Automotive Ignition Testing... Let the fun begin.

In the previous blog you gat some theory. Now, get ready for some practical knowledge from the confines of automotive diagnostics....

 

IGNITION TESTING STRATEGY

The following procedure is a step by step ignition system testing strategy meant to be used as a 15 minute vehicle test routine. Each step must be followed exactly and no steps should be skipped to prevent diagnostic errors.

There are 5 basic types of misfireselectrical, lean, rich, density and primary missed trigger. All can be detected with the following ignition tests.

Electrical misfire is related to the distributor cap, rotor, wires and plugs. This is called the fixed resistances of an ignition circuit. Carbon tracking is the most common of all electrical misfires next to defective secondary components. A carbon tracking problem will never show up at idle. It is important to stress the ignition system, by preloading or doing a WOT test, so as to identify any problems.

Lean misfire is related to an excess of air or lack of fuel condition as in a clogged injector, vacuum leak, dirty MAF, low fuel pressure, etc. If propane is added to the engine and the misfire does not go away, then the problem is not caused by a lean condition. A density misfire is the probable cause of the missing cylinder.

Rich misfire is related to a lack air or an excess fuel condition as in a leaky injector, diluted oil, filled canister, high fuel pressure, etc. If a vacuum leak is created the rich misfire should go away.

Density misfire is related to a jumped timing belt, ignition timing, EGR valve or mechanically related condition, whereby cylinder compression usually drops. Propane should always be added to the engine while verifying that the misfire still persists. If the misfire goes away when propane is added then the misfire is caused by a lean condition and not a density problem.

Primary trigger misfire is related to any sensor responsible for triggering the ignition spark as in a crank sensor, cam sensor, pick-up coil, etc. The use of an amp-probe is needed to identify a defective triggering device. A bad triggering component will show a missing coil current hump on the scope waveform. A defective triggering device will also cause the coil turn-on point in the spark primary waveform to fluctuate back and fourth more than 500 yS. As long as those fluctuations are bellow 500 yS, they are considered acceptable due to timing belt/chain, distributor or sensor wear. It is also highly recommended that the time base be set high on the scope, so as to pack a lot of trigger pulses on the screen. This will make it easy to see a missed triggering device. Most technicians tend to spread out the time base too much. In this case the idea is to do the opposite and pack as many pulses on the screen as possible.

With the exception of the electrical misfire, the other types of misfires (lean, rich, density, and trigger misfires) are considered variable resistance type misfires because said resistances change according to external conditions.

All ignition system tests take the following format. Test 1 through 5 will identify mostly fixed resistance misfires and test 6 identifies lean, rich and density misfires as well as trigger misfires or variable resistance. Together with the regular capacitive probe, an amp-probe is also connected to channel 2 or B of the scope and synched to the #1 cylinder TDC. By doing so, a spark and current waveform can be seen both at the same time on the scope screen and a determination can be made as to proper triggering device operation. . If a triggering device is suspected, it is highly recommended that the time base be set high on the scope so as to pack a lot of trigger pulses on the screen. This will make it easy to see a missed triggering device.

 

IGNITION TESTING PROCEDURE

Perform a simple visual inspection first. It is absolutely important to verify that the spark is reaching the spark plug first, before any tests are performed. Make sure that there are no illegal air gaps, shorted secondary wires, or carbon tracking in the secondary ignition wiring system. An open or shorted secondary wire will not show up so readily on the scope if the problem is after the spot where the capacitive clip is connected, unless the problem is so severe as to show up on the firing KVs or spark duration values.

1. Cranking KVs A parade ignition pattern is set on the screen. The engine is cranked at WOT to cut injector pulsation and raise combustion chamber pressures to its highest level, thereby, stressing the system to its maximum potential ( raising KV demand ). Any compression problems will be identified with this test by not showing the high KVs needed to fire the lean air/fuel mixture. Any low firing line KVs indicates low compression. The more air molecules in the combustion chamber, the higher the resistance and the more the KV demand. The more the amount of fuel in the combustion chamber, the lower the resistance and the less the KV demand. Air acts like an insulator and fuel as a conductor.

2. Idle Firing Line KVs The engine is let to idle and a parade ignition waveform is analyzed. All firing lines are compared for uniformity and proper KV amplitude. If a triggering problem is identified, attention should be shifted to that particular cylinder or group of cylinders. (Normal KV firing lines should be between 8 and 12 KV ). It is also important to analyze the individual firing KVs, spark line KVs and spark duration as well. (Refer to test #6). The firing KVs, spark KVs and spark duration are closely related and should be taken into account as a group.

3. 2500 RPM KVs The same analysis of the spark waveform is made as in step #2. Since the engine is now at 2500 RPM, the system is stressed more.

4. WOT snap A WOT snap test is made to verify cylinder compression and mechanical integrity. All firing KVs should go up (off the scale). A bar graph feature is desirable when conducting this test. This test is done using a parade pattern and/or bar graph so as to be able to see all firing KVs at the same time. Clogged injectors tend to show up is this setting, in which case, the spark duration would go well bellow 0.50 mS during the test.

5. Pre Load KVs The engine is pre-loaded and the firing line is analyzed. Most ignition problems are identified while pre-loading the engine, due to the stress on the ignition system. It is very important to always stress the system to its maximum.

6. Individual Cylinder Spark Line This test is made to identify any lean, rich and density misfires. The spark line and the spark duration are taken into account very carefully in this test. The system should be tested at idle no load and loaded, 2500 RPM no load and pre-loaded. It is always important to pre-load or stress the system while testing. The more air molecules in the combustion chamber the higher the resistance and the more the KV demand. On the other hand, the more fuel in the combustion chamber the lower the resistance and the less the KV demand. As stated before air acts like an insulator and fuel as a conductor.

In a distributor ( DI ) system ( 2 air gaps ) around 2 to 3 KV should be seen at the spark line and a minimum of 1.3mS spark duration.

In a DIS ( EI-waste spark ) system ( 2 air gaps ) GM type-II, around 2 to 3 KV should be seen at the spark line and a minimum of 1.0mS spark duration.

In a COP system ( 1 air gap ), around 1.5 to 2 KV should be seen at the spark line and a minimum of 1.5mS spark duration.

 

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