Tech Center
Vacuum Gauge Engine Performance Testing
A vacuum gauge shows the difference between outside atmospheric pressure and the amount of vacuum present in the intake manifold. The pistons in the engine serve as suction pumps and the amount of vacuum they create is affected by the related actions of:
- Piston rings
- Valves
- Ignition system
- Fuel control system
- Other parts affecting the combustion process (emission devices, etc.).
Each has a characteristic effect on vacuum and you judge their performance by watching variations from normal. It is important to judge engine performance by the general location and action of the needle on a vacuum gauge, rather than just by a vacuum reading. Gauge readings that may be found are as follows:
Normal Engine Operation
At idling speed, an engine at sea level should show a steady vacuum reading between 17" and 21" HG. A quick opening and closing of the throttle should cause vacuum to drop below 5" then rebound to 21" or more.
General Ignition Troubles Or Sticking Valves
With the engine idling, continued fluctuation of 1 to 2 inches may indicate an ignition problem. Check the spark plugs, spark plug gap, primary ignition circuit, high tension cables, distributor cap or ignition coil. Fluctuations of 3 to 4 inches may be sticking valves.
Intake System Leakage, Valve Timing, Or Low Compression
Vacuum readings at idle much lower than normal can indicate leakage through intake manifold gaskets, manifold-to-carburetor gaskets, vacuum brakes or the vacuum modulator. Low readings could also be very late valve timing or worn piston rings.
Exhaust Back Pressure
Starting with the engine at idle, slowly increase engine speed to 3,000 RPM, engine vacuum should be equal to or higher than idle vacuum at 3,000 RPM. If vacuum decreases at higher engine RPM's, an excessive exhaust back pressure is probably present.
Cylinder Head Gasket Leakage
With the engine Idling, the vacuum gauge pointer will drop sharply, every time the leak occurs. The drop will be from the steady reading shown by the pointer to a reading of 10" to 12" Hg or less. If the leak Is between two cylinders, the drop will be much greater. You can determine the location of the leak by compression tests.
Fuel Control System Troubles
All other systems in an engine must be functioning properly before you check the fuel control system as a cause for poor engine performance. If the pointer has a slow floating motion of 4 to 5 inches - you should check the fuel control.
Recap
- Engine problems can affect transmission performance.
- If you suspect an engine problem, connect a vacuum gauge to the intake manifold.
- Note the location and action of the vacuum gauge needle.
- Use the information in this article to determine the engine problem.
- Correct the engine problem before doing extensive calibration work on the transmission.
Performing A Vacuum Gauge Test
Connect the vacuum gauge hose as close to the intake manifold as possible and start engine. Run engine until normal operating temperature has been reached and then allow to idle. On V type engines having two manifolds, make separate tests on each manifold. If required, set the idle adjustment on the carbuetor so that the engine idles smoothly.
NOTE: Vacuum readings will vary according to the altitude in different localities. From sea level to 2,000 feet elevation all normal engines should show a vacuum reading between 17 to 21 inches. Above 2,000 feet elevation the vacuum reading will be about one (1) inch lower per each 1,000 feet rise in elevation.
Valve Lash Adjustment
- Rotate Engine Until Exhaust Rockers Begines to Open Exhaust Valve Set Intake Lash To Spec.
- Rotate Engine Until Intake Rocker Begins To Return From It's Full Lift Position. Set Exhaust Lash To Spec.
- Repeat For Remaining Cylinders Following Correct Firing Order For Your Engine.
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