Detecting Vacuum Leaks
Words and Photos By: Mark Yanochko
Before the widespread application of electronics in automobiles, engine vacuum played a much larger role in the operation of the vehicle. For instance, vacuum was often used to open the secondary jets on a 4-barrel carburetor. Also, engine vacuum was used as a signal to shift gears in an automatic transmission.
Much has changed over the last few decades. Carburetors aren’t used anymore and an electronic signal is used to upshift or downshift; but, vacuum still plays a significant role in today’s vehicles, and vacuum leaks can still cause major driveability problems. Vacuum is used for the EGR valve, PVC valve, power brakes and evaporative emissions. Let’s review the steps—that can be done right in your garage—to detect a vacuum leak that might be causing problems.
Before we begin our discussion about vacuum leaks, let’s check out the VECI (Vehicle Emission Control Information) label, A, shown in Figure 1. This underhood sticker shows the basic vacuum line routing, lists general engine information (such as spark plug and engine size) and has a simple engine diagram which shows the relative location of some of the emission control devices. This sticker can help you identify the vacuum-controlled devices on the vehicle.
As we will see, vacuum is most often routed through hoses that are made from materials such as neoprene. Breakdown of the hose material is common and is the most common cause of vacuum leaks. And, if there is a vacuum leak on a vehicle, it most likely will occur in one of the components mentioned in the second paragraph. Look for cracks, splits and other signs of deterioration in these hoses.
The fuel system on the modern automobile is a closed system. An evaporative control (EVAP) system prevents the emission of fuel tank vapors. This system includes a non-vented fuel cap, a vapor canister, B (Figure 2), to collect fuel vapors and a purge solenoid that allows the vapors to be drawn out of the canister and into the throttle body to be mixed with air/fuel mixture. The vapors are drawn out through the vacuum hose. Look for cracks and splits in this hose, C, that would indicate the hose is deteriorating.
On the top side of the engine, there are a couple of emission control devices that 
use vacuum. The first is the PVC valve, D, shown in Figure 3. The blowby gases from the crankcase are drawn into the valve through the vacuum line, E, into the air/fuel stream.
The second emission control device that uses vacuum is the EGR valve, F, shown in Figure 4. The vacuum line, G, comes into the side of housing, drawing in exhaust gas that is used to control combustion temperature.
Often, there is a vacuum line that comes off the intake manifold just after the throttle body. This line, H, is shown in Figure 5. Notice that the hose part of this line is short and is connected to a steel line, J. If this hose breaks down, it can separate from the metal line. When looking for vacuum leaks, always check this hose-to-metal line connection.
One of the major uses of vacuum is the power brake system. A vacuum line, K, is connected to a fitting on the power brake booster, L (Figure 6). The other end of the hose, M, is connected to the engine (Figure 7).
Up to this point we have discussed vacuum being routed through hoses and metal lines. Break down of the hoses and damage to the metal lines will interrupt vacuum flow and disable the function of the device requiring vacuum. But vacuum leaks can occur on the intake side of the engine between mated surfaces.
These mated surfaces can be between the throttle body housing and the mounting flange on the intake manifold. A common location is the mated surface between the intake manifold and engine block. The gasket between the block and the intake manifold, N, (Figure 8) can deteriorate. This causes a vacuum leak and the resultant driveability problems.
The recommended method to find the leak is to spray water along that parting line, N (you can spray water at the throttle body-manifold mating surface, too). If the idle speed changes, you’ve located the leak. But it can be difficult to spray water at the parting line along the bottom side of the intake manifold.
