Diagnose Rough Idle (Only)
Start the engine and monitor the EOT - the EOT is to be maintained within -15 °C (5 °F) during the test. The test must be performed while rough idle is occurring. Record the MFDES reading before interrupting any injectors; this will be the baseline for the rest of the test. The MFDES reading will not steady and will fluctuate rapidly. Take a five second snapshot of the PID information and record the high and low MFDES readings on the Injector Interrupt Tool Enhanced diagnostics sheet, using only MFDES numbers that have appeared at lease two times during the recording. After recording a baseline, interrupt each injector individually and repeat the above steps to obtain the high and low readings while each injector is interrupted.
These numbers should be recorded on the Injector Interrupt Tool Enhanced diagnostic sheet.
The enhanced diagnostic sheet is intended to be photocopied and then completed for each vehicle that is tested. A completed performance diagnostic sheet and enhanced diagnostic sheet must accompany any returned warranty parts and cores.
The following example illustrates the use of a scan tool to monitor the MFDES, as described in the paragraph above.
| Injector Off | MFDES | Average MFDES | Deviation | ||
|---|---|---|---|---|---|
| Baseline = 0 | 10.1-10.1 mg | 10.1 | - | ||
| 1 | 10.7-10.9 mg | 10.8 | 0.7 | ||
| 2 | 10.6-10.9 mg | 10.9 | 0.7 | ||
| 3 | 10.5-11.0 mg | 10.8 | 0.7 | ||
| 4 | 9.8 -10.5 mg (1) | 10.2 | 0.1 | ||
| 5 | 10.5-10.9 mg | 10.7 | 0.6 | ||
| 6 | 10.2-10.6 mg (1) | 10.4 | 0.3 | ||
| 7 | 10.4-11.0 mg | 10.7 | 0.6 | ||
| 8 | 10.6-10.8 mg | 10.7 | 0.6 | ||
EOT = 176°F mg = milligrams |
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Complete the enhanced diagnostic graph to illustrate the difference between cylinders and help identify weak cylinders/injectors. To start this process, derive the average for each of the MFDES readings by adding the high and low numbers together and dividing by two. For example, cylinder No. 5 has a reading of 10.5-10.9. Therefore, the equation would be: 10.5 + 10.9 = 21.4 divided by 2 = 10.7 (if needed, round the number up to the nearest tenth). Record this number on the enhanced diagnostic sheet for the appropriate cylinder. Now, subtract his number from the average MFDES recorded for the baseline. For example, cylinder No. 5 has an average MFDES of 10.7 and if the baseline MFDES was 10.1, the equation would be: 10.7-10.1 = 0.6. Therefore, 0.6 would be the deviation from the baseline. Perform these calculations for all cylinder readings.
In order to get the cutoff number, average all of the deviations. For example, as in the sample, we would add the following numbers: 0.7 + 0.7 + 0.7 + 0.1 + 0.6 +0.3 +0.6 + 0.6 = 4.3. Divide the sum by eight and round to the nearest tenth: 4.3 + 8 = 0.5375 ~ 0.5. Therefore, 0.5 would be the cutoff line for weak cylinders/injectors. By using this test, cylinders No. 4 and 6 have been identified as being weak.
Perform a relative compression test using a scan tool or check compression with a mechanical gauge on the weak cylinders and one or two of the strong contributing cylinders. Compare the weak cylinder readings against the strong contributing cylinders to verify that the power cylinder is mechanically sound. If the compression readings are within 20% of each other, the power cylinders can be considered good.
After injector replacement, test drive the vehicle for at least 20 miles and confirm that the rough idle concern has been corrected, do not retest.
If a rough idle persists, the test should be performed again. After all tests are run, all DTCs should be cleared before returning the vehicle to the customer.