2. A vacuum gauge is attached to the throttle space to measure the pressure directly on the intake manifold. Do not connect a vacuum gauge to accessory hoses.
3. Vacuum diagnostics should be combined with other diagnostic methods.
4. 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).
5. Before starting the tests, warm up the engine completely, block the wheels.
6. Check the vacuum gauge reading. On a serviceable engine idling, the vacuum gauge should show a vacuum of 430–560 mm Hg, and the gauge needle should be practically motionless.
7. If the vacuum gauge readings 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 leak in the injector.
8. If the arrow regularly deviates by 50-100 mm Hg, then the cause is valve leaks. Check the compression in the engine cylinders.
9. If the arrow irregularly deviates towards low readings, or trembles shows a low vacuum, then the cause is increased resistance to valve movement, or misfiring. Check cylinder compression and inspect spark plugs.
10. If at idle the needle fluctuates rapidly within 100 mm Hg, and the engine is accompanied by smoke from the muffler, then the valve guides are worn out. If the arrow fluctuates rapidly with a simultaneous increase in engine speed, 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 misfires.
11. Weak fluctuations of the arrow within 20–30 mm Hg. indicate unstable ignition operation.
12. With large fluctuations of the arrow, check the compression in the cylinders.
13. A slow change in vacuum over a wide range indicates a blockage in the boat's ventilation system, an abnormal mixture, and a leaking manifold gasket.
14. Bring the engine speed to 2500 rpm and release the pedal. The vacuum gauge readings should drop to almost zero, then rise and exceed the control readings corresponding to stationary idling by about 125 mm Hg, after which the vacuum should be restored to its previous level. A different vacuum behavior indicates wear on the piston rings. If these readings change too slowly, then check the cleanliness of the exhaust system passages (especially the neutralizer).