Finding and Fixing Engine Vacuum Leaks

Fuel, Emission and Carb

By Rodney (Mechanics)

Have you ever tried fixing up an engine, but it just won’t idle smoothly or run properly? Or maybe you’ve dealt with an engine that always seems off, no matter what you’ve tried? Well, chances are, you’re facing an engine vacuum leak.

Sometimes, a vacuum leak makes itself obvious with a whistling or hissing sound. But other times, it plays tricks and seems like an issue with ignition or fuel, making it hard to figure out. Either way, a vacuum leak is trouble because it lets in air the engine isn’t expecting, messing up the air and fuel balance.

Engine Vacuum Leaks Test

How do you identify if a vacuum leak is causing trouble? Look out for these common symptoms. If your engine shows any of these signs, a vacuum leak could be the reason.

Symptoms of an Engine Vacuum Leak

If your engine is idling too fast and you can’t get it to slow down, even after trying to adjust the carburetor or air bypass, then air might be sneaking past the throttle somewhere. This could be happening at places like the gaskets, hoses, or fittings in the carburetor or throttle body, or even around the fuel injectors. Another thing to check could be a worn throttle shaft.

If your engine is running rough or stalling, it might not just be because of a fancy camshaft or valve overlap—it could also be due to a vacuum leak. When there’s a serious leak, it can mess up the air/fuel mixture so much that your engine won’t even idle. An EGR valve stuck open or the wrong PCV valve can have the same effect. This rough idle is called “lean misfire,” where the fuel mixture is too lean to ignite properly, causing misfires and high hydrocarbon readings in the exhaust, which could make your vehicle fail an emissions test.

Feeling a hesitation or misfire when you step on the gas? It could be because of a vacuum leak, but it might also be caused by a weak accelerator pump in the carburetor, dirty injectors, or ignition issues like a cracked coil, worn spark plugs, or plugs with the wrong gap.

If you’re trying to adjust the idle mixture on an old carburetor engine and the idle speed doesn’t change much no matter what you do with the adjustment screws, you might have a problem with the carburetor or a vacuum leak.

Remember, vacuum leaks mostly mess with idle speed. When you’re driving faster or stepping on the gas, they don’t make as big a difference because there’s already a lot of air entering the engine.

TIP: If you’ve got a scan tool, check out the Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT) values. Usually, they should be around plus or minus 8. But if they’re higher, like +10 or more, your engine might be running too lean. Try revving the engine to 1500 to 2000 rpm and holding it for a bit. If the STFT value goes back to normal, it’s likely there’s a vacuum leak at idle. If it doesn’t change much, then it’s probably a fuel delivery problem like a weak fuel pump or dirty injectors, rather than a vacuum leak.

Vacuum Diagram

What Is Intake Vacuum

You’ve got intake vacuum in the intake manifold because of how the engine’s pistons pump and the throttle valve restricts airflow. If it weren’t for the throttle cutting off air flow, there wouldn’t be much vacuum in the intake manifold, kind of like in a diesel engine. But having intake vacuum isn’t all great—it causes pumping losses and makes the engine less efficient.

In older carbureted engines, vacuum helps to draw fuel into the engine, pulling it through different circuits. If there’s a vacuum leak, the engine might show signs of running too lean, like misfires, hesitation, stalling, or a rough idle. But these symptoms can also happen because of a clogged catalytic converter, a leaky EGR valve, or issues with valve timing, all of which reduce intake vacuum.

Engines with Multiport Fuel Injection and Gasoline Direct Injection don’t rely on vacuum to bring fuel into the engine. Instead, fuel is sprayed in under pressure. However, even though they work differently, most of these engines still have a throttle to control airflow and engine speed. And just like older carburetor engines, the throttle body on these engines also creates a restriction in airflow, which leads to vacuum inside the intake manifold.

How Much Range Intake Vacuum Is Normal?

For most engines, the intake vacuum should stay steady between 16 and 22 inches of Mercury (Hg). If it’s lower, there might be a vacuum leak or one of the other issues we talked about. If the reading drops slowly while the engine is idling, it’s likely because of an exhaust blockage. And if the vacuum reading keeps going up and down, it could mean there’s a leaky valve or worn-out valve guides that are letting vacuum escape.

What if There’s a Leak and Intake Vacuum Is Low??

Even though fuel-injected engines don’t need intake vacuum to bring in fuel, vacuum leaks can still mess up the balanced air and fuel mix by letting extra air sneak in. This can cause similar problems to a vacuum leak in a carburetor engine, like misfires, hesitation when accelerating, a rough idle, and sometimes even stalling. Common places for leaks include injector O-rings, intake manifold gaskets, the idle air control circuit, and the throttle shaft.

Fuel-injected engines also need intake vacuum to control the pressure of fuel behind the injectors. To get the right amount of fuel, there needs to be a steady pressure difference. That’s why the fuel pressure regulator is connected to intake vacuum. When vacuum decreases, it opens a bypass in the regulator, sending excess fuel back to the tank. This keeps the fuel pressure steady, adjusting it as needed to maintain the right air and fuel mix. If there’s a vacuum leak, it messes up this balance by reducing vacuum and increasing fuel pressure.

Measuring Intake Vacuum

You can measure vacuum using a vacuum gauge. Most of them show measurements in inches of Mercury (Hg), but you might also find some that use inches of water (H2O), kilopascals (kPa), or bars. For example, one inch of vacuum in inches of Mercury equals about 13.570 inches in water, 0.4898 pounds per square inch (psi), or 3.377 kilopascals (kPa).

How To Find A Vacuum Leak

A good way to spot a vacuum leak is by checking all the vacuum hoses and connections directly (visually). Check and see if any hoses are disconnected, loose, cracked, or if there are broken fittings. Sometimes, you might get lucky and find the problem quickly, but other times, it can take a lot of time and effort—it’s like searching for a needle in a haystack. And if the leak isn’t in a hose but in something like a gasket, worn throttle shaft, or injector O-rings, you might never find it just by looking.

To quickly find intake manifold vacuum leaks, use propane. Attach a rubber hose to a propane bottle, open the valve for a steady gas flow, then hold the hose near suspected leak spots while the engine idles. If there’s a leak, propane gets pulled in. This alters the air/fuel ratio, causing a noticeable change in idle speed and smoothness. Remember to disconnect the idle speed control motor on engines with computerized idle speed control.

In the same way you can also use aerosol carburetor cleaner. But be careful! The solvent can catch fire easily, so avoid smoking or using it near anything that sparks, like arcing plug wires. Just spray the solvent on suspected leak spots while the engine is running. If there’s a leak, the solvent will be pulled into the engine, causing the same effect as propane. You’ll notice a sudden change in idle speed and it’ll smooth out.

Aerosol Carburetor Cleaner

TIP: If you have a scan tool, keep an eye on the Short Term Fuel Trim (STFT) value while you’re using carb cleaner or propane to check for vacuum leaks. If there’s a leak and some of the cleaner or propane gets pulled in, you’ll see the STFT reading briefly drop. This confirms you’ve found a leak (but remember to keep checking because there might be more than one!).

Finding Engine Vacuum Leaks with a Smoke Machine

A safer method is using a smoke machine. These machines blow fake smoke into the intake manifold, sometimes mixed with a special dye to make leaks easier to spot. Next, you keep an eye out for smoke coming from hoses, gaskets, or cracks in the manifold, or you can use a UV light to spot the leak. This equipment is often necessary to find tiny air leaks in the EVAP (evaporative emissions) control system. Smoke machines can be expensive, ranging from $600 to $2000 or more, so they’re mostly used by professional technicians.

Finding Leaks With An Exhaust Analyzer

You can also use propane with an exhaust analyzer (just don’t use carburetor cleaner because it can harm your analyzer!). Vacuum leaks in the engine usually cause HC (hydrocarbon) readings to go up and down, so an infrared exhaust analyzer can (1) confirm if there’s a leak and (2) pinpoint where it is using the propane method.

With an analyzer, you can diagnose two types of vacuum leaks. The first kind is a general leak, like from the PCV hose or brake booster, which makes the mixture too lean. You’ll see a very low CO reading and slightly higher fluctuating HC reading, with a high O2 reading. The second type is a “point” leak that only affects one or two cylinders, such as a leaky manifold gasket or a crack in the manifold runner. For this, you’ll get a normal or low CO reading but high fluctuating HC readings, with high O2 levels again.

To find a leak, try using propane at suspected spots until you see the idle getting better or notice changes in the HC/CO/O2 readings. Once you find the leak, the idle should get smoother, HC and O2 levels should go down, and CO should go up.

It’s worth mentioning that having an idle mixture that’s too lean can also cause HC readings to fluctuate, just like a vacuum leak. But there’s a simple trick you can try to figure out which one it is. Temporarily make the idle mixture richer by putting a clean shop rag over the top of the carburetor until you get about 1.5 to 2.0% CO. If the engine runs smoother and the HC drops and stays steady, then the issue is likely a lean idle mixture adjustment. However, if the HC still fluctuates, it means the engine is still too lean in one or more cylinders, suggesting a vacuum leak.

Electronic Vacuum Leak Detection

If you’re into gadgets, there are electronic devices made for finding vacuum leaks. These detectors beep or flash when they pick up ultrasonic vibrations that signal a vacuum leak. They work by listening for specific noise frequencies using a sensitive microphone. While they’re very sensitive, sometimes they might pick up on small leaks that aren’t really an issue or detect “false” leaks like the sound from arcing inside the distributor cap or regular bearing noise in the alternator.

Using Air Pressure To Find An Engine Vacuum Leak

Another way to find a tough vacuum leak is by pumping about three pounds of regulated air into the intake manifold. You can do this by connecting a regulator to your air compressor hose and attaching it to a vacuum fitting or the PCV valve fitting on the intake manifold, carburetor, or throttle body. But be cautious not to use too much pressure, or you could make more leaks! Then, with the engine off and air flowing in, spray soapy water on suspected spots. If bubbles appear, you’ve found the leak.

You can use a “Smoke Machine” that uses mineral oil vapor just like the others. It pushes the vapor gently into the intake manifold at a light pressure of 3 PSI. If you see any “smoke” coming out of a hose or gasket connection, you’ve found the leak. You can also mix ultraviolet dye with the vapor to make the smoke easier to spot when you shine a UV light on it.

You can use a hand-pump to apply vacuum to different hoses and circuits to check if they hold vacuum. However, this method requires tracing the whole circuit to find its end and disconnecting and plugging any parts that don’t stop at a diaphragm or valve.

Replace The Vacuum Tube

How To Repair A Vacuum Leak

Replace leaky vacuum hoses. If a hose’s end is loose or cracked, cutting it off and reattaching it might temporarily fix the leak. But if the hose is old or brittle, you should replace it. Cutting hoses shorter could cause new problems, like rubbing against other parts or coming loose due to engine movement and vibration. Make sure to use the right type of replacement hose—either PVC hose or vacuum hose that can handle fuel vapors and vacuum without collapsing. Also, ensure the replacement hose is the same size and length as the original.

For vacuum leaks at the carburetor or throttle body base gasket, first try tightening the mounting bolts. If that doesn’t fix it, replace the gasket underneath. If there’s a heat insulator or adapter plate underneath, it might need replacing too, depending on its condition. While the carburetor or throttle body is off, use a straight edge to check if the base is flat, along with the manifold. If you find any warped surfaces, they might need resurfacing or replacing the component.

Vacuum leaks around the carburetor or throttle body throttle shaft can only be fixed by resleeving the shaft, which basically means replacing the carburetor or throttle body with a new or remanufactured one.

For vacuum leaks around the intake manifold gasket, start by tightening the intake manifold bolts again. Work your way from the center out, following the recommended tightening sequence. If that doesn’t work, you’ll need to remove the intake manifold and replace the gaskets. Sometimes, the mating surface of the intake manifold or heads isn’t flat, so check both with a straightedge. If they’re warped, you’ll have to resurface them on a milling machine. Another thing to watch out for is if the heads have been milled or resurfaced to increase compression. In that case, you’ll also need to machine metal off the bottom of the manifold where it connects to the block to maintain proper alignment. If you don’t, the manifold will sit too high, and the ports and bolt holes won’t line up.

If the EGR valve isn’t closing properly because of carbon buildup on the stem or valve seat, cleaning it might fix the issue. If not, you’ll need to replace the EGR valve with a new one.

If your power brake booster is leaking, replace it. But before doing that, double-check to ensure it’s the booster itself leaking and not just the hose or check valve.