Vacuum Gauge Testing to Diagnose Engine Performance Problems

2-04 Analyze vacuum issues to diagnose engine performance issues.

When measuring vacuum either mechanically or electronically with a pressure transducer and digital storage oscilloscope, a properly operating engine will produce a steady reading based on engine size and altitude at idle. Typical running vacuum is 15–22 inHg (50–75 kPa). Cranking vacuum, closed throttle, the engine should produce a minimum of 3–5 inHg (3.4–6.8 kPa). Low or no cranking vacuum will prevent the engine from starting.

Two separate vacuum tests are available: cranking vacuum and engine running vacuum. To perform the tests, connect the vacuum gauge as close to the intake manifold as possible, using the largest and most centrally located vacuum port (FIGURE 2-11). On engines with limited vacuum ports, use a vacuum tee to install the gauge. The tee allows the vacuum gauge to be installed while still supplying vacuum to an existing component (FIGURE 2-12). A cranking vacuum test is a quick way to check engine compression and breathing for a no-start or drivability concern.

TECHNICIAN TIP

Verifying correct vacuum is also necessary on computer-controlled vehicles. Low vacuum can significantly affect performance on vehicles using a manifold absolute pressure (MAP) sensor or a mass airflow (MAF) sensor for fuel and timing control, transmission engagement, shift points, and shift feel.

FIGURE 2-11 The vacuum gauge shows the difference between the atmosphere pressure outside the intake and the atmosphere that is present inside the intake manifold.

FIGURE 2-12 Accessing the vacuum port on the intake manifold allows the technician to verify the mechanical integrity of engine.

TECHNICIAN TIP

Before starting any test, always verify proper operation of equipment. Verify that the vacuum gauge being used operates accurately on a known-good engine. All vacuum gauges, if working correctly, should vibrate slightly since no engine has a perfectly steady vacuum. If the gauge is too steady and fails to vibrate slightly, it will not be able to pick up the small variances required for diagnosis.

General Guidelines to Follow When Using a Vacuum Gauge on a Gasoline Engine

The length and condition of the vacuum hose used to connect the gauge to the engine are important. Old deteriorating vacuum hoses may collapse slightly during testing, affecting the gauge reading (FIGURE 2-13). An excessively long vacuum hose will slow, and possibly cancel, any quick changes in engine vacuum. The ability to monitor fluctuations or changes in vacuum is crucial to finding valve sticking or valve sealing (burned valves) concerns. The larger the vacuum gauge face, the easier it is to notice small changes in manifold vacuum.

FIGURE 2-13 Utilizing a vacuum tee to allow the vacuum component to operate while the technician can measure the vacuum source to that component.

The table below gives some common states of vacuum on a running gasoline engine. These are just guidelines, because sea level affects an engine’s ability to create vacuum. Make sure to take into account the location of the repair facility and where the vehicle will be operated when diagnosing on this topic.

TECHNICIAN TIP

With a running engine, if the vacuum gauge is displaying no or very low vacuum, check the installation of the gauge. Verify that the gauge is connected manifold vacuum. A quick test is to raise the engine rpm. If the vacuum gauge is now reading vacuum, the gauge is attached to ported vacuum, not manifold vacuum. Move the gauge to a manifold vacuum source and retest it.

Testing Cranking Vacuum

Testing cranking vacuum involves a quick test performed to check the valves and piston rings’ sealing ability. To execute the test and achieve the most accurate results, the engine should be at operating temperature and the throttle closed (minimum airflow). Graduation indicators on most vacuum gauges for automotive use are in inches of mercury (inHg) and millimeters of mercury (mmHg). Combination gauges usually have vacuum on one side of the dial and pressure in psi on the other. To aid in diagnosis, also monitor cranking speed when checking cranking vacuum (FIGURE 2-14) Skill Drill 2-2.

FIGURE 2-14 Vacuum lines that are falling apart need to be replaced because they can cause lean issues with the engine operating. Along with lean situations, they may cause the component that they operate to not work.

TABLE 2-4 Engine Running Vacuum Test Results

Reading

Indication

17 to 21 inHg and steady at idle at sea level, less than 1 inHg per 1,000 feet increase in altitude above sea level

A 1 to 2 inHg variation at idle may be normal

A good reading

The range of most modern engines is 19 to 21 inHg. If the reading varies slightly, this may be normal, so do not be overly critical here when looking for obvious faults

Low but steady at idle, between 12 and 15 inHg (below 17 inHg)

1 to 2 inHg at variation may be normal

Vacuum leak, retarded valve timing, valves are too tight, late ignition timing, a positive crankcase ventilation (PCV) valve/engine breathing fault, and a worn engine

Consistent 1 to 2 inHg vacuum drop, more than normal needle vibration range

A moderate to severe sealing/breathing issue may show 4 to 8 inHg variation

One cylinder is not efficient, low air charge from worn rings or valves that are not opening or sealing correctly, and a burned or leaking valve

Erratic 1 to 2 inHg drop

Misfiring cylinder or valve sticking open slightly, rich or lean mixture

Snap acceleration needle drops to zero and then only reads 20" to 23" of vacuum; should be much higher—around 27"

Possible worn rings

Normal at idle speed

As the engine speeds up to a steady 2,500 rpm, the needle slowly goes down and may continue to drop. If monitoring while driving, it is initially fine at higher rpm but drops slowly while driving

Possible restricted exhaust system

Extreme rhythmic drops in vacuum

Burned intake valve or valve sticking wide open

Needle oscillates slowly, or drifts, 3 to 9 inHg below normal

Intake system leak

Normal at idle speed, but excessive vibrations at higher rpm

Weak valve springs; valves sticking in guides

TECHNICIAN TIP

If a vacuum gauge is not available and the vehicle is equipped with a MAP sensor, substitute the MAP PID in place of the gauge, for vacuum testing and diagnosis. Select MAP as the only PID, speeding the update time from the PCM to the scanner. Set the scan tool to measure only MAP in kilopascals. Kilopascals are much more accurate than psi or inHg. Graph the data over time. Being able to see extended time period provides the ability to notice small changes in engine vacuum. These changes are useful in detecting a valve sealing or operating issues. Perform vacuum testing the same way as testing the gauge. A cylinder mechanical concern results in a lower vacuum.

SKILL DRILL 2-2Testing Cranking Vacuum
  1. Get the vehicle up to operating temperature.
  2. Shut the engine off, disable the fuel, crank the engine, and allow all the fuel to be used in the fuel rail.
  3. Install a battery charger so that the battery is not killed during this test.
  4. Verify that the throttle plate is fully closed.
  5. Connect the vacuum gauge to a non-restricted manifold vacuum source. The maximum length of the hose should be 3 ft (90 cm).
  6. Connect a scan tool and select the rpm PID.
  7. Vacuum changes with load; therefore, verify that all accessories are off.
  8. Crank the engine for 10 to 15 seconds, and observe the vacuum gauge and scan tool.
  9. Crank the engine at closed throttle. The vacuum should be a steady 3–5 inHg (10–17 kPa) at a consistent rpm.
  10. Cranking vacuum less than 3 inHg (10 kPa) indicates either a compression or breathing concern, commonly resulting in a no-start condition.
  11. Remove the vacuum gauge and reinstall the fuse for the fuel system.
TECHNICIAN TIP

The cranking vacuum test also functions as a quick test of the PCV valve. The test checks only for restrictions. Note the initial vacuum while cranking. Next, while cranking the engine, pinch off the hose to the PCV valve by using pliers, or cover the end of the valve with a finger. If the PCV system is clear, the vacuum should rise. If the vacuum fails to increase, inspect the system for restrictions. An increase in vacuum does not verify the correct PCV installation or flow rate, only that the installed PCV valve is functioning.

TABLE 2-5 Cranking Vacuum—Analyzing the Results

•  If cranking speed is slow, verify the following:

•  The battery is fully charged.

•  The engine is at operating temperature.

•  The starter draw is not excessive.

•  Excessive voltage drop in the battery cables or connections will reduce starter performance and cranking speed.

•  If cranking speed is fast, the entire engine has little or no vacuum/compression:

•  The timing chain or belt has broken.

•  The teeth on the belt or sprockets have been stripped.

•  If cranking vacuum is low with a steady rpm, verify the following:

•  There is no large external vacuum leak

•  High lift, or high valve overlap performance camshafts, produce slightly lower vacuum and an erratic gauge reading. This is normal.

•  Throttle plate, if equipped, is not fully closed.

•  Cranking vacuum and rpm are erratic, so the cylinders are not pumping air equally:

•  One or more cylinders has little to no vacuum/compression.

•  Suspect a large vacuum leak past the rings, valves, or head gasket.

•  leaking valves, which are failing to seal

•  worn piston rings or cylinder walls, which are failing to seal or create proper compression

•  To locate the suspect cylinder(s) more in-depth testing will be needed:

•  cylinder leakage, compression testing, or both

•  Normal to high cranking speed with low, slightly uneven vacuum:

•  Check for retarded valve timing or low compression.

•  Check for jumped timing chain or belt.

•  Check for bent valve or pushrod.

•  Check for stretched timing chain.

•  Check for stuck variable valve timing (VVT) actuator.

•  Vacuum is steady but craning rpm is unstable or erratic:

•  Suspect a mechanical engine concern, a faulty starter, or a damaged flywheel.

TECHNICIAN TIP

For a no-start concern, a quick test can help eliminate a mechanical concern that limits the engine’s ability to create vacuum and build compression. Begin by placing a hand over the throttle body inlet while cranking. A strong suction or pull normally indicates that enough manifold vacuum is present for the engine to start. If there’s little pressure or little to no suction, the engine more than likely has a mechanical fault causing low manifold vacuum.

Inches of Vacuum Adjusted for Altitude

Vacuum readings vary with altitude. Typically, at sea level up to 2,000 ft (305 m) the normal vacuum reading is 17 inHg to 21 inHg (57.6 to 71 kPa). As altitude increase from 2,000 ft (610 m), the vacuum reading will be approximately one inch (1 inHg, or 3.4 kPa) lower for each 1,000 ft (305 m) rise in elevation.

TECHNICIAN TIP

A common saying for results when performing a vacuum test is, “if the vacuum reading is unstable, something touching oil is at fault.” In other words, using a vacuum gauge can find mechanical faults.

Abnormal Engine Running Vacuum Test Results

The results obtained from vacuum testing provide a relatively quick and easy method to determine relative engine health. A definite distinction exists between a steady vacuum gauge reading and a fluctuating reading. A steady but low reading indicates a concern common to all cylinders. Suspect a worn engine, incorrect cam timing on a single camshaft engine, or incorrect ignition timing. An irregular fluctuating needle indicates a problem is present in one or just a few cylinders. Combining a power balance test with the vacuum gauge test can help in finding which cylinder(s) are responsible for excessive needle movement Skill drill 2-3.

TECHNICIAN TIP

Ignition systems, fuel systems, sensors and actuators, valves, and pistons and rings all affect the combustion process. Each mechanical fault occurring in an internal combustion engine has a distinct effect on performance. Assess their performance by observing the variations on the gauge compared to normal. When determining the cause of an engine performance fault, evaluate the general location and action of the gauge needle instead of just the actual vacuum reading.

SKILL DRILL 2-3Testing Running Vacuum
  1. Start the engine, and bring it to its normal operating temperature.
  2. Shut the vehicle off, and install a vacuum gauge to a non-restricted manifold vacuum port
  3. Start the engine, and check vacuum at idle; low-speed cruise, steady throttle (1,500 to 2,000 rpm); and high-speed cruise, steady throttle (2,500 to 3,500 rpm). Record the results.
  4. The typical reading of a normally operating engine at idle is a steady 17 to 21 inHg (57.6 to 71 kPa) at sea level (some engines may be as low as 15 inHg, or 51 kPa).
  5. The engine should react as explained below:

    •  When checking at a steady 1,500 to 2,000 rpm, note the vacuum and record the reading, then increase the engine speed to 2,500 to 3,500 rpm:

    •  Vacuum should remain steady once the target rpm has been reached.

    •  The vacuum normally drops somewhat when initially opening the throttle to raise the rpm, and then increase slightly over idle vacuum before remaining steady.

    •  If rpm drops steadily, check for an exhaust restriction.

    •  If the rpm drop is irregular, it indicates a cylinder sealing issue, probable causes for which include valve-related or piston ring/cylinder wall faults.

    •  During a snap throttle test, when quickly opening the throttle, the vacuum should drop to between 0 and 5 inHg (0 to 17 kPa) and then rebound (overshoot) from 21 to 27 inHg (71 to 91 kPa).

    •  When initially releasing the throttle, the vacuum should drop rapidly to near zero, then spike about 25% above the running rpm vacuum when releasing the throttle, before returning to normal idle readings.

    •  If the vacuum fails to spike or return to previous levels, suspect a restricted exhaust system or worn internal engine components, rings, valves and cylinder walls.

    •  Lower vacuum readings should be expected and considered normal under the following conditions:

    •  altitude has increased, which causes the barometric pressure to decrease

    •  an aggressive camshaft is being used (high lift and overlap)

    •  when testing an Atkinson cycle style engine.

TABLE 2-6 Inches of Altitude Vacuum Typical

•Sea level: 1,000 ft (305 m) 17–22 inHg (57.6–74.5 kPa)

•Sea level: 1,000–2,000 ft (305–610 m) 17–21 inHg (57.6–71 kPa)

•Sea level: 2,000–3,000 ft (610–915 m) 16–20 inHg (54–67.7 kPa)

•Sea level: 3,000–4,000 ft (915–1,220 m) 15–19 inHg (50.7–64.3 kPa)

•Sea level: 4,000–5,000 ft (1,220–1,525 m) 14–18 inHg (47.4–61 kPa)

•Sea level: 5,000–6,000 ft (1,525–1,830 m) 13–17 inHg (44–57.6 kPa)

TABLE 2-7 Vacuum Testing Results

A rapid vacuum fluctuation of three or more inches of vacuum from normal indicates low compression from a possible valve train concern (FIGURE 2-15)

•  Failure of the valve to seat properly causes a vacuum leak.

•  Every time the valve fails to seat, the needle will fluctuate or flutter.

•  To pinpoint the faulty cylinder(s), perform additional testing for compression and cylinder leakage.

•  Broken compression rings, damaged cylinder wall.

•  Head gasket in one or two cylinders.

FIGURE 2-15 Most vacuum gauges measure positive pressure (PSI) and negative pressure (Vacuum).

Low, steady vacuum at idle (FIGURE 2-16)

•  If checking a performance engine, this is normal operation on a high overlap camshaft

•  Retarded cam or ignition timing (approximate 8 to 14 inHg, or 27 to 47.4 kPa),

•  One or more camshafts are retarded:

•  a vacuum leak past a gasket, seal or vacuum hose

•  tight valves

•  low compression on all cylinders or blowby from worn piston rings or cylinder walls

•  EGR valve sticking open

FIGURE 2-16

•  Low vacuum can also indicate a restricted exhaust

•  Run the engine at 2,500 rpm for 30 seconds

•  If vacuum drops while at 2,500 rpm, suspect a restricted exhaust

•  A fluctuating needle (typically 10 to 22 inHg, or 34 to 74.5 kPa), when increasing rpm, normal at idle (FIGURE 2-17).

•  weak valve springs

FIGURE 2-17

•  A fluctuating gauge needle of 1 to 2 inHg (3.4 to 6.8 kPa) (FIGURE 2-18).

•  points toward a misfire concern

FIGURE 2-18

•  An abnormally high vacuum reading (FIGURE 2-19).

•  air inlet restriction reducing the air pressure available in the intake manifold

•  excessively advanced cam timing

•  overly advanced ignition timing

FIGURE 2-19

Finding Exhaust Restrictions with a Vacuum Gauge

Low vacuum can also indicate a restricted exhaust. Verify exhaust restrictions by slowly increasing engine speed to 2,500 to 3,000 rpm and noting engine vacuum. The vacuum attained should be equal to or slightly higher than at idle. While raising the rpm, vacuum may dip slightly when initially opening the throttle and then recover; this is normal. If vacuum decreases as rpm increases or if it fails to rebound and overshoot the idle reading by 25% when releasing the throttle, suspect excessive backpressure from an exhaust restriction Skill Drill 2-4.

TECHNICIAN TIP

The described vacuum gauge testing is accurate on only a naturally aspirated engine. Turbocharged engines develop pressure in the intake manifold that increases with rpm, making vacuum testing inaccurate.

SKILL DRILL 2-4Testing Exhaust Restriction
  1. Position the vehicle in a well-ventilated area.
  2. Attach the vacuum gauge to the engine on a manifold port.
  3. Start the engine, allow it to idle, and then document where the vacuum gauge is.
  4. To verify whether there is a restriction, increase the engine speed to approximately 2,500 rpm and watch the vacuum gauge.
  5. Release the throttle quickly from 2,500 rpm. The vacuum should overshoot the initial reading by a minimum of 25% if the exhaust is not restricted.