Techinques for locating vacuum leaks?

jaymac

jaymac

New Member
Feb 18, 2004
995
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Northern Mass
#1
I was listening to my engine today at idle, and very faintly I heard a hissong coming from the back of the bay. It will get louder and softer, even though the engine RPM isn't really loping. It stays right around 900.
I sprayed some starting fluid on th obvious hoses, PVC, FP. MAP, some spos on the upper to lower, and a few general sprays down to the back of the intake, and the RPM didn't change. I also took a little length of vacuum hose to my ear and put it around other vacuum hoses and a few other areas, but didn't pick up on the hissing any louder.
Are there any other tools, tips, techniques I can use to pinpoint where this is coming from and if It's afecting performance?
Thanks,
Jeremy
 

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#4
I small leak a gauge isn't going to tell much. One way to find the leak is to wiggle the lines and when the tone changes, you have found the line.

Remember that a downside of plastic is that it echoes the sound of the air moving around. Many vehicles use plastic intakes, and they can be noisy, the fords are the worse.
 
#5
Jeremy, if your Mac is SD, the vac signal is pretty critical. A stockish (meaning unchipped/tuned) SD motor should pull almost 20" hg of vac at idle. Much less at idle and the puter thinks you are accelerating when you are not.

If you have a Miti vac, I have used mine to check out lines and systems. Are you real sure the PCV is well seated?

No great ideas from me.

Good luck bud.
 
#7
Daggar - AC is pulled...

JT - I don't have or know what a Miti vac is. Once again, i will have to be schooled on the matter :)
I did double check the PCV, and wiglling, pushing, pulling on it made no effect on the hissing. Of course, I'll check it again in the course of things. The grommet is a little more than a year old, so should still be good.

90- I wigled and pulled on and sprayed most of the lines, to no real avail, but obviously I'm overlooking something.

I guess I will attack it all again tomorrow, and if anyone has any info on a vacuum gauge and how to use it, maybe i can try that!
thanks again,
 
#9
I was thinking about this too considering I have an 96 and most of my vaccum lines are brittle as piss..does anyone sell a new vaccum hose kit with the original new plastic lines?
 
#10
Nitro, I think you meant 86 (though the lines on my 94 are vulcanized themselves). I keep spare line of as many sizes and types around as I can. Then I just grab some of whatever I need. That is likely the easiest and cheapest (one should be able to replace most lines - sans ones with funky bends or I.D. changes from one end to the other - for like 10-15 bucks).

Jeremy, no bigs on the Miti vac. It is a handheld vacuum pump which I find QUITE useful for bleeding systems and checking vac signals. Some folks have them and forget they have them, which was why I asked. Otherwise a plain ol' vac gauge should be cheap (if it isnt, I would get a MV instead, since it is a vac guage and much more). MV's are 30-40 bucks and generic siblings can be had for a bit less on sale.

I am thinking that if you check your manifold vac signal and it is near 20" hg, the hissing noise is likely something else.

Good luck bud.
 
#11
Use a propane torch, without the flame of coarse. Probe around with the end in areas that might be a potential leak, as it sucks in the propane the engine will speed up slightly and you have found your leak. Just be careful!
 
#12
The sound doesn't appear to be coming from the TB, but of course more inspection wouldn't hurt. It sounds like it's cominf from behind and below the whole lower intake area.
I'e replaced pretty much all the vacuum lines back there in the recent past, so they shouldn't be cracked. I guess I will have to go over each one w/ a littl emore time and attn. to detail, then look into the vac gauge and propane, etc.
 
#14
check the hose going to the PCV , it could have popped off , it faces downward so spraying starting fluid on it might have been going by the hose too fast for it to suck it back into it to notice a change in idle speed , the PCV is dead center of the intake sticking down in the lower intake with a larger 3/8 hose from underneath the upper intake ....I have seen some of these pop off , I would bet money its that , and suggest changing the PCV and breather element while you are back there , its easier to get to if you pull the upper intake off ..........
 
#18
Hey JT, I got a mitivac ( :) ), and checked the vac off the tree. The tube wasn't a super-snug fit on the nipple, but I held it tightly against it, and it pulled 19".
Sound about right, or should I search around until I pull 20"?

Thanks for your help!
 
#19
19" hg is great. Good call with getting that tool. As you have probably read the book, you can use it for lots of stuff (brakes, getting excess automatic trans fluid out, etc etc. Great thing to have in your tool box, IMHO).

I just had one quick thought, which might have been covered or not apply. Are the smog pump hoses in good shape and clamped tight?

I really cant think of much else to check for your hissing noise though. But you do know you have good manifold vac, which is nice to know (I dont see a reason to continue to hunt for a vac leak, though someone else might have a reason to).

Good luck bud. :nice:
 
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