A vacuum gauge provides valuable information about what is going on inside the engine at a low cost.
You can check for many internal engine problems such as: You can check for many internal engine problems such as: n rings n valves n leaking intake manifold gaskets n restricted exhaust n improper ignition n valve timing n ignition problems.
Vacuum system problems can produce, or contribute to, many drive ability problems. such as: Deceleration backfiring Detonation Hard starting Knocking or pinging
n Overheating n Poor acceleration n Poor fuel economy n Rich or lean stumbling n Rough idling n Stalling n No start engine cold
Both the absolute readings and the rate of needle movement are important for accurate interpretation. Most gauges measure vacuum in inches of mercury (in-Hg).
One thing to remember is that atmospheric pressure decreases as you go up in elevation
Your vacuum will drop as you climb upwards. Your vacuum will drop as you climb upwards. Rule of thumb over 2000 feet you will drop one inch of vacuum for each one thousand feet you go up. Rule of thumb over 2000 feet you will drop one inch of vacuum for each one thousand feet you go up.
n Make sure that you tee into a intake manifold source That is below the throttle plate or throttle valves. Not a ported or venturi vacuum source.
n Also make sure that all vacuum lines are connected or you will get a false reading. The engine should be at normal temperature.
Normal Reading n If the engine is OK the vacuum gauge should hold steady with a reading of 17-22 inches. If you perform a snap acceleration it should drop close to zero but then climb up to 25-28 inches.
Low, drifting reading n If the needle floats about three to eight inches below normal, suspect an intake manifold gasket leak at an intake port or a faulty injectors (on port-injected models only).
Low steady reading n This usually indicates a leaking gasket between the intake manifold and carburetor or throttle body, a leaky vacuum hose, late ignition timing or incorrect camshaft timing.
Regular drops n If the needle drops about two to four inches at a steady rate valves are probably leaking. Perform a compression or leak down test.
Irregular drops n An irregular down-flick of the needle can be caused by a sticking valve or an ignition misfire. Perform a compression or leak down test and read the spark plugs
Rapid vibration n A rapid four inch vibration at idle combined with exhaust smoke indicates worn valve guides. Perform a leak down test to confirm this.
n If the rapid vibration occurs with an increase in engine speed, check for a leaking intake manifold gasket or head gasket, weak valve springs, burned valves or ignition misfire.
Large fluctuation n Perform a compression or leak down test to look for a weak or dead cylinder or a blown head gasket.
Slow hunting n If the needle moves slowly through a wide range, check for a clogged PCV system, incorrect idle fuel mixture, carburetor/throttle body or intake manifold gasket leaks.
Quickly snap the throttle open until the engine reaches about 2,500 rpm and let it shut. Normally the reading should drop to near zero, rise above normal idle reading (about 5 in-Hg over) and then return to the previous idle reading. If the vacuum returns slowly and doesn't peak when the throttle is Snapped shut, the rings may be worn. Quickly snap the throttle open until the engine reaches about 2,500 rpm and let it shut. Normally the reading should drop to near zero, rise above normal idle reading (about 5 in-Hg over) and then return to the previous idle reading. If the vacuum returns slowly and doesn't peak when the throttle is Snapped shut, the rings may be worn.
n If there is a long delay, look for a restricted exhaust system (often the muffler or catalytic converter). An easy way to check this is to temporarily disconnect the exhaust ahead of the suspected part and redo the test.
n Remember you learn by doing. If you never hook up a vacuum gauge you will not know how a good car reacts when the timing is changed or when you remove a vacuum line so you will never know how a car reacts when it does have a problem.