Unfortunately, the readings of the vacuum gauge are difficult to interpret and the results of the analysis of readings can be erroneous, therefore, it is advisable to combine vacuum diagnostics with other methods.
The initial factors by which the readings of the vacuum gauge are analyzed and the most accurate conclusions about the state of the engine are made are the absolute reading of the device and the nature of the movement of the instrument pointer (dynamics of indications). The scale of most vacuum gauges is graduated in mm. rt. pillar. As the vacuum increases (and, accordingly, the pressure drop) the instrument reading increases. For every 300 m above sea level, the absolute readings of the vacuum gauge will differ by about 25 mm Hg.
Connect the vacuum gauge directly to the suction manifold (See photo), but not to other holes through which vacuum is created, separated from the manifold by a channel of a certain length (e.g. to openings in front of the throttle valve).
Warm up the engine completely before testing. Block the wheels and put the car on the handbrake. When the gear lever is in neutral position (or in the Park position on vehicles with automatic transmission) start the engine and let it idle.
Attention! Before starting the engine, carefully check the condition of the fan blades (the presence of damage or cracks on them). When the motor is running, do not put your hands too close to the fan, keep the appliance at a sufficient distance from the fan and do not stand in line with the rotating impeller.
Check the vacuum gauge reading. On a serviceable engine, the vacuum gauge should show a vacuum of 430-560 mm Hg, and the gauge needle should be practically motionless.
The following is a description of the nature of the readings of the vacuum gauge and the methodology for determining the condition of the engine based on them.
1. Too low a vacuum usually indicates a leak in the gasket between the intake manifold and the throttle chamber, the vacuum hose, as well as too late ignition or the wrong opening and closing of the valves. Before removing the toothed belt covers and checking the alignment of the timing marks, check the ignition setting with a stroboscope and eliminate all other possible causes, guided by the test methods described in this Chapter.
2. If the readings of the vacuum gauge are 75-200 mm Hg. below normal and are unstable (arrow twitches), then this indicates a leak in the gasket at the inlet of the suction manifold or a malfunction of the injector.
3. If the arrow regularly deviates by 50-100 mmHg, then the cause is not tightness of the valves. To confirm this conclusion, check the compression in the engine cylinders.
4. The needle is irregularly deflected towards low readings, or trembling indicates low vacuum. The likely cause is increased resistance to valve movement, or interruptions in the operation of the cylinders. Check cylinder compression and inspect spark plugs.
5. If at idle the arrow quickly fluctuates within 100 mm Hg, and the engine is accompanied by smoke from the muffler, then the valve guides are worn out. To verify this conclusion, it is necessary to test the combustion chambers for leaks (with air pumping). If the arrow fluctuates rapidly and at the same time an increase in engine speed is observed, then it is necessary to check the tightness of the intake manifold gasket, the elasticity of the valve springs. Such indications can also be due to burnt valves and interruptions in the operation of the cylinders (ignition failures).
6. Weak fluctuations of the arrow (within 20-30 mm Hg in both directions) indicate unstable ignition operation. Check all foreseen settings and adjustments, if necessary, connect an ignition system analyzer to the engine.
7. In case of large fluctuations of the arrow, check the compression in the cylinders, or carry out leak tests, as the causes of the malfunction may be an idle cylinder, or a leak in the cylinder head gasket.
8. If the readings of the device change slowly over a wide range, then check the cleanliness of the pipelines of the forced crankcase ventilation system, the correct adjustment of the combustible mixture, the tightness of the gaskets of the throttle body, or the intake manifold.
9. Open the throttle sharply, and when the engine speed reaches 2500 rpm, release the throttle. The shutter should slowly return to its original position. The vacuum gauge should drop to almost zero, then rise and exceed the control readings corresponding to stationary idle by about 125 mm Hg, after which the vacuum should be restored to its previous level. If the vacuum is restored slowly, and there is no excess of the control reading when the damper is suddenly opened, then the cause may be worn piston rings. If the vacuum recovery is extremely slow, check the cleanliness of the exhaust tract (usually muffler or catalytic converter). The easiest way to do this is to open the exhaust tract in front of the suspicious area and repeat the test.
3.6 The use of a screw-in compression gauge is more preferable than those devices that are pressed by hand to ensure tightness during measurement. Make sure the throttle is locked in the fully open position
Checking compression in the engine cylinders
1. The results of the compression test in the engine cylinders make it possible to judge the condition of a group of parts in the upper part of the engine (pistons, rings, valves and cylinder head gaskets). Namely, a decrease in compression may be due to leaks in the combustion chambers due to wear of the piston rings, damage to the valve heads and seats, burnout of the cylinder head gasket.
Note. To obtain accurate test results, the engine must be warmed up to normal temperature and the battery fully charged.
2. Start by cleaning the areas around the spark plugs by blowing out with compressed air (in the absence of a compressor, blow the areas with an automobile, or even bicycle, pump). This is necessary in order to prevent dirt from entering the cylinders when measuring compression.
3. Remove the spark plugs (see Ch.1).
4. Fully open the throttle and secure in this position.
5. Disconnect the central high-voltage wire from the cover of the ignition distributor, connect it to ground on the cylinder block. For reliability, connect to ground using a special shorting piece of wire with alligator clips at both ends. It also does not hurt to remove the fuse for the electronic fuel injection system on the mounting block, which will ensure
complete shutdown of the electric fuel pump when measuring compression.
6. Insert a compression gauge into the spark plug hole.
7. Turn on the starter and turn the crankshaft a few revolutions, watching the pressure gauge readings of the compression gauge. On a healthy engine, the pressure should build up quickly. Low pressure after the first piston stroke and a slow build-up on subsequent compression strokes indicates wear on the piston tracks. If the pressure is low after the first piston stroke and does not increase on subsequent compression strokes, then the cause is a leak in the valves or a leak in the cylinder head gasket (the cause may also be the formation of cracks in the head). Compression loss can also be caused by carbon deposits on the valve heads. Write down the highest compression value.
8. Repeat the measurement procedure for the remaining cylinders, compare the results with standard data.
9. Through the spark plug hole, inject a little engine oil into each cylinder (about three full syringe nipples), then repeat the test.
10. If, after the introduction of oil, the compression increased, then we can make an unambiguous conclusion that the piston rings are worn out. If the compression increases slightly, then the leakage occurs through the valves, or the cylinder head gasket. Leakage through valves can be caused by burnt valve seats and/or bevels, as well as deformation or cracking of valve stems.
11. If the compression is equally low only in two adjacent cylinders, then the most likely cause is a burnt gasket between these cylinders. This conclusion will be confirmed by the appearance of traces of coolant in the combustion chambers or in the crankcase of the crankshaft of the cylinder block.
12. If the compression value in one of the cylinders is 20 percent lower than in the other cylinders and the engine is idling unstable, then the cause may be wear on the camshaft cam that controls the exhaust valve.
13. If the compression value exceeds the norm, then the combustion chamber is covered with carbon deposits. In this case, the cylinder head must be removed and carbon deposits removed.
14. If the compression in all cylinders is low, or is very different for different cylinders, then it is necessary to test the combustion chambers for tightness, for which you need to contact a specialized workshop. As a result of the tests, leak locations must be accurately determined and a quantitative characteristic of the leak given.
Dismantling the power unit - methods and safety precautions
If it was decided to dismantle the engine for a major overhaul, or repair of the main components, then certain preparatory measures must be taken.
It is extremely important to outline the place in which the work will be carried out. Undoubtedly, the best place is the workshop. It is very important to have an equipped working platform, as well as a place to store the car. If there is no workshop or garage, then at least a flat and clean concrete or asphalt area will be required.
Flushing the engine compartment and power unit before starting dismantling will keep the tool clean and in constant working order.
You will also need a floor lift or hoist. Make sure these devices have enough capacity to lift the engine and transmission. Compliance with safety measures plays a paramount role here, since lifting the engine from the car is a potentially dangerous operation.
If work on dismantling the engine is carried out by an inexperienced person, then an assistant is needed. Consult and ask for help from persons with experience in such work. There are many examples of how attempts to single-handedly dismantle the engine with the rise of the latter from the engine compartment ended unsuccessfully.
Plan your activities ahead of time. Before you start work, rent or purchase all the necessary tools and equipment. Some of the tools that provide safety during dismantling and installation of the engine, as well as reducing labor costs include (besides the lift) a trolley jack of sufficient capacity, a full set of wrenches and mandrels, wooden blocks, rags and solvent to clean up the inevitable puddles of spilled engine fluids. If the lift will be rented, then agree on this in advance, having completed all the work in which this mechanism is not required. This will save money and time.
Keep in mind that you will not be able to use the car for a significant amount of time. To perform some work that is not available at home due to the lack of special equipment, you will have to contact a car service workshop. These facilities are on schedule and it would be wise to consult there prior to dismantling the engine in order to accurately estimate the time required to repair and remanufacture parts.
Always be very careful when dismantling and installing the power unit. Careless actions can cause serious injury. Think ahead about your actions. Do not spare time for this, because the main thing is work without injuries.