Cylinder Balance Diagnostic Test Mode (SEFI Only) Description

PURPOSE
The cylinder balance test is used to aid the technician in finding a weak cylinder. Upon initiating the test the Powertrain Control Module (PCM) automatically performs the test and records the results as output codes.

HOW TO INITIATE
The test is entered by depressing and releasing the throttle within two minutes after the Key On Engine Running (KOER) self test codes have been outputted. Refer to Cylinder Balance Test. Cylinder Balance Test


OPERATION
Once the test is entered, the idle speed control duty is fixed and the engine is allowed to stabilize. The engine speed is measured and stored for comparison with the other readings received later in the testing.

Then the fuel is shut OFF to the last cylinder in the engine. After the engine speed stabilizes, the rpm is measured and stored. The injector is turned ON and the process is repeated for each cylinder.

OBTAINING RESULTS
At this point, the maximum rpm drop is selected from the table of drops for each cylinder. This maximum drop is now multiplied by a calibrated precentage. The resulting number (rpm) is now used as the minimum rpm that each cylinder must have dropped to pass the test. (Example: 150 rpm x 65% = 98 rpm).

If all cylinders drop at least this amount, then a code 90 is output indicating a pass (Analog Volt/Ohm Meter (VOM) or Digital Volt/Ohm Meter (DVOM) still connected). No further testing is necessary. If a cylinder did not drop at least this amount, then the cylinder number will be output. (Example: 30 is output for cylinder #3). This indicates that cylinder 3 is weak or non-contributing.

The test can now be repeated a second time if the throttle is depressed and released within two minutes of the last code output. This time the maximum rpm drop that occurs is multiplied by a lower percentage. This number is now used as the minimum rpm drop for each cylinder. (Example: 150 rpm x 43% = 65 rpm).

If all of the rpm drops are greater than 65 rpm, then a code 90 is output. If cylinder number 3 had failed the first level and passed the second, then cylinder 3 is considered to be weak. If cylinder 3 fails again, the code 30 will be output again.

This test can be repeated a third time by depressing and releasing the throttle within two minutes of the last code output. This time the maximum rpm drop that results is multiplied by a still lower percentage. This number is now used as the minimum rpm drop for each cylinder to pass this test. (Example: 150 rpm x 20% = 30 rpm).

If all of the rpm drops are greater than 30 rpm, then a code 90 is output. If cylinder number 3 had failed the previous two levels, but passed the third, then it is considered to be a very weak cylinder. If cylinder 3 again fails the test, then a code 30 is again output. In this case, cylinder 3 is considered a non-contributing cylinder.

The cylinder balance test may be repeated as many times as desired by depressing and releasing the throttle within two minutes of the last code output. All further testing will be done using the third percentage level.