Here are the steps to performing a safe and reliable Compression Test on a gasoline engine.
Step #1 - Warm the engine to normal operating temperature.
Step #2 - Disable the Fuel and Ignition System
Step #3 - Remove ALL spark Plugs
Step #4 - Hold the throttle plate wide open
Step #5 - Install compression tester and crank engine through five compression cycles
Step #6 - Record the "first puff" and final compression pressure reading for each cylinder
Step #7 - Analyze your test results
There are several hazards associated with compression testing.
CAUTION! Removing spark plugs from an engine at operating temperature can cause a technician to severely burn his or her hands or arms. Most often this is from touching a hot exhaust manifold. If you get burned, immediately run cool water over the burned skin. The sooner and longer you do this, the better. Clean your skin gently but thoroughly to hasten your healing. Special gloves are available that will cover your hands and arms if you must work close to hot exhaust manifolds.
CAUTION! Removing spark plugs from an engine at operating temperature can cause the aluminum threads to strip out. This can ruin the cylinder head. Do not remove any spark plug that continues to offer resistance after turning it more than one half a turn. If this happens allow the engine to fully cool down. If it is till difficult to remove the spark plug it is best to notify the customer that attempts to repair this vehicle may require removal of the cylinder head and will cost $$$$$$
CAUTION! Removing spark plugs from an engine can allow dirt into the cylinders. Dirt that gets into the spark plug hole can cause the cylinder walls to get scored or scratched and this will create poor compression in a cylinder that was working normally. To prevent dirt and contamination from getting into the cylinder blow around each spark plug with compressed air prior to removing the spark plug. If the engine is particularly dirty, pressure wash around the spark plugs before removing them. ALWAYS wear safety glasses and a face shield when using compressed air and/or a pressure washer. Do Not pressure wash the entire engine as it will damage electrical wiring and connectors.
CAUTION! Cranking the engine with spark plug wires disconnected can damage the ignition system. To protect the ignition coil from shorting out, disconnect the primary wires that go to the ignition coil before you perform a compression test.
Step #1 - Warm the engine to normal operating temperature. The simplest way to ensure normal operating temperature is to use an infra-red thermometer aimed at the thermostat housing. 180 - 200 degrees is about the normal coolant temperature for an engine that is warmed up. Normally the upper radiator hose will stay cool to the touch until the thermostat begins to open. It will quickly warm up after this. Feeling the upper hose can confirm the thermostat is open, however you should verify the temperature with an infrared thermometer. A second method to verifying operating temperature is to hook a scan tool up and watch the Engine Coolant Temperature (ECT) sensor PID value.
Upon occasion you will want to do a compression test on an engine that will not start. You can do the compression test on a cold engine however the compression pressures will be low when compared to specifications. A cold engine with very low compression (or no compression) in all cylinders may well have a broken timing belt or chain. A non-starting and cold engine with no or very low pressure in some cylinders, and higher compression in others will need major engine work.
Diagnosing cylinder to piston ring sealing using the "First Puff" or Wet Compression Testing is not valid on a cold engine.
Step #2 - Disable the Fuel and Ignition System. CAUTION! If the fuel injectors spray fuel while the spark plugs are removed there is the potential for a fire in the engine compartment. Also any fuel that dribbles into the cylinders can change the compression readings and will make for inconsistent and inaccurate readings.
You can disable the fuel system by turning off the fuel pump however there will still be pressure in the lines until the injectors have sprayed out this pressure. IF you use this method, be sure to run the engine after turning off the fuel pump and then cranking the engine for a few more seconds to let the pressure drop off.
A better method for disabling the fuel system is to turn off the injectors. This can be as simple as unplugging the injectors (especially easy on throttle body injection) or removing the appropriate fuse. I recommend trying to start the engine after removing any fuse to make sure you unplugged the correct one!
CAUTION! Failure to disable the ignition system will cause the ignition coil(s) to fire with an open circuit in the secondary ignition system. If you fail to disable the ignition, the coils will develop extremely high voltage while cranking. This can partially short out the coil and cause a future misfire that only occurs under high engine load. It can also cause carbon tracking at any point in the secondary ignition system. Carbon tracks can cause a misfire only under high engine load, or a misfire that occurs only under damp operating conditions.
There are many ways to disable the ignition. On some vehicles the same fuse will turn off both the ignition and fuel. Disconnecting crankshaft position sensors (CKP) and/or camshaft position sensors (CMP) will turn off the ignition and fuel. For all vehicles simply disconnecting the low voltage primary wires that lead to the coil or coil packs will turn off the ignition. To be absolutely sure, test the ignition using a spark tester while cranking the engine to ensure there is no high voltage sparking.
Step #3 - Remove ALL spark Plugs. This is either the easiest or the hardest step. It depends upon your engine. Be careful when working close to the exhaust manifold. On a warmed up engine it can easily be over 300 degrees and will instantly give you a second degree burn if any bare skin touches the exhaust.
Step #4 - Hold the throttle plate wide open. A closed throttle plate will limit how much air can get into the cylinders and cause low compression readings. You can easily use a brake pedal depressor (used for completing a wheel alignment) to hold open the throttle plate.
Steps #5 and #6 - You will need to install the compression tester in one cylinder at a time and record the test results. The key to getting the most information out of your compression test it to accurately record your results. Have a paper and pen ready to write down the results as you complete each cylinders compression test. You will clearly hear each time that cylinder comes up on the compression stroke.
Record the pressure reading for the "First Puff" which is the first compression cycle. This is useful for determining where the compression is leaking should it be low.
Crank the engine for five full "puffs" or compression cycles. Then record the final compression reading. Your gauge will hold the final or highest compression reading until you release it.
Record the first puff and final compression pressures for each cylinder. To be consistent be sure to crank each cylinder for the same number of compression strokes!
Pay attention to the battery voltage as you complete this test. If the battery is weak, the final cylinder will be cranking slower than the first cylinder tested. If you neglect battery voltage you may be fooled as slow cranking speeds cause low compression. You may think you have a weak cylinder when the cause is a weak battery.
If your final one or two cylinders are lower than the first one or two cylinders, repeat the test on the very first cylinder. If that cylinder is now low, the cause is low cranking speed and not a weak cylinder.
Battery voltage should always stay above 10 volts during cranking. You may wish to use a battery charger during your testing however low amp chargers can be damaged if left turned on while cranking the engine. High amp chargers can raise the battery voltage too high and potentially hurt the vehicle computers! Another option is to use a booster battery during your test.
Step #7 - Analyze your test results. Your cylinders should be close to the specified compression pressures specified by the manufacturer. More important is even compression between cylinders. If the lowest cylinder is more than 20% less than the highest cylinder it can cause the engine to run rough.
The First Puff should be at least 50% of the fifth puff on that cylinder. If the first puff is well over 50% of the final reading, but the final reading is low, that cylinder is likely leaking past the valves.
If two adjacent cylinders have low compression it is often caused by a head gasket that is leaking between these two cylinders. If there is a very low first puff, and it gradually builds to an overall low pressure, that cylinder likely has bad rings or a worn cylinder wall. You can confirm this with a Wet Compression Test.
Wet Compression Testing is done only when a cylinder(s) read unacceptably low. To do a wet compression test use a small oil squirt can and put one or two squirts of oil in the cylinder. Then repeat the compression test. The oil will help seal around the pistons and cylinder walls, but not around the valves. On a normal cylinder, a wet test will read slightly higher compression than when tested without the oil. If the pressure goes up markedly after squirting in the oil you know that the rings and or cylinders are worn out. If there is only a slight change, the compression leak is probably in the valves. A Cylinder leakage test can be done to help identify where the compression is leaking from.
One final note on the wet test. It only works when the oil gets spread over the top of the piston. A wet compression test is not valid on a horizontal cylinder as the oil will have a hard time spreading all around the piston. These type of engines are often called "Boxer" or Flat or Horizontally Opposed engines. Subaru is the most common automobile manufacturer to use this style engine.
Compression pressures higher than specified are likely caused by carbon build-up inside the combustion chamber. It is a common maintenance procedure to run a Motorvac type of injector-intake cleaning procedure. This can be very effective at removing carbon buildup on valves, intakes and the combustion chamber.
A simpler procedure that is effective at removing carbon from the cylinder is to mix water and top engine cleaner at a 50-50 ratio. Warm the engine to operating temperature. Connected a SMALL diameter vacuum hose to manifold vacuum and place the other end in your mix of water and top engine cleaner. Run the engine at about 2,000 RPM and let it slowly suck in this liquid. (If you allow too much liquid in it can really damage the engine!) The water will "steam clean" the engine while the top engine cleaner helps knock loose the excessive carbon. This procedure will create quite a bit of smoke out the exhaust!
Carefully re-install the spark plugs.
Install a VERY SMALL amount of anti-seize to the spark plug threads.
Hand tighten each spark plug into the cylinder head. A piece of vacuum hose can make it easier to start the spark plug into the cylinder.
NOTE: DO NOT force a spark plug into the cylinder head.
You should be able to screw the plug almost all the way into the cylinder head without using a ratchet.
Torque the spark plugs according to the manufacturer specification. (As you are using anti-seize, torque to the lowest value.)
After completion, run the engine to ensure there is no misfire. (Misfires after replacing spark plugs are often caused by mixing up spark plug wires, grease on the spark plug gap, or cracking the ceramic insulation of a spark plug)
Graphical version of Phil Krolick's Website: cf.linnbenton.edu/eit/auto/krolicp/