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Old 09-02-2011, 12:27 PM   #1
porphyre
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Default How to find exhaust leak?

Hey guys.

My state requires annual emissions inspections in order to renew license plates. Took my car in (FWD, 4 cylinder, GM sedan) and they said it had an exhaust leak.

Now, the exhaust on this car broke about 3 years ago. It broke right after the mounting flange of the cat converter - cat back join. It broke on the cat back side. A local shop fixed it in about two minutes by welding a bead around it. Good as new. Literally charged me $5. Took longer for the lift to raise and lower than it took him to weld it.

The inspector said the weld had cracked and was leaking.

So, my preferred exhaust shop is closed for the holiday. I took it to the shop referred by the inspector. That guy said the weld was not leaking, it was the gasket in the flange. He also said the flex-pipe was trashed and would break when he dropped the exhaust to replace the gasket. He wanted $150 to do the gasket and replace the flex pipe. He said the weld wasn't leaking, it was the gasket, and it'd be $150.

For $150, I'll wait until my regular shop is open and spend $200 for a whole new cat-back (which is needed anyway, my exhaust is trashed)

My question is, how do I find the leak myself? I don't trust this guy because he didn't find a leak I KNOW is there downstream by the muffler (seen it drip and bubble condensation in the mornings). To my eye, the weld does look cracked on one side.

The guy waved his hands over the pipe while a helper revved the motor.
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Old 09-02-2011, 12:35 PM   #2
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Default Re: How to find exhaust leak?

Quote:
Originally Posted by porphyre View Post
The guy waved his hands over the pipe while a helper revved the motor.
That is how I find a leak, if no noise indicating where it is located
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Old 09-02-2011, 12:37 PM   #3
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Default Re: How to find exhaust leak?

That water dripping smoke hole in the muffler may have been factory installed. They do that so the condensation doesn't rust the muffler out from the inside.
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Old 09-02-2011, 01:10 PM   #4
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Default Re: How to find exhaust leak?

Looking for soot trails will give you the general area of the leak. Using your hand as the shop did is how you locate the exact spot of the leak. You don't have to rev it to find it though, it just makes it easier.
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Old 09-02-2011, 01:49 PM   #5
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Default Re: How to find exhaust leak?

Smoke machine is the best way. that I know of to find an exhaust leak. The cheap way is to use a lit candle and have someone hold their hand over the exhaust. The leak will move the flame or blow it out. It also doubles to find fuel leaks but that normally isn't pretty if you find one. lol
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Old 09-02-2011, 01:58 PM   #6
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Default Re: How to find exhaust leak?

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Originally Posted by mrholeshot View Post
Smoke machine is the best way. that I know of to find an exhaust leak.
That's how I usually find them, smoke machine and a bright light. Though if you've got a home-built MechanicNamedJohn style machine I wouldn't recommend doing it that way. Party style smoke machines usually use a glycol based smoke solution, and glycol is one thing that kills catalytic converters.
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Old 09-02-2011, 02:03 PM   #7
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Default Re: How to find exhaust leak?

Now don't go nuts on me guys.. but isn't there a trick with adding in some tranny fluid
to a running engine, into the carb, and watching for the smoke? Or is that an
old shade tree mechanic trick? By the way, I have never done this, but have heard
of it..

(Putting on my flame suit)
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Old 09-02-2011, 02:28 PM   #8
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Default Re: How to find exhaust leak?

Thanks for the insight, guys. Guess I'll spend some time under there with a candle and looking for soot.

The exhaust is loud by design, so finding the leak by ear is impossible.

The weld is literally right next to the flange. Practically on it. Guy seemed sure it was the gasket and not the weld.

wornoutoldman - True that, but that's not the case here. The dripping is on the weld where the OEM piping was cut. OEM was a dual exit muffler which was cut off and replaced with a single exit 18" resonator.
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Old 09-02-2011, 07:24 PM   #9
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Default Re: How to find exhaust leak?

I used to put on a heat resistant glove and hold a rag over the tailpipe and have someone listen for the leak (old school I know, but it worked).
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Old 09-02-2011, 08:44 PM   #10
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Default Re: How to find exhaust leak?

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I used to put on a heat resistant glove and hold a rag over the tailpipe and have someone listen for the leak (old school I know, but it worked).
best way ive found.
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Old 09-02-2011, 08:51 PM   #11
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Default Re: How to find exhaust leak?

Intake and exhaust leaks are two of the first things you look for if your old harley isn't running quite right. To find an exhaust leak put a teaspoon of oil in the suspect cylinder and the motor will make smoke for a couple of minutes after you start the engine. Some smoke will come out of the leak. Make sure you do this outside or you will smoke the place out. I have done this to my bike and it does work for that.

Chris

Last edited by Shadowdog500; 09-02-2011 at 08:59 PM.
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Old 09-02-2011, 08:58 PM   #12
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Default Re: How to find exhaust leak?

Plug with air fitting and regulated air, inserted in the tail pipe. Listen/feel for leak. Engine off, of course. You'll get some air leaking into engine through open ex. valves; but enough pressure in exhaust system to find the smallest of leaks.

Haven't tried it myself, but it's a method used by the powertrain garages on the proving ground.

Not sure of the pressure, start low and work your way up slowly.

Good luck.

Open flame under the car, be careful and have a helper standing by with a proper extinguisher and ready to pull you out by your legs when you knock yourself out after banging your head.
Just kidding, but be careful if you try that method.
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Old 09-02-2011, 10:14 PM   #13
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Default Re: How to find exhaust leak?

Quote:
Originally Posted by BigAl62 View Post
I used to put on a heat resistant glove and hold a rag over the tailpipe and have someone listen for the leak (old school I know, but it worked).
this is the way ive always done it too, it'll either build enough pressure to push your hand off or you'll hear it leak
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Old 09-02-2011, 10:26 PM   #14
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Default Re: How to find exhaust leak?

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this is the way ive always done it too, it'll either build enough pressure to push your hand off or you'll hear it leak
Please describe in detail this method. I havent worked with many old timers and I would greatly appreciate someone explaining it to me. Thank you.
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Old 09-02-2011, 10:26 PM   #15
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Default Re: How to find exhaust leak?

Best to do the handcheck method as soon as you start a cold engine, that way you can get your hand right at the subject crack and you can feel it without burning yourself. Another way that works if you live in a cold climate, you can observe any leaks "puffing" out of any cracks. But, you only have about five minutes to do that at start up.

My favorite two methods.
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Old 09-02-2011, 10:37 PM   #16
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Default Re: How to find exhaust leak?

150 is reasonable.
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Old 09-02-2011, 10:40 PM   #17
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Default Re: How to find exhaust leak?

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Originally Posted by dan1120 View Post
Please describe in detail this method. I havent worked with many old timers and I would greatly appreciate someone explaining it to me. Thank you.
start engine, go to back of car, put hand over pipe to "block" it and listen for a hiss, not much more to it
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Old 09-02-2011, 11:04 PM   #18
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Default Re: How to find exhaust leak?

I have used almost all of these methods over the years except for the candle. A manifold vacuum line dropped into a container of tranny fluid will show every pinhole. It will also look like you are trying to smoke out the neighborhood. A couple rags and duct tape or bailing wire can also be used to block the tailpipe if you don't have a helper to listen for the hiss.
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Old 09-03-2011, 07:41 AM   #19
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Default Re: How to find exhaust leak?

I'm surprised nobody's mentioned using a stethoscope yet. Go over every joint and weld with a stethoscope with the motor idling and you'll hear any leaks pretty well.
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Old 09-03-2011, 08:06 AM   #20
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Default Re: How to find exhaust leak?

Sea foam works really well. if your exhaust is solid all the way back it will exit the end. if you got a leak well there will be alot of smoke from that one area.
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