Four engines shake at idle for four different reasons. Compare what happens when electrical/A/C load is applied or engine speed increases. Test before you replace.
Find the cause of an idle complaint, not just a PID outside the usual range. Observe three controlled conditions, perform at least two distinct tests including the discriminating tests, select the supported cause and next action, and explain why a look-alike cause is less consistent with the evidence.
Teaching simulation only. All values and responses are invented for a generic, four-cylinder port-injected gasoline engine with electronic throttle control. Fuel-trim values are illustrative closed-loop snapshots, not universal limits; OEM idle-control strategies and specifications differ. The A/C load and raised-RPM conditions are simulated, not instructions to perform unsafe tests.
The light dashed reference line shows the selected condition’s commanded speed. The orange line shows an illustrative actual-speed trace, not a scan-tool recording.
| Condition | Command | Actual mean | STFT tendency | Cylinder 2 misfire count | Throttle actual |
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Progress lasts only while this page stays open. This simulation does not issue grades or service authorization.