In the course of helping my friend sort out a lean condition with his Mercedes GLK280 we have to pinpoint the source of a vacuum leak in the intake plumbing.

The interwebz tend to recommend having the car taken to a mechanic so that they can perform a smoke test.

I don't know of anyone in my city who owns a smoke machine, individuals or garages alike.

Surely, there must be alternative means to determine the exact source of a vacuum leak? I'm open to all suggestions that are readily available to a home mechanic.

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    Possible duplicate of Finding a possible vacuum leak
    – Xen2050
    Commented Apr 28, 2016 at 1:18
  • Just a friendly suggestion. If you are to use the pressurized method, be aware that if you have dry rotted fittings or hoses, then you have the possibility of created more issues then you originally had as the pressure may cause more cracks and even split bad hoses.
    – Warren
    Commented May 16, 2018 at 17:44

10 Answers 10


There are several methods to detect vacuum leaks of unmetered air into the fuel delivery system

One method is to use soapy water in a spray gun.

The real issue here is how are you going to detect a vacuum leak and how do you know you've found it.

What is a vacuum leak in this context related to a fuel delivery system?

Unmetered air in a fuel system is considered to be between the butterfly in a throttle body or a carburetor and the cylinder head. The components involved are:

  • Throttle body or carburetor

  • Gasket between the throttle body and carburetor and the intake manifold

  • Intake manifold

  • Intake manifold gasket between the intake manifold and the cylinder head

  • Vacuum lines to operate various components that require vacuum such as the brake booster

This is a pretty big list and finding an efficient method to test all of the joining points plus the accessories that may require a vacuum can be daunting. Efficiency in troubleshooting this issue is important.

Symptoms of a Vacuum Leak

  • High Idle

  • Rough Idle

  • High Idle dropping rapidly to a stall

  • Stalling without driver intervention. (Difficulty Idling)

Issues with Air/Vacuum Leaks

Unmetered air in a fuel system adds oxygen to the mix. This changes the air fuel ratio the components are trying to control after the control mechanism, the throttle body or carburetor. The additional oxygen changes the air fuel ratio making the car run poorly. The damage that unmetered air can do to the overall system includes:

  • Increased combustion temperature due to more oxygen

  • Increased temperature of an exhaust valve due to more oxygen

  • Burned valves due to overheating

  • In rare cases, points in the combustion chamber and piston that become molten during high temperatures leading to complete failure of the engine

Soapy Water Test

The Soapy Water Test has you putting water in a spray gun with a bit of soap on areas of the intake system that are suspected air leak points. You may need to occasionally rev the motor a bit to keep it idling or increase the idle RPM while troubleshooting to prevent stalling if that is one of the problems you are experiencing.

As you spray the soapy water on suspect points the idle with flatten out and become normal as the soapy water blocks the air leak or crack temporarily. Continued spraying on the suspect points to temporarily block the unmetered air will ultimately reveal the culprit of the air leak.

Once the failing component or vacuum leak is discovered you would fix it accordingly and then repeat the tests to ensure success.

Tools Necessary to Complete Troubleshooting


One tool overlooked here is an ultrasonic leak detector.

They work by listening to the noise of a leak. Obviously they would be better used in a totally quiet environment, but it's a possibility. I've used them to determine automotive refrigerant leaks, and for water leaks from outside to inside the passenger compartment of a car.

For vacuum, dunno how well they'd work, but it may be worth a shot. Basically you hold a microphone device to suspect areas and the system listens for the "micro" noise (like escaping air) Here's a reference.

For the body water leak issue, we used a ultrasonic sound source inside the car, closed all the windows and doors and checked the entire car for sealing.

Obviously that isn't optimal for engine vacuum, but you get the idea. In the alternative you could hook up a vacuum source to the intake manifold with the car totally off and then do a totally safe and sane vacuum leak test. (I still shudder when I thing about getting my hand caught in an alternator cooling fan on a vehicle I was working on.. Thwack!) Please be careful out there.


Some people have stated listening for a vacuum leak noise and if located within a vicinity disconnect and add a new line of the same diameter and get a cigar and blow smoke from the cigar in the line. After blowing smoke in the new line cork the end and look around to see if any smoke is coming through.

If you don't smoke you could always look into getting a food smoker:

enter image description here

  • 1
    Given the context of my problem, I find the name of the food smoker in the image rather amusing
    – Zaid
    Commented Apr 27, 2016 at 19:04

One other way to look at this, sort of from a different direction. As I get older I really like being able to do more with less and do so safely. I think its sometimes awesome that an old guy with barely any tools can out perform the young guys with an expensive tool box. (And yeah, I used to have many of those tools... now I'm trying to downsize a bit.)

I used to design vacuum parts for Detroit based OEM. My take was it was much easier to just do a really good physical inspection on all the parts involved then it was trying to use fancy tools to find the leak.

And because you can do the inspection with the vehicle off, its much safer. You need to physically look at each and every vacuum component in the entire system from engine back.

  • Where every there was a soft joint in the vacuum system you verify that the rubber is soft and pliable and that the connection is solid.
  • I will say the rigid nylon vacuum lines rarely have issues.
  • One problem area to inspect is the brake booster, because the vacuum stuff is hidden. Best test there is to use a portable hand held pump and test that independently (again with engine off.)
  • And then there are the Heating Ventilation Air conditioning (HVAC) automotive systems from Mid 1970's till 2000 or so. Those are definitely different beasts, but generally a hand held pump would go a long way for troubleshooting those parts.

And there was one part that always made me laugh. One part I was responsible for was a vacuum reservoir. What was funny was that the bigger it was the less it held. (Yeah, I know, it sucks. Stupid engineer joke. ha!)

  • 1
    Together with a mirror & flashlight, this should find any hoses that were knocked/fell off or are cracked/falling apart, loose/cracked parts, etc. Should really be step zero for almost any mechanic work (if only engines weren't usually so dirty & greasy)
    – Xen2050
    Commented Apr 28, 2016 at 1:36

@DucatiKiller gives a very good assessment of how to look for leaks. Using these same efforts, you can also use gas from a propane torch around the same areas described. When the gas finds a vacuum leak, you'll find a definite change in engine RPM due to the fuel which is found. Mind you, you are opening the valve and using the gas itself, unlit. You don't even have to use enough, as the vacuum will find the gas and make it known.

Also, along the lines of what @DucatiKiller is stating ... you don't even have to use soapy water, just water in a spray bottle. This may actually be a better alternative because plain old water does not leave a soapy residue behind when it dries. Also, you can test the same area several times. When the water dries out, it goes back to the way it was running, then throw another dose onto the affected area to recheck your findings.

  • 2
    Are there webpages or mechanics that really recommend spraying unlit propane around a car engine? It sounds insanely dangerous. I've seen propane grills explode in a fireball, and engines have plenty of sparks & hot spots when they're running, I can't imagine spraying propane around is a safe idea. If it leads to a fire or injury, insurance would probably deny coverage too (or at least try to)
    – Xen2050
    Commented Apr 28, 2016 at 1:00
  • 1
    Wow, I did find a mechanic recommending using unlit propane to check for a vacuum leak, he says to use a rubber hose to direct the propane, and even says it's safer than spraying carb cleaner around the engine - but both methods have the potential to start a fire, definitely have a fire extinguisher nearby. So +1 I guess
    – Xen2050
    Commented Apr 28, 2016 at 1:10
  • @Xen2050 - You really aren't using that much propane ... just enough to affect how the vehicle runs, but point well taken. Commented Apr 28, 2016 at 1:11
  • @Xen2050 Propane is probably the safest thing you can use for this method, as it will disperse quickly. Unlike carburetor cleaner and other flammable sprays, which can set your things on fire if used incorrectly. Commented Apr 28, 2016 at 8:05
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    Just for the record: Don't smoke your cigar while doing this. Or, do it, with video, and sign a waver first.
    – 3Dave
    Commented Dec 2, 2016 at 23:12

This is how I diagnosed a vacuum leak in my car. I removed the air filter from the intake pipe (mine was a turbocharged lancer evolution with an after market intake pipe) started the car. (Make sure your surroundings is not dirty and is very quiet). After starting the vehicle, I placed a book to block the air being sucked into the intake pipe (do not place a plastic bag as it might get sucked it) and obviously the vehicle died in few seconds. I could hear a buzzing noise which was the vacuum leak (as there will be vacuum inside the system) and I just tried to pin point the source of the leak (which was the brake master cylinder O-ring). There are many ways to check for a leak and this is one of the cheapest ways.



Another way that has not yet been mentioned is artificially creating a vacuum or pressure in the intake. Vacuum is naturally there when the engine is working, but often the iconic hissing sound a leak makes is overwhelmed by the natural sounds of a working engine in the case of tiny leaks. If you reproduce the vacuum or create pressure with the engine off - you will have a much easier time locating the leak.

This can be done by either pumping or sucking the air out of the intake, for example by mounting a foot pump, compressor, vacuum cleaner (has to be far enough to not overwhelm the hissing, obviously) to the air pickup by the use of homemade adapters or simply tape. However do note that pressure/vacuum that is too high can damage other parts of your intake tract, so take extreme care not to use more pressure/vacuum than necessary.

Combined with the cigar, soapy water, hose stethoscope and a mirror this can be an extremely effective method.

  • 1
    I like this suggestion. For forced induction vehicles the problem is usually the opposite... they need to pressurize the system to manifest a boost leak. The approach is just as applicable to naturally aspirated vehicles.
    – Zaid
    Commented Apr 28, 2016 at 8:39
  • 2
    A friend used the hose from his compression tester, which happened to have an air compressor-friendly quick-disconnect fitting on the end. Hooked it up to the compressor, threaded it into a spark plug hole, made sure the valves in that cylinder were open, and pressurized the thing to 100PSI. All vacuum leaks were then easily identifiable (audible). Kind of wanted to smack him in the back of the head. Smartass.
    – 3Dave
    Commented Dec 2, 2016 at 23:14

If it's a vacuum leak, it will be sucking in air, and you'll probably be able to hear it. With the rest of the engine making noise, using something to pinpoint the sound would help:

  • Take a piece of rubber hose and hold one end up against your ear, moving the other end around the engine listening for the hissing/sucking sound.

It's kind of like using a doctor's stethoscope or a mechanic's stethoscope if you could take off the "end" piece and just have an open hose.

These two mechanics have decent YouTube videos on finding vacuum leaks:


There are usually two parts to this diagnosis:

  1. Establish that you have a vacuum leak

  2. If confirmed, find out the source of the malaise

Confirming the presence of a leak

My favorite trick to establish the presence of a vacuum leak involves physically blocking the air intake(s) using your hands or something suitable. It usually helps to remove the air filter(s) as it reduces the cross-sectional area that needs to be blocked.

If the engine continues to run after blocking the intake, this is a clear indication that the engine is drawing air from an alternative source, i.e. a leak.

Pinpointing the leak

Others have outlined several different techniques in their answers. Here is one more that is useful for someone who has access to an air compressor:

  • block off the air intake
  • introduce pressurized air into the intake
  • spritz soapy water from a spray bottle on suspect areas. The presence of bubbles will signify air escaping from the intake system

This approach is aptly demonstrated by Jafro in this video.


I've used a stick of burning incense. Cheap and simple!

  • 1
    how do you get it into the system? Commented Apr 27, 2016 at 21:21
  • This was done from the outside - not injected into the intake. Place the stick around the easily accessible areas. For those that are not easily accessible just gently waft/wave the smoke near possible areas. The key is gently - since the leak will be most likely be strong enough to accelerate the air faster than you are waving the smoke. I've used this method as well as the soapy water method. I found the incense method more obvious since the bubbles from applying the soapy water can confuse you with possibly being the leak.
    – Dave Black
    Commented Apr 27, 2016 at 21:32
  • The fan when the engine is running would seem to be problematic for this solution. Commented Apr 27, 2016 at 21:35
  • True. luckily, I didn't run into that problem since the engine hadn't been on for very long. If the fan was on, you could just disconnect the fan harness.
    – Dave Black
    Commented Apr 27, 2016 at 21:36

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