Cracked Engine Vacuum Hose

Vacuum Leak Symptoms & Causes

In Engine by 12 Comments

A 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 the leak the fastest way.

Let’s begin by checking to see if 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 also be a lot of other symptoms from a vacuum leak, because of the importance of a closed system. The air flow meter measures the air entering the engine and if you have a vacuum leak, the engine will inject uncalculated air and the air/fuel mixture will become faulty.

1. Rough Idle/High Or Low Idle RPM

Vacuum Leak Symptoms

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 trying to control it and it will result in strange idle symptoms

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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 normally results in a lean mixture and lean mixture can result in slow acceleration.

3. Check Engine Light/Trouble codes

Vacuum leaks 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 often get trouble codes about a lean mixture or misfire trouble codes.

4. Misfires & Backfires

You often get misfires & backfires from a vacuum leak. When you have a vacuum leak, the air/fuel mixture can get so lean that your car cannot 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 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 find where the sound comes from.

How to Find a Vacuum Leak

Autool Smoke Machine

There 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 borrow one from a car repair workshop or buy 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 borrow 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 is wrong. The airflow meter will calculate the amount of fuel that is injected into the engine, then it calculates 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 become lean and it can result in trouble codes and rough runnings.

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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 controlling 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 below. If you have any other car questions, you are welcome to ask them at our homepage.

12 thoughts on “ Vacuum Leak Symptoms & Causes ”

  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. Getting cylinder 4 miss fire, changed spark plugs and coil packs still getting engine light coming on any ideas why

  6. I’ve usually overlooked vacuum leaks as being the culprit of a lot of error codes stored in the cars ecm. When we skip checking for vacuum leaks, those stored codes can and usually do take us down an expensive road. Trying to fix something that’s not broken can be a real headache, not the forget that all those electrical parts are usually none returnable items. I’ve been down that road too many time. $5.00 worth the vacuum hose can save you $250.00 worth getting parts you do not need. What I’ve learned is while your vehicle is running well, take the time then to check your vacuum line system. We tend to either skip certain things or we end up getting in over our heads because we’re now trying to fix two problems at the same time. That is why I usually go over my vehicles before an issue arises simply because I know me and I do not work well when my mind is all boggled up with all those false positive error codes, while I tell myself that I should have checked these lanes a month ago…..

  7. Cant find vacuum leak left side 2005 Cadillac deville. Replaced intact manifold gaskets and pcv hose to manifold. Still hear vacuum leak. Replaced coil pack #2 cylinder. Replaced injector#2 cilinder.

  8. I have a Holden Astra 2003 model. I have had it looked at by 4 different mechanics. My traction control light has been staying on and the car stalls at traffic lights now and then or chugs while driving once a day. Holden diagnosed it with s code P0210 Throttle Position Sensor. My dad got a reconditioned one and the car wouldnt run properly with it. The mechanic said it was not the throttle position body ant the accellotator sensor tested ok as well. So no one has been able to diagnose.

  9. Some people use propane to find the leak. Same as using carb cleaner, just let a small amount of propane escape from your unlit torch around the intake. When you hear the idle go up, and revs get higher, that is where your leak is located. Be careful, it could catch fire.

  10. I have got an engine check light which comes on after driving for a few kilometres and sometimes it doesn’t come on, also when l reach a speed of 100km/h the engine gets jumpy like it’s about to switch off and the temperature gets too hot such that the engine switches off on its own and you’ve to wait for 45 minutes or more to cool and start it again. What might be the causing this? please help

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