Air leak?

[craze]

My car ('92 325i) seems to run fine and puls pretty hard. Not to mention it has been in two different shops in the past month for various other issues and neither of them have said anything about this. When I have the hood up and the engine running there is a sound which can only be described as a rush of air as in an air leak of some sort and it seems to go away when the car is revved. Has anyone had any kinda experience with this.... is it normal? A couple different (non-mechanics) have noticed the issue and we all thinks its a bit odd.

--j

[Creo]

I have that same question, any ideas?

[Jared at Pelican Parts]

Hmm.. Have you checked the vacuum hoses for leaks?

[Creo]

To be honest, as lame as this is, I really don't even know how/where to check for vacume leaks. On that same note, I've heard that the intake man. is just loud sometimes.

[GeoPappas]

Originally posted by Creo
I really don't even know how/where to check for vacume leaks.

When the engine is running, pop the hood and try to track down the specific area where the noise is coming from. This can be a little tricky, but if you try it a few times, you will probably find the problem.

Check that all rubber hoses are in good condition (i.e., that there are no cracks). Squeezing them a little bit sometimes helps to see how pliable they are.

Also, check that all rubber hoses are connected to something, and that the connection is tight.

You can also buy a can of carb cleaner and spray it whereever engine parts connect, and hoses connect.

If the idle drops consistently while spraying it on a specific part, then you have probably found a vacuum leak.

[Dave at Pelican Parts]

A fair number of cars in general are pretty noisy when air goes through the idle bypass in the throttle body. I know my 914 certainly is!! My Honda is less so, but there is a noise if you listen for it with your head in the engine bay. These sounds either get quieter as the throttle is opened, or are overwhelmed by the rest of the engine noise.

I don't know about the E36 (never went looking in Mom's car), but it's possible that this is quite normal.

You can isolate where it is coming from by using a mechanic's stethescope, or just by using a length of vacuum hose. Hold the one end of the hose to your ear, then put the other end of the hose in various places and see if you hear the noise in that one ear.

--DD

[craze]

it seems like it come down the centerline of the engine.... where the fuel rail is....

[Jared at Pelican Parts]

Im going to second Dave on this one. Im thinking it may just be the sound of the bypass valve..

[Creo]

What exactly does the bypass valve do? Sorry for asking so many questions, just like to know.

[Jared at Pelican Parts]

The idle bypass valve allows a certian amount of air to pass the throttle butterfly to keep the ngine idling.

[th@nks!]

Strange to hear..... This might not apply but my family owns a 94 volvo 940 turbo and all of a sudden there was a loud hiss kidna noise that was on whenever the engine was on, and particularly loud at idle. Try this test:

Get your hands on some carb cleaner and spray it around the area in which you hear a leak. it does no harm to spray it anywhere. It might be subtle but if there is a leak somewhere on your intake whether it be a hose or a gasket the engine will rev a little bit higher for a second, you'll either hear it or you should have someone look at your tach when you're doing it. If it happens, you found a leak! As for diagnosing exactly WHAT it is you're on your own. Hoses are easy to replace but it could be a little more difficult. In my case on the ol' volvo, it was the gasket between the intake manifold and the head on one cylinder ergo i bought a new gasket. Also look for leaks around your injector seals. I did the gasket and the injector seals in about 3-4 hours. (i had some mishaps though.... but when is it that you work on a car and EVERYTHING goes as planned? hehe)

Try the Carb Cleaner Test and post back. Ask questions if you think i can help.

[th@nks!]

and i just now read what geopappas said..... i feel silly

[Jared at Pelican Parts]

Originally posted by th@nks!


Get your hands on some carb cleaner and spray it around the area in which you hear a leak. it does no harm to spray it anywhere.

Yeah, be sure to spray a flammable petroleum-based cleaner in the general vicinity of the coil or fuse box, or better yet, spray it directly on a hot exhaust manifold.

DO NOT do this! Carb Cleaner is some of the most flammable liquid there is! Think about it, it's used to break down gasoline it's pretty wicked stuff.


If you want to check for air leaks. fill a spray bottle full of water and soap, then spray. The bubbling will tell you where the leak is.

This one is almost as bad as one I read where a guy wanted to clean out his muffler by filling it with gasoline and rocks, then shaking it.

Geez, use some common sense here.

[kingnine]

Right! Soapy water is the best way (besides the new smoke equipment) to check for leaks in an intake system. Besides, even if you spray carb cleaner where there are no leaks, it can find its way through a perfectly good gasket or seal, and cause a false reading. It can also melt or discolor some plastics and harm electronics.

[th@nks!]

worked fine for me jared..... i didn't say go overboard.... dont try to tell me that you've never played "flame thrower" with some hairspray and a lighter. In the future i'll say to be more careful.

[Jared at Pelican Parts]

Actually thanks, a friend of mine back in the day was permanently disfigured by doing the trick with the hairspray and the lighter. It exploded in her face. Have you ever seen the effects of a flashback explosion? I have. I've seen how they do skin grafts up close, it's not a pleasant experience. Glass eyes are also not attractive.


Under NO circumstances do I advocate ANYONE use carb cleaner to check for leaks. Carb cleaner has stuff like tolulene and zylene in it. this stuff is not only incredibly flammable, it melts rubber and plastic. There's a reason it's used to break down gasoline when you are rebuilding a carb. I honestly cant see ANY benefit from using carb cleaner, whereas soapy water will bubble and let you know exacty where the leak is.

I normally dont comment like this on the boards, however when I get a post like this that can possibly compromise the safety of people reading this, people who may have never worked on a car before, or not know a lot about engines, I feel responsible to make a comment saying that this is NOT the thing to do.

[Drews323i]

The soap and water is by far the way to go. But if its a vacume/air leak it shouldnt be blowing bubbles it should be sucking air/soap water in? If its blowing air out there is a major problem. Just a thought?

1980 323i
1985 325e
1994 325i

[th@nks!]

lol i wasn't gonna bother saying it drews. i think it will make a bubbling SOUND or something.

[Javier at Pelican Parts]

Gentelmen by all means follow Jareds advise on this one! he is CORRECT!
Alittle overspray onto the manifold could ignite and the rest is history, and remember its also the vapors that are also flammable.

[Javier at Pelican Parts]

Also as I read more indepth if ther is a leak or vac suction the water and soap trick will point this out, you just have to have eagle eyes, look were the liquid is falling. this is by far the best, easiest, cheapest, and safest method