Where to Look for Vacuum Leaks

2-06 Interpret vacuum leaks.

Finding a vacuum leak may be easy or challenging. Perform a visual inspection, looking for loose, split, cracked, disconnected, or incorrectly routed vacuum lines or hoses. Reroute any incorrectly run vacuum lines or hoses that prevent a repeat repair. Also, remember that oil causes rubber to swell and decompose, placing the PCV system high on the list of possible faults. Carefully inspect the PCV vacuum tube from the valve to the engine and the fresh air supply hose that provides filtered air to the PCV system. An inlet hose connects to a filtered air source, usually either at the air filter housing or at air inlet tube after the MAF sensor to the engine. Another common source of leaks is the fresh air inlet hose between the throttle body and air filter housing. The clamps will often be loose after servicing or inspecting the air filter.

At times, a vacuum leak is audible and easy to find. If a visual inspection fails to identify a fault, begin with the engine running, then listen for and trace the source of the noise until the leak has been identified. Technicians can use a stethoscope with the metal rod removed or a piece of vacuum hose placed near their ear. Move either tool slowly over suspected leak areas, such as two mating surfaces or any hose connection, to narrow down the source of the noise. Electronic leak detectors that flash or beep when sensing an ultrasonic vibration (vacuum leak) are also available to use in place of a stethoscope.

Other times, finding a vacuum leak may prove difficult. Sometimes a vacuum leak is not audible; instead, it disguises itself as a misfire. Depending on the severity of the vacuum leak, the accompanying symptoms may differ. Some of the symptoms that accompany a vacuum leak are a loud hissing sound that is worse at idle, a high idle speed, a rough idle or stalling, or a hesitation on acceleration. Hard-to-find leaks include a leaking engine gasket, injector O-ring, worn throttle body shaft, improper PCV valve (excessive flow), and an improperly seated oil dipstick. Even harder to find is a hairline crack between two runners of a plastic/nylon intake manifold that changes with temperature.

One source of a vacuum leak that can prove challenging is the power brake booster. Generally, boosters leak only internally, making routine testing procedures inadequate for finding the fault. Pinching the vacuum hose (rubber hoses only, never pinch off plastic, nylon, or steel lines), supply hose with the engine running while listening for a change in the engine or monitor short fuel trims (SFT) on a scan tool to verify the leak. When pinching off the hose, do so as close to the booster as possible, to eliminate the hose and check valve as potential leaks. Boosters also may leak only when the brake pedal is depressed, making the diagnosis even more challenging. If a leak does not appear while sitting in the bay during the testing procedure, monitor the SFT while applying the brake. A sudden change that indicates a lean condition (fuel trims will respond positively) is a clue that the booster may be at fault. Repeat the test with the boost supply hose pinched off near the intake and retest. If the lean condition no longer exists, verify the brake booster hose’s integrity, then replace the booster and recheck.

Regardless of the difficulty in finding the leak, the result is the same: Any unmetered air entering the engine can alter the air-fuel mixture. Vacuum leaks make maintaining the correct air-fuel ratio difficult for the engine management system. MAF vehicles measure all the airflow into the engine. Air that enters the engine downstream of the MAF is unmeasured and unaccounted for, leaning out the air-fuel mixture. The severity of the leak determines the result. Increased emissions and a lean misfire, hesitation, or stumble on acceleration as the throttle opens are usual results.

TECHNICIAN TIP

A PCM can compensate for minor vacuum leaks by reducing airflow into the engine. The PCM controls airflow by closing the throttle plate on an electronic throttle control (ETC) throttle body. If equipped with an IAC valve, the PCM moves the pintle more toward the closed position (the IAC PID will be near 0 instead of the normal 25 to 30 counts at idle) on a cable-actuated throttle plate. If the PCM can compensate for the vacuum leak by controlling airflow, it will not reduce measurable manifold vacuum noticeably, if at all. By reducing bypass air around the throttle plate, intake vacuum remains stable in the intake manifold. Using PID data for the previously mentioned systems can help find a vacuum leak that a vacuum gauge or that fuel trims may not.

To eliminate a vacuum leak quickly, do the following:

• Block the drive wheels.
• Apply the emergency brake.
• Place the vehicle in reverse, adding a load, then raise the rpm off idle to approximately 1,500 rpm.
• If the symptom disappears, it signals that an engine vacuum leak is present.
• Small vacuum leaks typically create a drivability issue only at idle and low rpm, when the throttle is closed.
• Opening the throttle, under load, provides additional air to enter the engine, negating the effects of a small vacuum leak.
• Large vacuum leaks, however, make the engine run extremely lean during most operating conditions.
• An engine running too lean increases the possibilities of detonation, or engine damage, by burning a piston or a valve.

TECHNICIAN TIP

If a throttle body block-off adapter is not available, another way to seal the air inlet is to wrap the air cleaner in clingy plastic wrap and secure it with the air inlet hose, a clamp, or tie-down strap. This method is not as good as plugging the throttle body, but it will work. Caution: Remember to remove the plastic wrap when testing is complete; otherwise, the vehicle will not start.

Using a Smoke Machine

The safest method for finding a leak is the smoke machine. These machines pass a low-pressure mineral vapor of artificial smoke into the intake manifold. Most smoke machines mix an ultraviolet dye with the smoke to facilitate leak detection (FIGURE 2-24). Begin the test by finding a large vacuum source that enters the intake manifold. The chosen location must feed the intake, not just the throttle body and air inlet tube. Typically, the brake booster hose or PCV inlet are excellent places. Most smoke machines also come with several adapters to connect to the throttle body inlet. While the throttle body is an excellent location, the air inlet tube would need to be removed, which is a frequent cause of leaks on older vehicles that occurs from the constant flexing due to engine torque. Therefore, if testing at the throttle body, also smoke test the air inlet tube to verify that it is not the source of the leak.

FIGURE 2-24 Cranking vacuum waveform recorded after performing a chemical induction service to remove the carbon buildup. Note the evenness of the peaks and valleys. On an engine with no faults, the pattern should be similar and should continue to repeat. The highest vacuum pulled is about 90° after top dead center (ATDC) of the intake stroke as the piston descends in the bore.

After connecting the smoke machine hose to the engine, block off the throttle body, if it is not being used as the test point. Sealing the intake traps smoke in the manifold. Next, turn the machine on, and begin looking for smoke vapor seeping out of hoses, gaskets, O-rings, cracks in the manifold, and any mating surface. Most smoke contains ultraviolet (UV) dye that leaves a visible stain highlighting the source of hard-to-find leaks. The dye is easier to spot when the glasses and light match the UV qualities of the dye.

When testing with smoke, understand that any component that passes vacuum to another part will not be visible at the engine. The purge solenoid, integral PCV valve, and brake booster are examples. Disconnect the purge line that runs from the purge solenoid outlet to the fuel tank to monitor the purge valve for a leak. A purge valve that fails to seal will show a leak at the outlet of the normally closed valve. Replace the valve if smoke is present. PCV systems that are integral to the valve cover or located inside the oil filter housing also can prove challenging. Do not assume that smoke from the oil fill cap is an indication of a PCV fault. Remember that at least one intake valve will be partially open, allowing smoke to pass into the crankcase.

TECHNICIAN TIP

Some engines, such as the VW 2.5 L, use two crankcase breather valves (PCV system). One of the valves is integral to the valve cover, and the other is part of the oil filter housing. If either the diaphragm or the valve fails, manifold vacuum will be present in the crankcase, resulting in idle quality issues and various lean codes. To test, remove the dipstick, install a vacuum gauge on the dipstick tube, and then check for excessive crankcase vacuum of no more than 1–2 inHg (3.4–6.8 kPa). After finding an excessive vacuum with the gauge still connected, block off the vacuum tube to the valve cover breather valve. Blocking off the tube eliminates the vacuum source to the integral PCV. If the crankcase vacuum returns to normal, near 0 inHg (0 kPa), the valve cover breather has failed. Therefore, replace the valve cover and retest. If a change does not result when blocking off the vacuum source for the valve cover, locate and disconnect the vacuum tube from the oil filter housing to the intake manifold. If the crankcase vacuum is now less than 1 inHG (3.4 kPa), replace the oil filter housing. In some cases, a loud whining or moaning noise from the engine may be present. A loud whining or moaning sound emanating from a running engine indicates excessive crankcase vacuum is pulling outside air in past the engines seals. To make an accurate diagnosis, remove the oil fill cap, with the engine running. If the noise disappears or changes, a breather valve is at fault.

Finding a Vacuum Leak without Using a Smoke Machine

A vacuum leak affects an engine’s operation differently, depending on which method of intake airflow is used. Normally a MAF engine will idle low with a vacuum leak and show reduced MAF. A MAP sensor (speed density) on a vehicle will idle high and more richly than normal with a vacuum leak. Vacuum leaks can be either internal or external to the engine.

Finding External Leaks without a Smoke Machine by Using an External Hydrocarbon

Check and monitor the following PIDs, with the engine running, while spraying carburetor cleaner around the intake manifold, PCV lines, vacuum lines, and their connections as well as the air inlet tubing after the MAF sensor. Watch the rpm, short-term fuel trim percentage(s) and oxygen (O₂) sensor voltage. When a vacuum leak has been found on a vehicle equipped with a MAF sensor with a hydrocarbon source, the rpm will increase. Along with the rpm increase, the following should also occur: a corresponding increase in the MAF sensor output due to the increase in airflow; short-term fuel trim (STFT) will go negative; and the O₂ voltage will move rich. A vehicle with a MAP sensor will respond with a decrease in MAP voltage; the rpm will decrease; and the fuel trims will move leaner.

SAFETY TIP

Propane and carburetor cleaner are a potential fire hazard. Use caution when spraying any flammable material around a running engine.

•  Check for DTCs if this has not already been done.

•  Look for lean exhaust codes and fuel trims biased positive above the standard ±10%.

•  Monitor the following PID data using the following—either manufacture-specific or generic OBDII variants (FIGURE 2-25):

•  long-term fuel trim (LTFT)

•  STFT

•  rpm and desired rpm

•  O₂ sensor or wide band oxygen sensor

•  electronic throttle plate position and desired throttle plate position, if equipped

•  MAP or MAF sensor

•  IAC, if equipped.

FIGURE 2-25 A smoke machine creates smoke that is then pressed through the system with an inert gas so that it does not create an even more flammable environment.

Using Propane

A proven and often used technique of locating vacuum leaks is to use propane. Attach a short piece of hose to the propane bottle. Open the valve on a propane canister to produce a steady flow. With the engine at idle; slowly move the hose along all vacuum hoses, connections, mating surfaces, seams, and flanges (FIGURE 2-26). Keep the flow of propane regulated to help isolate the leak. With the engine running, propane will be siphoned into the suspected leak area, causing a noticeable change in the idle speed and/or smoothness. A change in rpm occurs because the propane temporarily corrects the lean air-fuel mixture.

FIGURE 2-26 Picture 1 shows a normal running engine with no faults. Picture 2 shows the changes in the PID data with the introduction of a vacuum leak. When diagnosing a vacuum issue, understanding the data PIDs facilitates a quicker diagnosis.