The problem symptoms due to poor idling are classified into two groups.
• The engine does not smoothly turn and the engine vibrates.
• The idling speed is abnormal (the speed is high, low, hunting). When dividing the procedures for troubleshooting for poor idling, pay attention to the following point.
• In the case of idling trouble, the troubleshooting method or the area of malfunction cause is greatly different depending on the malfunction occurrence condition. It is important to perform the complete malfunction occurrence condition confirmation and narrow down the malfunction whether rough idle or idling trouble.
1. Verify poor idling
Regarding poor idling, the cause of the malfunction differs depending on “rough idle” or “abnormal idling speed.” So, narrow down the cause of the malfunction by understanding the condition of the poor idling.
Rough idle
Rough idle is a symptom when the engine does not smoothly turn and vibration is occurring.
Abnormal idling speed
Abnormal idling speed is a symptom when the engine speed is not in the specified range.
Specific examples:
• The idling speed is high
• The idling speed is low
• Hunting
• The first idle is low
• Drop in the rotating speed when the load to the engine changes
etc.
• Even when the symptom is a rough idle, the malfunction sometimes may be caused by abnormal idling speed (idling speed is low).
• The engine vibration in the diesel engine is larger than that in the gasoline engine when idling if the technician cannot determine the cause, make a judge based on a comparison with another vehicle of the same model.
2. Conformity with the result of the DTC output
Even if the DTC indicates abnormality, the malfunction that the DTC indicates is different from the malfunction that the customer points out. Therefore, inspect the relationship between the DTC and the problem symptom.
• Normal DTC is displayed
The cause of the malfunction can be estimated in the area that has no DTC.
• DTC is displayed
Check the conformity between the output result of the DTC and the problem symptom.
3. Cylinder power balance inspection
Judge the malfunction as either “affects a specific cylinder” or “affects all the cylinders”.
Gasoline engine:
• When the malfunction affects only a specific cylinder, proceed to troubleshooting for the three engine elements in the cylinder.
• When the malfunction affects all the cylinders, proceed to the air fuel ratio.
Diesel engine:
• When the malfunction affects only a specific cylinder, narrow down the cause of the malfunction based on the difference whether the power in the cylinder is strong or not during the cylinder power balance inspection.
• When the malfunction affects all the cylinders, inspect the exhaust gas condition and narrow down the malfunction cause to either the three elements.
4. Inspect the gasoline engine’s three elements
When the malfunction affects only a specific cylinder, it can be considered to be in the one of the three elements in the engine. Narrow down the malfunction to either the ignition, fuel or compression element.
• Ignition system
When the ignition spark is weak or has no spark, the diagnosis code relating to the ignition signal or relevant area is not displayed. As a result, the malfunction is not judged to be in the ignition primary system but in the ignition secondary system.
• Fuel system
Judge whether the injector operates or not.
• Compression system
Measure the compressed pressure using a compression gauge.
5. Inspect the diesel engine’s three elements
When the malfunction affects a specific cylinder, narrow down the malfunction cause based on the power of the cylinder, that is, whether it indicates strong power or weak power, in the cylinder power balance.
(1) When one cylinder power is strong, narrow down the malfunction cause based on the exhaust smoke. Inspection methods
When the exhaust is normal
Combustion is considered to be normal, so inspect the fuel system to determine whether the amount of fuel injection from the injection nozzle is insufficient or nothing at all, etc.
• Fuel leak in the injection pipe, etc.
• Injection nozzle failure
• Injection pump failure
When the exhaust is white smoke
Misfi re is considered, so inspect the compression and glow systems that may possibly contain the cause of the abnormal combustion.
(2) When one cylinder power is strong, consider strong explosion pressure as a cause. An increase in the injection amount due to a fuel system malfunction is considered.
• Injection nozzle failure
• Injection pump failure
When the amount of fuel injection increases, the amount of black smoke increases under the conditions of sudden acceleration with no load and diesel knock may occur.