vacuum leak symptoms

Symptoms of a Vacuum Leak & The Causes

In Engine by Magnus Sellén5 Comments

vacuum leak symptomsA Vacuum leak around your engines intake manifold can cause a lot of different symptoms to your engine like rough idle or rough acceleration.

The good thing is that a vacuum leak is not hard to identify if you have the right tools and the right knowledge.
I work as a diagnostic technician and I will share my knowledge about the most common vacuum leak symptoms and how to find and fix it the fastest way.

Let’s begin to check the symptoms of a vacuum leak could be in a car engine.

Signs of a Vacuum Leak

The most common symptoms of a vacuum leak are rough idle, rough acceleration, misfires, idle problems and a check engine light. There could be a lot of other symptoms from a vacuum leak also, because of the importance of a closed system. The air flow meter is measuring the air entering the engine and if you have a vacuum leak, the engine will inject uncalculated air and the air/fuel mixture will get faulty.

1. Rough Idle/High or Low Idle RPM

If you have a big vacuum leak, you can often experience a rough, high or low idle RPM. This is because the throttle body is trying to get a stable idle by opening and closing the throttle. If you have a major vacuum leak, the throttle body will get problems to control it and it will result in strange idle symptoms

2. Rough/Slow Acceleration

A vacuum leak will often result in rough or slow acceleration because the engine will get the wrong air/fuel ratio when you have a vacuum leak. A vacuum leak does normally result in a lean mixture and lean mixture can result in a slow acceleration.

3. Check Engine Light/Trouble codes

Vacuum leaks do often result in a check engine light and stored trouble codes in the engine control unit. Because the air flow meter is reading one value, and the vacuum leak will cause the air/fuel mixture to get leaner, you do often get trouble codes about a lean mixture or misfire trouble codes.

4. Misfires & Backfires

You do often get misfires & backfires from a vacuum leak. When you have a vacuum leak the air/fuel mixture can get so lean so your car can not fire the cylinders properly. It will also often result in misfire trouble codes. Read the trouble codes to identify misfires caused by a vacuum leak.

5. High pitch sound from the engine

Broken vacuum hoses can often result in a high pitch sound because the engine is sucking air at all the time through the leak. If this is the case, you are lucky because it’s a lot easier to identify a vacuum leak if you have a high pitch sound. Just search where the sound comes from.

Vacuum leak from intakeRough/High/Low Idle RPM

Rough/Slow Acceleration

High Pitch sound

Check Engine Light

Misfires & Backfires
Broken Vacuum hose

Faulty intake manifold gasket

Faulty throttle body gasket

Faulty intake manifold
Replace any broken vacuum hoses

Replace intake manifold gasket

Replace throttle body gasket

Replace intake manifold

How to find a Vacuum leak?

autool smoke machineThere are some different techniques to find a vacuum leak. My best tip to find a vacuum leak is with an EVAP smoke machine. You could either lend one from a car repair workshop or one yourself. If you are interested in getting one you could check out our review site: EVAP Smoke machine Review.

If you do not have the money or the opportunity to lend one, you can go with the Soap + Water technique. Just mix soap and water into a spray can and spray all around the intake. If you have a vacuum leak you will often get bubbles from it at the leak area. Because the leak is a vacuum leak, you may not always get bubbles and if that’s the case you could try my other tip.

The other tip is to spray a flammable spray around the intake where you think the leak is located when the engine is at idle. One recommendation is to use brake cleaner because it’s highly flammable. Remember to be very careful, because it could result in a big engine fire if it ignites! When you spray it at the leak, the engines idle will raise up and you will find out in which location the vacuum leak is.

Why is the car running roughly from a vacuum leak?

The car is most often running roughly from a vacuum leak because the air/fuel mixture will get wrong. The airflow meter will calculate the amount of fuel that is injected into the engine, then it does calculate how much fuel the engine will require to get perfect combustion and inject the perfect amount. If you have a vacuum leak, it will inject air from a location after the air-fuel meter and the air-fuel mixture will get lean and it can result in trouble codes and rough runnings.

Large vacuum leaks can also cause high or rough idle because the throttle body is controlling the Idle RPM and when you have a vacuum leak, the engine will suck air from the leak and the throttle body will have problems to control the idle properly.

If you have a vacuum leak from the intake manifold gasket, near a cylinder, just one cylinder can get a faulty air-fuel mixture and it could cause misfires at that cylinder, which could be recognized from trouble codes in the engine control unit on that specific cylinder. To read the trouble codes you should use an OBD2 scanner.


  • The most common symptoms of a vacuum leak are rough/high idle, misfires and a check engine light.
  • A vacuum leak tricks the air-fuel meter and will result in a lean mixture
  • A vacuum leak tricks the throttle body and will result in a high/low idle RPM.
  • Vacuum leaks can be found with a smoke machine, soap + water or with a flammable spray.
  • Vacuum leaks are often coming from the intake manifold gasket, broken vacuum hoses or the throttle body gasket

If you have any other questions about vacuum leaks or want to tell us your story of how you found your vacuum leak, comment down below. If you have any other car questions you are welcome to ask them at our homepage.

Hello I'm Magnus, the owner and the writer of this website. I have been working with cars since I was 16 and I'm specialized with in-depth Automotive diagnostics. Also been driving drifting for the last 6 years. I'm here to give you answers to all your automotive questions and I hope that you enjoy our content.

  1. i suppose just having an untightened hose could cause this problem also?

  2. Even if the throttle body gasket has been changed, if the throttle body hasn’t been tightened down so it has an air tight fit, it can cause a misfire and/or a lean code on one or two banks in my experience. Gaskets can dry out or split, but not tightening down the throttle body can also cause some issues. Have an 05 Mustang GT. The throttle body was removed to be cleaned, along with the MAF sensor. New gasket put in throttle body. It was not tightened down enough so it threw a P0171 (bank one too lean) and P0301 Cylinder 1 misfire. Upon finding the throttle body wasn’t tightened down properly, once done, both codes went away. Sometimes it is just the obvious.

  3. Cheapest way to find a leak is with a cigar. You just breath out the smoke into one of the air intake hose…

  4. A sudden EML has thrown up a lean mixture on a diagnostic test and no fault codes. On lifting the bonnet a hissing of air can be he!rd. Is it now just a case of locating the leak

  5. Due to a very high demand and high ammount of comments, you have to wait for some time for your car questions to get answered. If you want to get fast answers from a certified master technician you can ask your questions here:
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