19.6.
Engine Vacuum Test
The intake stroke increases volume and decreases pressure, while the compression stroke decreases volume and increases pressure. The intake stroke produces a vacuum, while the compression stroke produces high air pressure. Both strokes depend on the sealing of the cylinder to achieve the desired results. To achieve overall engine performance and economy, it is important that no leaks exist to decrease the amount of vacuum in the cylinder or intake manifold. The intake manifold delivers the air or the air-fuel mixture to the individual cylinders. The vacuum produced by the intake stroke of each cylinder determines the amount that is delivered by the intake manifold. If there is a difference in the amount of vacuum formed from one cylinder to another, the amount of air delivered and the amount of power produced by the cylinders are different. Bad valves, leaking gaskets, and poor piston rings cause a low production of vacuum. High-compression engines produce more vacuum than do low-compression engines. On an engine with a compression ratio of 8:1, the expected pressure, formed during the compression stroke, is greater than eight times atmospheric pressure. On the intake stroke of the same engine, the piston increases the volume eight times the original amount, moving from
TDC to BDC. This decreases the pressure by more than one-eighth of atmospheric or about 12.4 kPa. This pressure is less than atmospheric and is therefore a vacuum.
In places of high elevation, the atmospheric pressure is also lower. Vacuum, therefore, is defined as any pressure lower than atmospheric. The strength of the vacuum is proportional to the difference between low and atmospheric pressure.
The amount of air that enters the cylinder is determined by a number of things. At low engine speeds, the piston is moving rather slow. The duration for which the intake valve is open determines the time the air has to get into the cylinder. The slower the engine speed, the more time the air can enter the cylinder. When the difference between pressure in the cylinder and in the atmosphere is more, the force of the air to enter the cylinder is greater. Therefore, the greater the vacuum produced by the piston, the faster the outside air attempts to fill the cylinder.
Engine vacuum is needed not only for the distribution of the air to the cylinders but is also used to operate other engine systems and some automobile accessories, such as power door locks, power brakes, air-conditioning ducts etc. The difference between atmospheric and engine air pressures is used to perform these jobs. By separating the two pressures with a diaphragm or movable piston, the force caused by the difference of pressures causes the piston or diaphragm to move. By attaching a lever or rod to the piston, work can be performed by the force. These accessories cannot operate if air is allowed to enter into the low pressure side, or if there is no atmospheric pressure present on the other side. The pressure differential must be available to perform work.
The feeding of the cylinders also requires the presence of both high and low pressure. Monitoring the production of vacuum not only helps to determine how well the accessories or engine systems work, but also indicates how well the cylinders work as a vacuum pump. A cylinder’s ability to form and hold a vacuum is also its ability to form and hold compressed air. If there is leak in the cylinder, it also shows up on the compression test, the cylinder leakage test, the cylinder power balance test, and the engine vacuum test.
19.6.1.
Engine Cranking Vacuum Test
To conduct an engine vacuum test, all that is needed is a vacuum gauge, which gives readings of the low pressure present at the point it is attached to the engine. The vacuum gauge should be connected to the intake manifold below the throttle plates for most testing. A connection here gives the information needed to determine the condition of the engine and its related systems. When making the connection, it is best not to disconnect any hoses that lead to a system or accessory. Use a “tee” fitting in the main vacuum hose at the manifold to connect the gauge if possible. Some engines are fitted with a removable plug in the manifold to permit the testing of vacuum. Avoid connecting the gauge to an accessory, because the readings at accessories are not always the true reading of the engine’s vacuum. True manifold vacuum is best measured at the manifold itself.
Manifold vacuum is measured under two conditions: while the engine is cranking and while the engine is running. The first tests to be conducted are with the engine cranking. To prevent the engine from starting, disconnect the battery lead to the ignition coil. To get an accurate reading of manifold vacuum during an engine cranking vacuum test, the throttle plates should be adjusted fully closed. Crank the engine and observe the reading on the vacuum gauge. There are typically no specifications for the reading, but what one should expect is a reading of some vacuum. If the engine produces zero vacuum during cranking, it is very likely that the engine
fails to run. Some vacuum must be present to draw in the air-fuel mixture. If the reading is high enough and the needle is steady, it is an indication that the cylinders are producing the same amount of vacuum. If the needle moves up and down the scale, but spends more time at one reading than the other, it indicates that one or two cylinders are not producing as much vacuum. The pulsating motion of the needle indicates the same condition of the engine as done by the cylinder power balance test, i.e. an unbalanced engine. If the needle seems to spend an even amount of time at each of the readings, it indicates that half of the engine is operating under different conditions that the other half. These needle movements are more important than the actual reading of the gauge.
While the ignition is disabled, pull out the PCV valve and cover the opening with the thumb. The PCV system is designed for a calibrated vacuum leak. This means that the rest of the engine is set up for the loss of vacuum through the PCV system. If the system is working correctly, plugging this leak should cause the cranking vacuum to increase. By pulling the PCV valve out, with its hose still attached, and putting the thumb over the opening of the valve this leak is plugged. If the vacuum does not increase, while cranking the engine, there is a fault in the PCV system. The problem could be a clogged PCV valve or a hose in the system is already plugged, and the present activity has no effect on it. Another possibility is that there is a hole or cut in the hose before the PCV valve. This is allowing air to leak in, and by plugging the end of the hose the vacuum leak is not stopped.
19.6.2.
Engine Running Vacuum Test
Engine manifold vacuum is also monitored when the engine is running. While running, the engine operates with its various systems. All of these systems can influence the reading on the vacuum gauge. Before taking a vacuum reading with the engine running there are two things to check. First check the engine’s idle speed. To obtain a correct indication of the engine condition, the engine must be set to specifications. Care must be taken not to disturb the present running condition of the engine. One should not attempt to smooth the idle with the mixture screws. Adjust the idle speed as close as possible to the recommendations of the manufacturer. The second item to check is the ignition timing. If timing is found off, one should not correct it at this stage. To have useful readings, one should be aware of the present timing.
When an engine is at idle and the ignition timing is changed, the engine’s speed also changes. Advancing the timing tends to increase the idle speed, while retarding the timing slows down the idle speed. Advanced timing causes combustion to be more efficient. Engine speed increases with an increase in combustion efficiency. Anything that increases engine speed at a fixed throttle position also increases vacuum, simply because of the increased number of intake strokes at the higher speed. Advanced timing causes the engine’s vacuum to increase with the corresponding increase in engine speed. Retarded timing causes the vacuum to decrease. The information on the present ignition timing helps to decide the expected normal vacuum reading from the engine. If the timing is advanced, one should expect a slightly higher than normal reading and a slightly lower reading if the timing is retarded. Before checking the vacuum of the engine at idle speed, bring the engine to its normal operating temperature. An engine at operating temperature, correct idle speed, and correct initial ignition timing should have a reading of more than 54 kPa. The reading should be steady. There are no specifications for the vacuum readings of different engines. The desired reading varies mostly from engine to engine as the compression ratio differs. The higher the compression ratio, the higher is the desired vacuum reading. To formulate own vacuum specifications for the engine, consider the ignition
timing and the compression ratio of the engine. If the timing is not correct, add or subtract 1 or 2 units to the expected reading of more than 54 kPa. If the engine has high compression, add 1 or 2 units to compensate for the high compression. Use final value for the desired vacuum specifications.
A steady needle on the vacuum gauge indicates that the cylinders of the engine are forming about the same amount of vacuum. If a reading on the gauge is lower than what is expected, it indicates a loss of vacuum. This may be caused by low compression or a vacuum leak at a common place to all cylinders, such as the mounting base of the throttle assembly. Poor piston ring seal, bad valves, and leaking gaskets are all possible causes for a lower reading on the vacuum gauge. To determine whether the low reading is due to a vacuum leak, raise the engine speed and observe the reading at a constant higher speed. If the reading decreases much from the reading taken at idle, the problem is probably in the engine. A slight decrease in vacuum indicates that there is a vacuum leak in the system. This occurs because as the throttle opens, the vacuum normally decreases. If there is a leak in the intake manifold, it has less effect on the vacuum. To locate the vacuum leak in the manifold, bring the engine speed to idle and squirt oil around the sealing surfaces on the manifold. When oil is introduced to the area of leakage, the vacuum gauge reading also rises temporarily.
It is very unlikely that all cylinders have bad valves. If all the cylinders of an engine have leaking valves, the vacuum reading would be low and steady. Typically, bad valves show upon the gauge as a wavering needle between a higher and a lower reading. Each time the cylinder with the bad valve begins its intake stroke, the needle on the gauge drops to the lower reading. The more cylinders that have a bad valve, the more often the needle dips to the lower reading. This is why there would be a steady low reading if all the cylinders have bad valves. The movement of the needle of the vacuum gauge fluctuates between a high and a low reading any time there is a difference between the amounts of vacuum formed by each cylinder of the engine.
If the engine has a leaking head gasket, only two cylinders are affected. Therefore, the vacuum gauge needle fluctuates between a high and a low reading. The gauge reacts much the same as if two cylinders had bad valves. To identify the problem conduct a cylinder leakage test or a compression test. A cylinder power balance test identifies, which cylinders are affected.
By observing the fluctuation of the needle, one can determine how many cylinders are affected. For four-cylinder engine, the needle spends as much time on the high reading as it does on the low if there is a problem with two of the cylinders. If there is one bad cylinder, the needle spends one-fourth of the time on the lower reading.
Often the needle of the vacuum gauge waivers quickly between two readings. This is caused by a slight variance in the vacuum formed by the cylinders. Because of this quick flutter, it is difficult to see the needle. By pinching the vacuum gauge hose, one can slow the needle movement and have a better indication of the engine’s condition. What should also be observed is the range of the flutter. The larger the range of the flutter, the variance is more between cylinders. The level at which the range of the flutter occurs should also be observed. If the flutter occurs in the normal area of the scale, it usually indicates that there is a slight imbalance in the fuel-and-air delivery system. Worn and leaking valve guides also cause the needle to flutter, but the reading indicates several kPa below normal. This type of problem does not show up in a compression test or a cylinder leakage test. Worn valve guides allow air to enter from the valve covers into the cylinder. This un-metered air reduces the amount of vacuum in the cylinder.
If there is a sticking valve, the needle of the gauge drops randomly. The needle responds to the decrease in vacuum caused by the random sticking of the valve. As the valve sticks open or hesitates before it closes the vacuum in the cylinder decreases. It is unlikely that a valve sticks open each time it begins to close, so the needle of the gauge drops on a random basis. Watching the gauge for a period of time one can observe the occurrence of this condition. This condition causes the engine to have an intermittent miss. Each time the valve sticks, the engine speed decreases, causing a decrease in vacuum. This type of problem is often difficult to diagnose. The use of the vacuum gauge aids in this diagnosis.
If the engine has excessive leakage past the piston rings, the vacuum gauge can be used to identify this. In an engine that has good sealing between the piston rings and the cylinder walls, the needle of the vacuum gauge drops to near zero and rebound to a reading atleast 17 kPa higher than the reading at idle, when the throttle is quickly opened and closed. This is the result of the pistons moving quickly through the intake stroke with the throttle closed. The movement of the pistons is faster than at idle, and with the throttle plates closed, the vacuum builds-up in the cylinder. The initial drop to zero, as the throttle is opened, is caused by the large amount of air that is quickly introduced. A cylinder with excessive leakage around the piston rings, cannot hold the vacuum, which is produced with the high piston speeds and the sealing of the throttle plate. For vacuum leaks past the rings, the gauge responds with a rebound reading of less than 17 kPa over the idle reading.
While quickly opening the throttle and releasing it, watch the vacuum gauge. If the needle vibrates with the increase of speed, it indicates that there is a problem with the valve springs. A weak or broken valve spring causes the valve to remain open for a short time, when it should be closed. The condition of the springs can be checked by holding the engine speed at about 2000 rpm and observing the gauge. If the needle fluctuates with the increase in engine speed, the valve springs are probably weak or worn out. If there is a random drop in the gauge reading as the speed is maintained, it indicates that one or more valve springs are broken.
While the engine is running at 2000 rpm, one can check for an exhaust restriction. An exhaust restriction prevents the exhaust from leaving the cylinders and reduces the engine’s vacuum, because the cylinders are not empty when the intake stroke begins. A collapsed exhaust pipe or a plugged catalyst can cause this exhaust restriction. To check the flow of the exhaust, observe the vacuum gauge while the engine is running at 2000 rpm. Hold the engine at this speed for at least 15 seconds. If there is a restriction in the exhaust, the vacuum gradually decreases. If the vacuum stays the same, the exhaust is flowing well. Often, the vacuum reading with a plugged exhaust drops to a level that is below the idle speed reading.
The proper use of the vacuum gauge can help to decide whether or not to conduct a compression or cylinder leakage test. It also indicates if the-cylinders are all forming the same amount of vacuum. This provides basically the same information as a cylinder power balance test. The vacuum gauge cannot tell the exact location of the compression losses, but it can tell if there is a problem and what to do to identify it. Low readings indicate that a problem does exist. Steady needle readings indicate that the problem is a common one, and a fluctuating needle indicates that the problem is not common. Performing an engine vacuum test is a good way to start a diagnostic tune-up. From the information gathered, one can decide, which tests should be conducted to diagnose the problem properly.
