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Cummins Engine Idle Diagnosis and Treatment

Aug. 08, 2019

Engine no idle speed

The engine has no idle speed. Generally, the throttle is turned off when it is placed at the idle position. When the throttle is slightly enlarged, the speed is increased quickly and cannot be stably operated at a low speed.


a. The governor idle spring is too soft or broken.

b. The governor sensing element is worn too much.

c. The plunger of the fuel injection pump is seriously worn.

d. The temperature is too low.

e. The cylinder pressure is too low.

Diagnosis and treatment

a. There is no idle speed at cold start, which is generally a normal phenomenon. This is because the temperature is too low, the viscosity of the oil is too large, the internal resistance of the engine is increased, the spray and evaporation conditions of diesel oil become worse, resulting in the engine can not maintain the minimum stable speed operation, when the throttle is raised, it will turn off quickly. In this case, the throttle can be adjusted to run at a slightly higher idle speed, and then resume idle operation after the engine heats up.

b. If the engine has been used for a long time, there is no idling condition, and the fuel consumption is not normal due to insufficient power. It is generally the plunger wears too much, and the oil leakage increases at idle speed, so that the oil supply cannot meet the idle speed requirement; or the cylinder pressure is too low, the fuel injection advance angle is too large, too small, resulting in poor engine combustion conditions. As long as according to the insufficient power treatment, the idle condition will naturally recover.

c. If the above conditions are normal, the idle working element of the governor should be considered for abnormality. Check if the governor spring is broken or the governor is too worn or the spring is too soft, so that the centrifugal force of the flying hammer during idle operation is much greater than that of the spring tension and reduce oil, or the maintenance personnel do not adjust properly. The fuel injection pump assembly must be removed and the maintenance adjustments must be repeated on the test bench.

d. When checking the governor, first adjust it with the idle screw. If the adjustment is invalid, adjust the pre-tension force of the idle spring. The idle spring adjusting nut can be slightly screwed inward or a washer is placed on the spring seat to increase the preload and increase the idle speed. If the spring is broken, it should be replaced and then adjusted to restore the idle working performance.

Cummins generator

Idle speed is too high

The idle speed of the engine is too high, which shows that the engine speed is still higher than the specified value of idle speed when the throttle is lifted.


a. The throttle lever is not properly adjusted.

b. The throttle return spring is too soft.

c. The idle limit block or adjustment screw is out of adjustment.

d. The idle spring is too hard or the preload is too large.

Diagnosis and treatment

Excessive idle speed is one of the easiest faults to check and troubleshoot. First of all, check whether the throttle is back to the minimum position, if not, check the throttle adjustment and throttle return position. Adjust the throttle wire limit screw, if the throttle still cannot return, and then check that the throttle return spring is too soft. If it is the fuel injection pump shortly after inspection and commissioning, it should be considered whether the idle speed adjustment is correct, and the idle speed spring preload force adjustment is too large. If the spring has been replaced, check whether the spring is too hard.

Idle speed is unstable

The form of idle instability of the engine is that it is running at idle speed, fast and slow, or vibrated.


a. There is air in the oil circuit.

b. Low pressure oil supply is not smooth.

c. The idle speed stabilizer is improperly adjusted.

d. The fuel supply of the injection pump is uneven.

e. The pin shaft and fork head of each connecting rod of the governor are excessively worn.

Diagnosis and treatment

When the diagnosis of idle speed is unstable, it should be analyzed and judged according to the engine service time and maintenance degree.

a. First of all, it should be checked whether the oil supply of the low-pressure oil circuit is unblocked, whether the filling of the diesel oil meets the requirements, the maintenance of the generator engine is timely, otherwise it should be cleaned, maintained or replaced.

b. If the diesel generator stops for a long time or the fuel tank diesel oil is not replenished in time, a small amount of air seeps into the oil circuit and should be exhausted.

c. If the genset has been used for a long time, the fuel injection pump has been debugged many times without checking the wear of the governor. During the commissioning, pay attention to whether there is excessive wear at the joints of the speed control element and the throttle lever. Otherwise, it should be replaced or welded. When welding the rotating parts, attention should be paid to the symmetry of the mass to ensure balance.

d. Idling speed is unstable and accompanied by vibration. It is caused by the uneven oil supply of the fuel injection pump. It can be checked by the oil-by-cylinder method. If the broken cylinder does not cause a change in the rotational speed, it indicates that the cylinder oil supply is insufficient or the injector atomization is poor. Check the injector first and check the fuel injection pump.

e. If the idle speed stabilizer is improperly adjusted, it should be re-examined on the test bench.

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