So my car was acting a little wonky for the last few weeks ever since I replaced a few vacuum hoses and fuel hoses (which involved taking the UIM off), and I suspected vacuum leaks, but I couldn't seem to find any.

Well, in case you don't happen to have a mechanics stethoscope (IMO enough hissing sounds already come from my engine, so trying this method seems futile anyway) or a can of carb cleaner (or starter fluid), or a fog machine used to find vacuum leaks, there are two simple tips I have for finding minor vacuum leaks (don't expect to find any of the horrifically, devious, hard to find ones that aren't caused by typical causes).

First off, try pinching all the small vacuum hoses you can get to... doing so will isolate at least part of the hose from the engine. If you pinch a hose and the engine idle goes down, this indicates a vacuum leak. Try other hoses on the same vacuum line (not nessicarily the same hose, but other hoses down the same metal line for example). Replace any hoses that cause the idle to go down to eliminate them from the problem. Pinching a hose may also cause an otherwise unnoticable hissing sound to go away, or a change in the sound that the intake makes (if you've got a modified intake at least).

Second, if replacing any hose that causes a drop (or change) in idle does not fix the apparent vacuum leak (there are two sure signs of vacuum leaks: one, unplugging the BAC or jumpering the initial set connector does not lower the idle, two, the idle will not change much or at all when you adjust it, even to it's limits, three, a low, pulsating idle... it may pulsate a little or it may pulsate a lot. Yes, I know that makes three, but that usually goes together with the first two signs), and pinching these hoses still causes a change in the idle, then it's likely that shutting off that hose just happens to make another vacuum leak stronger, so you'll need to try the next technique to find the leak.

Finally, feel every hose you can see, especially those that come in large clusters of 5 or so. If you work on the intake system, there's a fairly large change that you may forget to plug one back in, especially one that's hard to see.) This should be very obvious, but it's quite easy to tell if one side of a hose is disconnected by tugging gently on it (tugging a bit on each side of a hose will also tell you if it's lose and needs to be replaced), or simply pushing it around with your fingers, and having it move instead of staying put like it should.

Here's hoping my annoying popping sound has gone away... but there's still a possibility that I have a few minor leaks since the idle still responds to the brake pedal (increasing the vacuum pressure makes leaks more apparent, although the idle changing because of the brake booster is normal to a certain extent, because of the massive vacuum it pulls).

If these methods fail, you will probably have to rely on the use of some sort of flammable liquid, or a more advanced fogger technique.

 

If you have a compressor, this is the quickest way to find vac leaks:

https://www.rx7club.com/2nd-generation-specific-1986-1992-17/water-sensors-water-pump-diff-models-583211/ (posts #11 & #16)

 

That's a very good way to do it, but my way is free

Also, if you happen to have a vacuum gauge, you can tell if the car is running well or not by looking at the vacuum at idle. It should be 15-20 or so PSI, and ideally it shouldn't drop when you give it any throttle (I think?), and it should also be steady at idle, if there's no leaks.

 

Heh, if you have 15-20psi at idle, you got bigger problems!!!!
Maybe 15-20 inHg at idle...

 

At WOT, im pretty sure vacuum should be close to 0 hg

 

Right, it's inches of mercury... and according to my gauge the normal range is something like 16-23.

I would try it out but the crappy plastic T it came with broke so I need to go get a metal one.

I'm thinking I might have one of the demonic vacuum leaks that doesn't come from a rubber hose being loose... sure hope it isn't the lower manifold or something.

Also, to make matters work, I apparently have an exhaust leak since there are fumes in my engine bay now.

 

ahhh crap. my vacum is like 7 psi at idol. bugger. I just replaced all my vac hoses and gasgets. ( except the LIM ) damnnnnnn

 

Not PSI... inches mercury (hg).

 

im using an After market gauge plumbed into one of the nipples for the oil bleed injectors
Im sure it was sitting around -20 ( on my PSI gauge ) before i blew it up

 

I gave up after weeks of using the free methods unsuccessfully. My tester found my leak the instant I opened the valve, and it was in a location the free methods would probably never have found (underside of TB). The $10-20 worth of parts were totally worth it. And if you don't have a compressor, a bike pump or tire pump will work just as well.

It should definitely drop when you open the throttle. The vacuum is caused by the huge restriction of the closed throttle. Opening the throttle lowers that restriction, so vacuum drops.

It should be steady at idle, but that doesn't necessarily mean there's no leaks.

The oil nozzles are fed air from in from of the throttle, so there can be no vacuum read there, only boost. If you saw vacuum then the oil nozzle air bleed line must be hooked up wrong, and they won't be working very well...

-14.7psig is absolute zero pressure. It's phyically impossible to go below that. Most aftermarket boost gauges with imperial units show vac in inHg and boost in psi. Dunno why, it's always seemed pretty dumb to me to changes units like that.

 

hmmmmm
well. that would explain alot.
I hooked them up as per the emission diagram. ( im talking bout the lines that go from the TB into the LIM. Ive ( S5 manifolds ) got the top nipple hooked up to the primary air bleed, the middle ( large ) hooked up to my oil injector 4 way splitter thingy and the bottom one hooked up to my secondary air bleed nipple facing the firewall with a vac hose going in between the one on the other side to the front secondary air bleed nipple.
The nipple in the middle, slightly lower is hooked up to my FPR


if this isnt how its supposed to be then im stumped. and it is reading vac, approx 5-7 inHg
what have i done wrong?

 

If you use a compressor that's too big, you'll end up making a lot more problems... I have a cheapo 12v-powered one that came with a highway safety kit, though...

Also, what do you need to hook up an end cap like that to a compressor? They didn't have any fittings like that when I went to hardware store to look for parts to make one of those a few months ago.

I wanted something that would hook up like a tire's stem valve, but they didn't have anything like that. Do they make hardware like that?

 

you basically need to find a fitting that can go into you compressor and go from there. im assuming it is one of those compressors with a fitting to pump up a tire? unless you can unscrew that or you cut up an old bike tube i dont think you can use that compressor

 

That all sounds fine, but where exactly is the boost gauge fed from?

A boost gauge is supposed to be connected to a line that's fed from a post-TB nipple. The easiest way is to tee into either of the BOV or MAP sensor lines. Or if you've removed your emission controls, the top and bottom nipples on the front of the UIM see vacuum (the middle one doesn't), so you can connect a boost gauge directly to one of them.

Not really, if you get silly with the pressure the worst you'll do is pop a hose off. I put a gauge on so I knew exactly how much pressure I was using, and just opened the compressor supply valve slowly unless I got the desired result, which onto took a couple of psi anyway.

That should work fine as long as it's not too loud to hear a hissing leak.

General hardware stores don't usually stock brass fittings like that. I got them from an engineering supply store.

Sure, but again you probably need an engineering supply store rather than a hardware store. A tire shop will have them too. Ask for a Schrader valve.

 

It's *way* too loud to hear hissing sounds...

I guess I'll just go home and use my dad's compressor... makes finding the proper fittings easier anyway.

I'm not sure if the pressure regulator works worth a crap on it though...

 

Don't worry about the reg. Just slowly crack open the ball valve on the compressor outlet and if there's a leak you will instanly hear a hiss.

 

haha, i swaped my boost gauge over to my BOV nipple and got a solid 20 inHg. Stoaked. my vavum issues are sorted. now i think my TPS is fucked. wasnt getting any reading when i opened the throttle right up. better get a new one aye

 

NZ. Am I correct that you pressurize the intake system to find leaks with the engine running?

 

I think i'm goign to try pressurizing the intake trick this weekend then RTV the crap outta the leak, i have emissions to do . .

how much PSI it too much for N/A??

 

Um, no.

You'd never find the leak that way anyway, not to mention the engine wouldn't run (not enough air).

 

I guess ya have to rotate the engine to find a spot that doesn't have an intake/exaust overlap. Possible in a bridge port motor?

Or are you just checking up to the TB?

 

You need very little pressure to show up a leak. Like I said above, I quickly found mine with only a couple of psi. Turbo or NA makes no difference.

Correct.

Yes. Even in a high-overlap motor, the actual amount of overlap is a small percentage of a full cycle.

No, it's right up to the (closed) ports. Air can still get through the TB even when it's closed, otherwise the engine wouldn't idle.

 

So I take it I need a male 1/4th inch (ish) coupling plug (that will hook up to the compressor hose), a female 1/4th inch (ish) ball valve, and a 1/4th inch male threaded peice adapter with a suitable bolt in order to bolt the item on to whatever cap is suitable?

IIRC the OD of the FC's AFM is 2.5 inches, but I'm not sure who carries 2.5" OD or 2.5" ID end caps (besides special ordering it or something).

If it's going to be slipped OVER the AFM it needs to be flexible, and if it's going to go into the intake hose with the AFM off, it needs to be hard.

I don't have any silicone couplers and I'd rather avoid having to order to many things off the internet...as a matter of simplicity.

(actually come to think of it, they AFM's sides aren't the same diameters, are they? The filter side is about 2.5 inches (I think), but the engine side might be upwards of 3?)

 

My tester replaces the TID, but the S5's 2.5" AFM inlet is a convenient place to connect. I'd buy a cheap 2.5" coupler (and clamps) from an autoparts store, take it to a plumbing store and find a PVC end cap that fits inside it. If you can't find one that fits well you might need to mix and match reducers and short lengths of pipe. PVC pipe sizing is a bit wierd and the walls are quite thick, so don't get hung up on the nominal sizes (my "50mm" end cap has an OD of 70mm). Use you imagination. Remember it has to seal perfectly. A leak tester that leaks is useless...


For the brass bits, go to an engineering supply store with the PVC bits and tell them exactly what you're going to do. They should be able to give you the bits you need.

Why do you have to order anything off the internet? Just go to a shop.