Engine Vacuum Test

Throttle plate opening controls the amount of air that can enter the engine. Movement of the accelerator pedal opens or closes the throttle plate, changing engine vacuum. For a stock engine, vacuum should range from 16 to 22 in. Hg at idle, and the needle should be steady (Figure 3.24). An engine has higher vacuum when it is operating under light load. Vacuum drops to zero under WOT.
• At idle, with the throttle plate nearly closed (Figure 3.25a), engine vacuum will be high.
• At medium throttle opening, when cruising (Figure 3.25b), engine vacuum will be in the neighborhood of 8 to 10 inches.
• At WOT (Figure 3.25c), engine vacuum is zero.

A leaking intake manifold gasket can cause an engine to idle rough, especially when cold, before oxygen sensor feedback begins. The oxygen sensor can compensate for small vacuum leaks to a certain extent. At speeds above idle, symptoms of a leaking

a

b
FIGURE 3.23 (a) A vacuum gauge reads pressure when the needle
moves clockwise and “vacuum” when the needle moves counterclockwise.
(b) Connect the vacuum gauge to an intake manifold vacuum source.

FIGURE 3.24 Normal engine vacuum

intake manifold diminish because the size of the leak is proportionally less as the engine breathes more air.

A leaking intake manifold gasket can result from sloppy cleanup of gasket surfaces during manifold installation, from failing to clean out bolt holes, or from bottoming out bolts that are too long. 

A car with an oxygen sensor feedback fuel system can run rough when cold but run fine after warm-up when the computer responds to the oxygen sensor signals by compensating with a richer

air-fuel mixture.

One vacuum test for a manifold gasket leak is to pinch off the two PCV valve hoses—the one to the intake manifold and the breather hose to the air cleaner—then run the engine. If there is vacuum at the oil filler opening, an intake manifold-tocrankcase vacuum leak is indicated.

To test for weak piston rings (Figure 3.26):

• Raise the engine speed to about 2000 rpm.

• Snap the throttle closed and watch for an

increase of 2 to 6 inches of vacuum above normal.

• Worn rings will not increase vacuum sufficiently during deceleration. Generally, the higher the rise, the better the condition of the rings.

FIGURE 3.26 Vacuum readings for weak piston rings.

Sticky valves are indicated when the needle drops quickly or drifts. This could be because valves are hanging up in their guides rather than closing freely (Figure 3.27). The movement will have no apparent rhythm. Leaking valves (Figure 3.28) are indicated when the needle drops at regular intervals. A power balance test will pinpoint the low cylinder.

FIGURE 3.27 Vacuum reading for sticky valves

FIGURE 3.28 Vacuum reading for leaking valves.

FIGURE 3.29 Vacuum reading for bad valve springs

Bad Valve Springs. Sometimes valve springs that are too short or too weak have been installed by accident. Otherwise, valve springs weak enough to cause this problem are rare. To test for weak valve springs with a vacuum gauge (Figure 3.29):

• Raise engine rpm slowly to 2000.

• Look for rapid fluctuations of the needle as speed increases.

• An exhaust analyzer will have a good hydrocarbon (HC) reading at idle but bad under acceleration.

Restricted Exhaust. To test for a restricted exhaust (Figure 3.30):

• Raise the engine rpm quickly to 2000 to cause the vacuum reading to go momentarily low and

then release the throttle quickly. • Vacuum should return smoothly and quickly to higher than normal levels.

• A slow, hesitating return can indicate a breathing restriction.

When an engine with weak valve springs is accelerated, hydraulic lifters can overfill (pump up).

There is no noise, but the engine will run rough when it returns to idle.

According to one Auto Engine Rebuilders Association (AERA) bulletin, incorrect valve spring tension

can result in a rough idle that is only apparent on initial cold startup. OBD II engine computer systems

sometimes sense this misfire and cause the malfunction indicator lamp (MIL) on the dash to illuminate.

Restricted Exhaust. To test for a restricted exhaust (Figure 3.30):

FIGURE 3.30 Vacuum reading for restricted exhaust

• Raise the engine rpm quickly to 2000 to cause the vacuum reading to go momentarily low and then release the throttle quickly.

• Vacuum should return smoothly and quickly to higher than normal levels.

• A slow, hesitating return can indicate a breathing restriction.

Other Causes of Low Vacuum. Port fuel injected engines use O-rings to seal each individual fuel injector where it enters an intake port (Figure 3.31).

FIGURE 3.31 O-rings seal the fuel injectors where they enter the

intake ports

When an O-ring leaks, the resulting vacuum leak and lean air-fuel mixture in that cylinder cause a rough idle (Figure 3.32).

FIGURE 3.32 Fuel injection O-rings can become hard and brittle
with age, resulting in a rough idle, especially during open-loop operation
when the engine is first started and the computer is not receiving
feedback from the oxygen sensor

If an engine is equipped with an air pump for emission control, be sure that it does not have a 

When an engine with weak valve springs is accelerated, hydraulic lifters can overfill (pump up). There is no noise, but the engine will run rough when it returns to idle.

According to one Auto Engine Rebuilders Association (AERA) bulletin, incorrect valve spring tension

can result in a rough idle that is only apparent on initial cold startup. OBD II engine computer systems sometimes sense this misfire and cause the malfunction indicator lamp (MIL) on the dash to illuminate.