Cylinder Drop Test

The cylinder drop test is performed to determine which cylinder is misfiring. When diagnosing the ignition system this test will help in determining which cylinder is not receiving adequate spark.

WARNING
Do not disconnect any spark plug (secondary) wires when the engine is running. The high voltage created in the ignition coil will damage the module that controls the ignition coil(s).

Cylinder Drop Test Set-Up

To perform a cylinder drop test, a piece of vacuum hose approximately one inch in length for each cylinder, a test light is and a tachometer is required. Having a notepad to record your observations is also helpful. If a tachometer is not used observe and record how the engine is running before shorting cylinders. Perform the cylinder drop test in a logical order. Follow the firing order or systematically work beginning with cylinder number one.

The vacuum hose will be installed between the distributor plug wire tower and the spark plug wire. When the vacuum hose is cut to fit be sure that it is long enough for the test light to touch the vacuum hose and fit between the wire and the distributor cap. If the hose is too long it can affect how the spark plug fires. Working one cylinder at a time:

1. Disconnect the spark plug wire from the distributor cap.
2. Insert the short length of vacuum hose into the boot of the spark plug wire.
3. Seat the plug wire with the vacuum hose on to the tower distributor cap.
4. Repeat this for each coil or distributor tower
5. Connect the tachometer to the last cylinder to be tested.
6. Connect the clip of the test light to a known good ground away from the battery.
7. Choose a location such as the engine block or intake manifold.

   CAUTION
   Do not connect the test light clip directly to the battery. Do not allow ignition the high voltage developed in the ignition systems to flow directly into the battery. Secondary voltage is significantly higher than the voltage of the battery. The battery can be damaged or even explode.

   CAUTION
   The cylinder drop test is performed while the engine is running. Follow all safety precautions and ensure that all test equipment is clear of any moving parts underhood.

8. Place the transaxle/transmission in P (PARK), engage the emergency brake, and chock the rear wheels to prevent vehicle movement.
9. Start the engine and let it idle.

Cylinder Drop Test

WARNING
Do not make contact with any metal parts of the car when performing a cylinder drop test. The voltage created in the secondary ignition system is very high (thousands of volts) and you can be shocked by it.

1. Take note of the engine speed displayed on the tachometer or the condition of how the engine is running.
2. With the engine running and while you are not touching any metal parts of the car touch the probe end of the test light to the vacuum hose.
3. Record the engine speed value displayed on the tachometer
4. Engine speed should decrease
5. The engine should sputter, run worse and may stall
6. If the engine stalls remove the test light from the vacuum hose and re-start the engine.
7. Repeat this for each cylinder and record the engine speed or how the engine performs.

NOTE: Before testing the cylinder that has the tachometer connected to it relocate the tachometer lead to another cylinder.

Cylinder Drop Test Analysis

1. Subtract the engine speed value for each cylinder from the engine speed value recorded before the cylinders were shorted. The difference is the drop in engine speed for each cylinder.
2. As an example, if engine speed at idle at the beginning of the test is 650 RPM and the shorted engine speed is 575 RPM, the difference is 75 RPM
3. Compare all of the calculated engine speed drops. The cylinders that have higher engine speed drop values are contributing more that those with lower values.
4. If a tachometer is not available and you have recorded how the engine performs when the cylinders were shorted, the cylinder(s) that had the least effect on idle quality is/are the cylinder(s) that are suspect
5. Inspect the spark plug wire, spark plug, for each affected cylinder and perform component diagnostics as required.