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What are vacuum leaks

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Old 11-18-04, 12:33 AM
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What are vacuum leaks

What are vacuum leaks? And how do you repair them
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Old 11-18-04, 12:41 AM
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Vaccuum leaks, in my best ability to describe them, are when your hoses are bad, or disconnected, and air leaks out, which means it doesn't get to the carb right.

I guess you fix them by finding them and either replacing or connecting the hoses.
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Old 11-18-04, 01:27 AM
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Engines literally suck air into them, creating a vacuum in the intake system. The vacuum generated is proportional (inversly) to load, and it's put to good use in many ways. One important way, in particular, are your brakes (the booster is vacuum operated)! EFI generally uses a vacuum (MAP) sensor to determine load or an Air Flow Meter (AFM) to measure air flow. If you have a vacuum leak, not only do you loose the vacuum "signal" but it means air is sneaking into the system that is unaccounted for by the Carb or ECU, so the car would run rough and/or stall from the lean AFRs. And your brakes may not work...
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Old 11-18-04, 08:07 AM
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I am thinking of getting an 84 gsl, and the seller is saying that the "Vacuum Lines need to be capped"

What does that mean???? And do you think it will cost a lot to do it?
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Old 11-18-04, 10:12 AM
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Old 11-18-04, 10:20 AM
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Originally Posted by soulja77
I am thinking of getting an 84 gsl, and the seller is saying that the "Vacuum Lines need to be capped"

What does that mean???? And do you think it will cost a lot to do it?
Sounds like he has probably either removed some stuff that wasn't needed and you need to plug holes or reroute some vacuum lines.

OR

He can't find a vacuum leak that is causing a problem and doesn't want to deal with it anymore.

either way if you can do the work youself it should be very inexpensive. Hiring someone to do it you will be just paying them for their time. It could get pretty expensive if they can't find the problem right away.
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Old 11-18-04, 11:08 AM
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Agreed. It can be tedious to find and fix but not expensive. And Blake pretty much explained what it is.
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Old 11-18-04, 11:43 AM
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Originally Posted by jays83gsl
Vaccuum leaks, in my best ability to describe them, are when your hoses are bad, or disconnected, and air leaks out, which means it doesn't get to the carb right.

I guess you fix them by finding them and either replacing or connecting the hoses.
Not exactly right....

Your engine is like a giant air pump. It's constantly sucking air (and fuel) in, and moving it out the tailpipe. Thanks to the sucking of the engine, you can create vacuum pressure in vacuum lines in order to run some things. Your brake booster for example uses vacuum so that you can apply more pressure to your brakes.

When one of your vacuum lines cracks or breaks, the engine can suck air in through that little hole.

Picture it this way: take a straw and a pingpong ball. Place one end of the straw in your mouth, and the other on the pingpong ball and suck. You can actually pick the pingpong ball up off the table with that suction. Now put a hole in the side of the straw and try it. It doesn't work all that well with the air leaking in from the hole.

Add to that the fact that the extra air that the engine gets doesn't have matching fuel necessarily, so it *should* lean out your mixture and cause all sorts of idle and low-rev running problems.

So it's air leaking IN, and not out that causes the problem

How do you fix 'em? Well you replace the 'straw' so to speak... In this case, you replace hoses that are part of the vacuum system which are leaking. One of the most common (and biggest) leaks is from a hose on the passenger side of your carb to the back of the intake manifold. It attaches to the carb's "Anti Afterburn Valve", and it's path takes it over the exhaust manifold. After years of heat from the exhaust manifold, it gets brittle and cracks, creating a huge vacuum leak.

You can check for vacuum leaks by giving it something other than air to suck in. If you take a propane torch (unlit of course) and wave it around the vacuum lines, you will hear a change in the sound of the engine when it sucks in propane instead of air. You can also do this if you have smoke/fog like from dry ice, you won't hear as much, but you'll see it.

Hope it helped,
Jon
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Old 11-18-04, 01:04 PM
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vipernicus.... that is the best explanation I've ever heard to describe a vacuum leak.. Awesome job.. You should write repair manuals..
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Old 11-18-04, 03:44 PM
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lol... repair manuals wouldn't be nearly as fun. I prefer freehand

Seriously though, I do enjoy writing, and helping people. This forum is a good way to put the two of them together.

Jon
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Old 11-18-04, 04:48 PM
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Carb cleaner , wd40 or starter spray can also help fing a vac leak . Just spray it along the lines ,engine running , when it hits the leqk it will suck in and the rpm's will pick up.
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Old 11-18-04, 05:52 PM
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Update, i went to check out the car... the guy took of the emissions controls since the vehicle is 20 years old and emission pass is not required anymore.

What he is going to do is use something called JB weld to clog those holes, this is the vacuum leak he was referring to.

Will this work?

He also has a 81 radiator installed and took of the oil metering pump and uses premix for fuel.

Thanks guys for the help
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Old 11-18-04, 06:10 PM
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no no.. tell him to just buy vacuum caps.. that way they can be taken off when needed be..
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Old 11-18-04, 07:14 PM
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where can you get vacuum caps? any mazda dealership?
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Old 11-18-04, 07:22 PM
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and what diameter, if you know that as well...


Thanks as always guys...
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Old 11-13-05, 08:18 AM
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To find a vacuum leak u can use shavning cream. Watch and see where it goes in.
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Old 11-13-05, 01:13 PM
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am i the only person who says stay the **** away frpom this car?!

HE TOOK OFF THE OMP! OMG!
you really need that... or a big freaking bottle of MMO/two stroke oil (like a keg) for your gas tank....
scary **** man.. my OMP was siezed, didnt know, and it injected idle amount of oil into a fast moving engine.. the engine lasted a little less than a year.
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Old 11-13-05, 06:16 PM
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And the cat came back,...
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Old 05-18-06, 09:05 AM
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my 82 first gen has a bolt in one of the vacuum holes around the carb, if i take it off the engine wont run but when its in it idles high, why??? and what is that bolt doing???

Last edited by rotoryheat; 05-18-06 at 09:07 AM.
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Old 05-18-06, 09:21 AM
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You DON'T need the OMP if you run premix. Just add 1oz. of 2cycle oil per gallon of fuel right before you pump the fuel, this will help it mix.
Don't use shaving cream to find your vacuum leak, start the car and spray some carb/choke cleaner around the vacuum lines, when the idle goes up, you found your leak.
As for the vacuum caps, check out this writeup by mazspeed.
http://www.ten15.net/Maz.tutorials/
That will walk you through the whole emissions removal process (except for the carb junk).
Justin
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Old 05-18-06, 09:22 AM
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Originally Posted by rotoryheat
my 82 first gen has a bolt in one of the vacuum holes around the carb, if i take it off the engine wont run but when its in it idles high, why??? and what is that bolt doing???
Can you post a pic?
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Old 05-18-06, 12:20 PM
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Originally Posted by rotoryheat
my 82 first gen has a bolt in one of the vacuum holes around the carb, if i take it off the engine wont run but when its in it idles high, why??? and what is that bolt doing???
Re-read the explanation of what I said a vacuum leak was.

When the engine is running, the engine is trying to suck air from anywhere it can. Through the carb is the biggest source, but it also sucks air through all of those little ports under the carb.

If one of yours is capped off, that means that something was removed at one point which needed vacuum to run. That's not a big problem, ALL of mine (except one) are blocked off because I've removed that whole rat's nest of tubing.

When you remove the screw, you open up another hole the engine can suck air through. That air bypasses the carb, and so it doesn't get any fuel, because only air going *through* the carb can suck fuel out of the carb. This means your engine now has way too much air, and not enough gas.

To make matters worse, having another source of air means that less air goes through the carb, which means less velocity through the venturis, and thus less vacuum signal to suck gas. So that compounds the problem even more.

Plug that hole the way it was and leave it that way. If you can provide a pic I could probably even tell you exactly what *used* to be connected there.

Jon
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Old 05-18-06, 04:14 PM
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another reason why vaccume leaks are bad....that air is un filtered... which means if you drive somewhere dusty, you can really screw up the durability of your seals.....
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Old 05-28-16, 11:03 PM
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Originally Posted by Bob Holton View Post
Carb cleaner , wd40 or starter spray can also help fing a vac leak . Just spray it along the lines ,engine running , when it hits the leqk it will suck in and the rpm's will pick up.
What if i cant get my car to start do to a vacuum leak? Then how do i check for leaks?
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Old 05-29-16, 10:22 AM
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took of the oil metering pump and uses premix for fuel.
Sounds like a pain in the *** to me.
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