Engine Running Rough at Idle

For the engine rough at the idle phenomenon, as a technician, we must identify and fix several systems to find the cause of the rough engine. We'll go over some of the systems that are best suited to identify the cause and provide a solution to the problem.

The cause of rough idle can be related to cracked engine mounts, the mixture being too rich or lean, induction air leak, plugged injector nozzles, uneven cylinder compression, faulty ignition system, fuel pressure too low, and lead deposits on spark plugs (Crane, 2018). First thing to do to solve the problem is checking the engine mount, check if the mixture is rich or too lean, and if so, adjust the idle mixture. The engine will always run at its best and smoothest when the mixture is rich, however it will run rough in the lean mixture. Different cylinders will operate at different air-fuel ratios, so engine roughness occurs when the mixture is maldistribution (Mike Busch. 2013). Then check for induced air leaks, and check gaskets, intake manifold, and bolts. If any leaks are found, replace or repair accordingly, any replacement or repair must be in accordance with the manufacturer's manual.

One of the most common causes of this symptom can be misfire and clogged fuel injectors. The cause of false ignition here could be a dirty/damaged spark plug or a broken ignition wire. Check the plug circuit to see if any plugs, high voltage conductors, or towers in the distribution block appear to be shorted or open. If plugged nozzle, clean with Methyl-Ethyl-Ketone, acetone, hydro-carbon cleaning solvent, or a chlorinated solvent. The large plug gap is also a cause, clean the gap then check all plugs. Test the ignition high-tension lead harness with a high-tension lead tester. Check the distributor block for cleanliness, tracking, and cracks (Thomas Ehresman. 1998). When the fuel injector is completely clogged, it will lead to a rough engine, EGT cold, and CHT dropping. If a clogged fuel injector is detected, release it by switching to a full-rich mixture and high boost (Mike Busch. 2013).

One of the causes of engine roughness that rarely happens is that the internal timing of the magnet is not correct, and the magnet or coil is weak. Check the E-gap. The magnet should be replaced if any internal problems are detected, remove the magneto, and fix the problem on the test bench (Thomas Ehresman. 1998).

When troubleshooting aircraft, technicians need to pay attention to check and read engine instrument. Pay attention to the differential pressure gauge, which represents the difference in pounds per square inch and indicates the difference between the two pressures. Next is the fuel pressure gauge, it is very important because we all know that the fuel pump pushes the fuel at a certain pressure so that the fuel can reach the engine. Finally, there is the tachometer, which displays the number of revolutions per minute of the engine. Monitor the tachometer to see if the engine is working properly with RPM while the technician is in the process of troubleshooting.

Troubleshooting is an important task for any mechanic, to be able to do it easily depends on the mechanic's experience and understanding of the engine. If we understand all the parts and components of the engine, the problem will be narrower and resolved faster. However, if we are not sure, it is necessary to spend more time on troubleshooting. Following the manufacturer's manual will contribute a great deal to finding the problem.

 

References

Crane, D. (2018). Aviation Maintenance Technician: Powerplant (J. Foulk & D. Scroggins,

Eds.). Aviation Supplies & Academics, Inc.

Mike Busch. 2013. Understanding CHT and EGT.

https://resources.savvyaviation.com/understanding-cht-and-egt-2/                    

Mike Busch. 2013. Rough Engine.

https://resources.savvyaviation.com/wp-content/uploads/articles_eaa/EAA_2013-11_rough-engine.pdf

Thomas Ehresman. March 1998. Piston engine troubleshooting. Aviationpros.

https://www.aviationpros.com/home/article/10389142/piston-engine-troubleshooting

 

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