19.4.
Cylinder Power Balance Test
A cylinder power balance test measures the amount of power that a particular cylinder contributes to the total power output of the engine. The total output of an engine is the sum of all the power produced by the individual cylinders. The cylinder power balance test uses the cylinder’s effect on the engine’s speed to determine this power. While the engine is running, either a spark plug or primary circuit is grounded. This prevents the spark plug from firing in the cylinder, which prevents combustion in that cylinder. Engine speed should decrease when the cylinder is inoperative. The drop of engine speed is recorded and compared to the drops that occur when the other cylinders are eliminated. If all the cylinders of an engine produce the same
amount of power, all engine rpm drops become equal during a cylinder power balance test. Unless it is necessary to do so, more than one cylinder should not be grounded at one time during this test.
If one cylinder is not contributing much to the total power output of an engine, the drop in engine speed during the power balance test becomes much more for that cylinder than for the other cylinders. When an engine is running rough, it is usually caused by one or more cylinders that are not producing as much power as the others. The cylinder power balance test identifies the less productive cylinders.
Most engine analyzers are equipped with a power balance tester. Some have buttons, which when depressed, short out the cylinders. Other testers have the feature of an automatic power balance test. The automatic test shorts out cylinders one at a time and displays the change in engine rpm as each cylinder is shorted. Power balance testers on engine analyzers short out the cylinders according to the firing order. The analyzers equipped with button controls for the selection of the cylinders have their buttons numbered from one to eight. Number 1 is the first cylinder in the firing order, and 8 is the eighth cylinder of the firing order. It is important to remember that the engine analyzers do not recognize the firing order of the engine that is being worked on. Therefore, all references to a cylinder are made according to the cylinder’s place in the firing order.
A cylinder power balance test can be conducted without an engine analyzer. All that is needed is a tachometer to measure the engine rpm. To conduct the test, a spark plug wire from the spark plug has to be disconnected and grounded. The change in engine speed with the cylinder disconnected is the amount of drop caused by that cylinder. The test is continued by disconnecting the wire to each cylinder, one at a time, and recording the change.
While conducting a cylinder power balance test, it is important to keep in mind that the cylinders that cause the least change in engine speed when disconnected are those that are producing the least amount of power. That means if there is no change in engine speed when a cylinder is disconnected, the cylinder is producing very little or no power. The cause of the problem could be mechanical or could be in the fuel or ignition systems. Further tests are necessary to determine the exact cause. If the engine being tested is fitted with an EGR valve, the vacuum line going to the valve must be disconnected and plugged. Changes in engine vacuum may cause the EGR valve to cycle on and off, which can very the engine speed, and hence this interferes the readings. The changes in engine vacuum are caused by the elimination of cylinders that are producing power.
If the car is fitted with a catalytic converter, any cylinder should not be shorted for longer than 15 seconds at a time. Without combustion taking place in the cylinders, raw fuel leaves the cylinders through the exhaust. Excessive amount of raw fuel in the catalyst can cause damage to the converter. Between the shorting out of each cylinder, the engine should be run on all cylinders for at least 30 seconds to allow the raw fuel to leave the converter, preventing fuel build-up in the converter.
To conduct a cylinder power balance test with an engine analyzer, first connect the analyzer according to the manufacturer’s procedure. Disconnect and plug the vacuum line to the EGR valve, if fitted with the engine. Now start the engine and let it run until it is at normal operating temperature. When the engine is warm, set the engine speed to its fast idle speed using the fast idle cam. To achieve this, open the throttle just wide enough to move the fast idle cam freely,
so that the adjustment screw rests on the next step of the cam (Fig. 19.15). This allows the engine to run at about 1000 rpm.
With the engine running at its first idle speed, press the button on the analyzer marked “1″. This shorts out or “kills” the first cylinder of the firing order. Looking at the tachometer on the analyzer, note the drop in engine speed. Release the button and record the rpm drop. The engine is now running on all cylinders again. After a short period of time, press the 2 button, which kills the second cylinder in the firing order. Record the rpm drop and continue the test, shoring out the other cylinders one at a time. While testing each cylinder, keep the cylinders shorted for minimum time needed only to get an accurate reading of the drop in speed.
If all cylinders receive the same amount of fuel, air, and heat, the rpm drops for each should be within 30 rpm of each other. If the difference between cylinders is greater than 30 rpm, the engine’s power is out of balance. The cylinders, which cause the engine speed to drop least, are not producing their share of power.
The cylinder power balance test is quick and easy to perform. It is important in identifying cylinders that have problems. It can also be used to determine the system that is responsible for the loss of power and/or rough running. The results of this test alone do not specify where the problem is. But when the test results are compared to the test results of a compression test and cylinder leakage test, a lot can be found out about the problem. The compression test determines the maximum pressure in the cylinder on the compression stroke of the piston. The cylinder leakage test measures how well the cylinder seals. Both of these tests are conducted on the compression stroke when the intake and exhaust valves are closed. The cylinder power balance test compares the contribution of each cylinder towards maintaining a particular speed. By comparing the results of each one of these tests to each other, one can identify what can be and what cannot be the problem. If an engine has good compression, proper cylinder leakage, and good power balance, the cylinders are well sealed and there are no problems with the syst as common to all cylinders, then the engine would probably run smoothly and efficiently.
If the results of the tests on an engine show that it has good compression, proper leakage, oat poor power balance, then the imbalance problem is not caused by leaky cylinders. Also the problem is not in the fuel system unless the car is equipped with port type fuel injection. The fuel system of most engines is common to all cylinders and therefore would not cause a power balance problem. The problem in this engine must be caused by a non-common system or by an ignition problem (typically in the secondary) or by a vacuum leak that does not affect all cylinders. This imbalance could also be caused by a mechanical failure or defect. The results of the compression test and the cylinder leakage test indicate that the cylinder seals are well, but do not specify anything about how well the valves are opening to let the mixture in or the exhaust out. If a push rod is bent, the valve still closes well but does not open as wide as it is designed for. This limits the amount of mixture that could enter or the amount of exhaust that could leave the cylinder, depending on whether it is an intake or exhaust valve that is affected. The same condition can also exist as a result of a broken rocker arm, a worn camshaft lobe, or a collapsed
Fig. 19.15. Setting the fast idle speed.
hydraulic lifter. All these faults do not affect the sealing of the cylinder, but affect the opening of the valves. To determine the exact cause of the poor imbalance, one should test the ignition system, and then follow the manufacturer’s guidelines for checking the valve-related items.
An engine with good compression, good power balance, and excessive cylinder leakage is typically an evenly worn high-mileage engine. The good results from the compression test are evidence for carbon build-up in the cylinders. The increased compression ratio hides the leakage in the cylinders. The leakage test results verify that there is excessive leakage in the cylinders. Because the engine is worn out evenly, the power balance test results are good. The complaint of the driver of this car would probably be one of poor starting or poor performance. The cure for the problem is to rebuild the engine, which is well beyond the scope of a diagnostic tune-up. However, it may be important to locate the source of the compression leak so that the customer can be informed.
While conducting a cylinder leakage test, the intake the exhaust valves must be closed before supplying air to the cylinders. If the camshaft timing is changed because of a stretched timing chain or belt, the valves still close and open properly, but they do so at the wrong time. An engine with this problem would have satisfactory cylinder leakage test results because as part of the procedure for conducting the test, it is required to close the valves. The effects of this problem are evident in the compression test indicating a low compression in all cylinders. Similarly, since the camshaft affects all cylinders equally, the power balance test provides good result with all cylinders performing just as poorly. An engine with poor compression, proper leakage, and good power balance usually has a valve timing problem.
Fig. 19.16. Checking valve timing.
The simplest way to check valve timing (Fig. 19.16) is to bring the number 1 cylinder to TDC on the compression stroke and align the timing marks on the crankshaft with the pointer used to time the ignition. With the marks aligned, remove the distribution cap. Notice the direction
of the distributor rotor. If the rotor is in the correct location, it points to the terminal for the number 1 cylinder. If the valve timing is off, the rotor does not point at number 1. Since the distributor drives off the camshaft, this can be used as a reference for the timing of the camshaft. If the relationship between the crankshaft and the camshaft is not proper, the valve and ignition timing becomes wrong. This simple check can be verified by removing the valve covers and observing the valves of the number 1 cylinder. Both valves should be closed when the piston is on the compression stroke at TDC. If the piston is at TDC during the exhaust stroke, both the intake and exhaust valves are open slightly. Rotating the engine past TDC should cause the exhaust valve to close and the intake to open wider. Another way to verify the change in valve timing is to remove the timing belt or chain cover and check the alignment marks on the camshaft and crankshaft gears or pulleys.
Using the combination of results from the compression test, cylinder leakage, and cylinder power balance test, one can identify the system that should be tested further.
