 10% leakage. An engine that's still in pretty good condition may show up to 20% leakage. But more than 30% leakage indicates trouble.

The neat thing about a leakage test (as opposed to a compression test) is that it's faster and easier to figure out where the pressure is going. If you hear air coming out of the tailpipe, it indicates a leaky exhaust valve. Air coming out of the throttle body or carburetor would point to a leaky intake valve. Air coming out of the breather vent or PCV valve fitting would tell you the rings and/or cylinders are worn.

A leakage test can also be used in conjunction with a compression test to diagnose other kinds of problems.

A cylinder that has poor compression but minimal leakage usually has a valvetrain problem such as a worn cam lobe, broken valve spring, collapsed lifter, bent push rod, etc.

If all the cylinders have low compression but show minimal leakage, the most likely cause is incorrect valve timing. The timing belt or chain may be off a notch or two.

If compression is good and leakage is minimal, but a cylinder is misfiring or shows up weak in a power balance test, it indicates a fuel delivery (bad injector) or ignition problem (fouled spark plug or bad plug wire).

Leakdown testers are a little harder to find for some reason. I got mine from aperaceparts.com for about $80

For more see http://www.aa1car.com/carleyware/library.htm

• jcwhitney.com - Compression Tester with 10mm, 12mm and 14mm adapters
• aperaceparts.com - $80 Leakdown Tester. You'll need adapters.
• eastwoodco.com - $100 Leakdown Tester. You'll need adapters for this as well.
• eastwoodco.com - $45 single gauge leak down tester.