The purpose of the cylinder balance test is to assist the mechanic in finding a weak or non-contributing cylinder. The test is entered by depressing and releasing the throttle within two minutes after the Engine Running Self-Test DTC's have been output.
Once the test is entered, the IAC duty cycle is fixed and the engine is allowed to stabilize. Engine rpm is measured and stored for later use. Next, the fuel is shut off to cylinder number 4, 6 or 8, depending on engine. After a brief stabilization period the engine rpm is again measured and stored. The injector is turned on again and the process is repeated for each of the injectors down to one. At this point, the maximum rpm drop that occurred is selected from the table of rpm drops for each cylinder. This maximum rpm drop is now multiplied by a calibratable percentage. The resulting number (rpm) is now used as the minimum rpm that each cylinder must have dropped to pass this 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. No further testing is necessary. If a cylinder did not drop at least this amount, then the cylinder number would be output. For example, 30 for cylinder number 3. This indicates that cylinder number 3 is either 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 to pass this test.
Example: 150 rpm x 43% = 65 rpm
If all 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 number 3 is considered to be weak. If cylinder number 3 again failed, the code 30 would be output again.
The 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 the rpm drops are greater than 30 rpm then a code 90 is output. If cylinder number 3 had failed the first and second level, but passed the third, then it is considered to be a very weak cylinder. If cylinder number 3 failed the third level then a code 30 would again be output. In this case, cylinder number three would be considered a non-contributing cylinder.
The Cylinder Balance DTM may still be repeated as many times as desired by depressing and releasing the throttle within two minutes of the last code output. All further testing (i.e. 4th, 5th pass) will be done using the third level percentage.
Trying to get smooth idle forever, 92 stock SC, changed head gasket, no smooth idle since.