Why Is Cummins Engine Idle Speed Too High Or Unstable

by Generator D. Diesel Generator

Today Starlight Power shares the reasons of Cummins engine idle speed too high or unstable. Hope the article is helpful to you.

Idle speed is too high

The idle speed of Cummins 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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About Generator D. Senior   Diesel Generator

101 connections, 1 recommendations, 632 honor points.
Joined APSense since, September 11th, 2017, From Jiangsu, China.

Created on Nov 4th 2020 23:57. Viewed 67 times.


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