Failure Diagnosis And Repair of Ignition System

In general, there are many symptoms caused by ignition system failure. The typical ones are hard starting, unstable working state, weak power, abnormal noise, backfire, malfunction of an individual cylinder, stall at the low engine speed and high speed, stall during acceleration, bad idling etc. In daily driving, we should stay alert if the engine can't run smoothly.

1. Test ignition system failure

1.1 Central high voltage line

1.2 Spark plug

2. Failure diagnosis and repair (Distributorless)

2.1 Engine misfire

2.2 Engine doesn't work (normal sparks)

2.3 Engine doesn't work (no sparks)

1. Test ignition system failure

The automotive ignition system always has problems. But the ignition system is relatively independent, so when we check the circuit, the function of the circuit can be used to determine the location of the failure.

When the car engine can not start, and we have confirmed that the starting system works without problems. The regular operation engine suddenly stops working, and the spark plug doesn't work as usual. The symptoms are likely to be caused by an abnormal ignition system.

In this article, we will introduce two methods to check whether the vehicle's ignition system is faulty when the engine can't start.

1.1 Central high voltage line

Here is a testing method that applies to ignition systems with distributors. Let's take a look.

Pull out the central high-voltage line from the distributor cap. Keep it 5-9mm away from the cylinder block after starting the engine.

Observe sparks at the end of the line. If there are no sparks, the ignition system is faulty. Check the sensor, ignitor and ignition coil. If necessary, put repair and adjustment on the agenda.

1.2 Spark plug

This testing method applies to the ignition system without a distributor.

Here are the specific steps. Remove the ignition coil. Separate the injector plug (to prevent fuel injection during inspection). Use the spark plug sleeve to remove the spark plug. Install the spark plug to the ignition coil. Latch the spark plug to the engine.

2. Failure diagnosis and repair (Distributorless)

The distributorless ignition system (is also known as the direct ignition system) replaces the distributor assembly with the electronic control device. It relies on computer control.

This all-electronic ignition system has no distributor, no mechanical wear, no adjustment. The ignition voltage is high. The distributorless ignition system is an ideal type. Here we are to describe the common troubleshooting skills of this ignition system.

2.1 Engine misfire

The failure of the engine misfire usually refers to the symptoms that the air-fuel mixture can't be burned in the cylinder due to various reasons during the engine operation.

The failures of the multi-cylinder engine misfire are dangerous to the vehicle and the environment. They should be promptly eliminated. There are many causes of engine misfire, and the specific causes and parts that need to be checked are as follows.

●  Fuel system

The main reason is that the combustible air-fuel mixture is too rich or too thin, which causes insufficient combustion in the cylinder. We can check the idle pressure to judge.

Check whether the air filter is clogged, whether the air intake pipe is leaking, whether the fuel line is clear, whether the oil pressure is too high or too low, whether the injector fails.

Test the oil pressure with the unique oil pressure gauge and pipe fittings. The pressure at idle speed should meet the technical requirements of the vehicle. The value is usually 0.25MPa.

The oil pressure should rise to 0.3MPa when the vacuum hose is pulled off. Otherwise, the oil pressure regulator should be replaced.

High Quality Fuel System Parts

●  Air distribution mechanism

Poor intake of the combustible air-fuel mixture or the incomplete discharged exhaust gas will cause the normal combustion in the cylinder.

In this situation, you can check whether the valve clearance is appropriately adjusted and whether the camshaft is deformed.

●  Poor cylinder sealing

On the one hand, the poor cylinder sealing causes the decreased compression process and the combustible mixture of temperature and pressure. This affects the ignition performance of the combustible mixture.

On the other hand, poor cylinder sealing causes the reduction of the combustible mixture, which reduces the cylinder pressure in the expansion process.

Check whether the seal between the piston, piston ring and cylinder wall is good, whether the seal between the valve and valve seat is good, whether the cylinder gasket is damaged.

●  Ignition system

The lack of sparks or improper ignition moment leads to abnormal combustion in the cylinder.

We should check whether the spark plug, ignition cable, ignition coil primary winding, electronic ignition components, and the electronic controller's relevant parts have problems. Check if the corresponding ignition signal control circuit is poorly connected.

●  No sparks

In the sparks inspection, if only to determine whether the work of an individual cylinder is good (that is, to determine which cylinder has no sparks), you can artificially stop the cylinder injector work. You judge whether it is a problem in terms of the cylinder in the stop injector before and after the change in engine speed.

Stopping the injector work can only determine whether the cylinder has sparks. To determine whether the ignition system causes it, you'd better use the ignition cable to test the cylinder.

High Quality Ignition System Parts

The specific method is to make the engine run steadily at low speed. Unplug the cylinder spark plug on the ignition cable. Keep it 5-9mm away from the cylinder block to the sparking test. 

Determine whether the ignition system is normal according to the ignition cable sparks, the test before and after the engine speed change, and the change's magnitude (use tachometer).

When using the above method, there are always two cylinder's spark plugs that have problems at the same time so that the test can check the work of the two cylinders.

2.2 Engine doesn't work (normal sparks)

In the failure troubleshooting, use the ignition cable to test the cylinder. If there are sparks, the corresponding ignition coil and its control parts work normally. Further maintenance methods are as follows.

●  No change in engine speed

If there is no change in engine speed during the sparking test, the two cylinders, including the defective cylinder, are not working at the same time. We should focus on spark plugs in these two cylinders for inspection.

●  Instantaneous increase in engine speed

If the engine speed in the test rises instantaneously than the moment of pulling down the ignition cable, but it can't rise to the original (before pulling the ignition cable) speed, the two cylinders that ignite at the same time (include the defective one) work properly.

●  Speed increases to original value

If the engine speed rises to the original value, the cylinder works with the defective cylinder  while working properly. The faulty cylinder does not work well.

●  Almost no increase in engine speed

If the engine speed doesn't increase in the test, the cylinder doesn't work any more. We should focus on the spark plug in the cylinder.

2.3 Engine doesn't work (no sparks)

When you troubleshoot the misfire failure, use the ignition cable to test the sparks. If there are no sparks, the corresponding ignition coil and the control parts don't work normally. The further maintenance methods are as follows.

First, check whether the high-voltage line is normal. Then check whether the ignition signal control line is normal.

If the line is normal and the ignition coil secondary does not have high voltage generation, the oscilloscope can be used to detect the ignition control signal between the electronic controller, ignition electronic assembly, and ignition coil.

If the signal is normal, the ignition coil may have a broken circuit.

Lauritz Carolsfeld        October 21, 2021

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