Vehicles produced since the mid-1990s have sophisticated on-board diagnostics called OBD II. Figure 3.21 shows a screen shot of a scan tool during an electronic power balance test. The scan tool disables fuel injectors rather than the ignition system. This eliminates the possibility of catalytic converter damage and oil dilution from raw fuel entering the crankcase during the test.
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| FIGURE 3.21 Electronic cylinder power balance. When an injector is | disabled, engine rpm should drop |
A cylinder that causes less drop than the others is not pulling its full load. Variations in rpm drop between cylinders should be less than 5%. A problem could be caused by the ignition system or fuel system, or the engine could have vacuum leaks or compression problems. Occasionally, rpm will rise as a cylinder is shorted out due to exhaust gas entering the intake manifold. This can be caused by an EGR valve that is open. (EGR should be closed at idle.) Like an air leak (also called a vacuum leak), EGR leaks cause a drop in manifold vacuum. But EGR leaks do not respond when you richen the mixture like air leaks do. The cylinder causing the rise in engine rpm is the one from which the exhaust gas for the EGR valve was picked up. Retest at cruise rpm and the problem will disappear.
The cylinder power balance test can also be done with the engine running at higher speeds than idle. Compare results at low and higher speed.
An engine with a burned valve will perform poorly at low engine rpm but would improve at higher rpm. A leaking valve does not have as significant an effect at higher speed as it does at low speed because the air coming into the engine and leaving it is moving too fast and has a much higher volume.
A restriction in the intake, like that caused by a worn cam lobe, with hydraulic adjustment will result in little change in engine operation at idle. The problem will become gradually more pronounced as speed is increased.
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