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Old 05-18-09, 02:45 PM   #1
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How is such a large vacuum leak so elusive?

I replaced my intake manifold and now I have a problem, a large vacuum leak. Even with the throttle body completed blocked off, the car idles at a 700ish rpm. Though it's hard to hear it clearly over the sound of the engine, I can hear the hissing of air being sucked in from where it shouldn't be. My instinct is to spray liquid around the manifold/plenum area and watch it be drawn in. I sprayed methanol, but I don't see or hear it being sucked in. It must be a giant leak to still be idling at 700RPM with the throttle body completely blocked.
What are common places of leakage for the intake manifold assembly? What other techniques might I try to pinpoint this leak?
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Old 05-18-09, 02:46 PM   #2
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can you make something to release smoke and see where it is being sucked in?
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Phobias are for irrational fears. Fear of junk ripping badgers is perfectly rational. Those things are nasty.
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Old 05-18-09, 02:47 PM   #3
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That would be optimal. I don't have a smoke machine though.
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Old 05-18-09, 02:50 PM   #4
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but maybe you could make something that smolders and realease smoke, or hold a cigarette or some such near there? Heck, you are at college, there should be kids smoking something everywhere man.
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Phobias are for irrational fears. Fear of junk ripping badgers is perfectly rational. Those things are nasty.
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Old 05-18-09, 02:53 PM   #5
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but maybe you could make something that smolders and realease smoke, or hold a cigarette or some such near there? Heck, you are at college, there should be kids smoking something everywhere man.
We had a communal heap of chewing tobacco located in the lounge of my dorm. I'm home now though, and I don't smoke. The idea of making smoke is good though.
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Old 05-18-09, 02:54 PM   #6
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How about a sniffer bottle, maybe from eye drops, nasal spray etc, and some talcum poweder? Make a small puffer.
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Old 05-18-09, 02:55 PM   #7
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Or contact the HVAC repair guy for the campus. He may well have an air current tester, smoke puffer.
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Phobias are for irrational fears. Fear of junk ripping badgers is perfectly rational. Those things are nasty.
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Old 05-18-09, 03:03 PM   #8
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Find 3-4 feet of small diameter flexible hose (borrow from chem or fluids lab) and use it like a stethoscope to find the source of the whistling. One end in your ear, the other end you wave around the engine bay and poke into the rats nest to find the open (loud) location.
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Old 05-18-09, 03:05 PM   #9
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Find 3-4 feet of small diameter flexible hose (borrow from chem or fluids lab) and use it like a stethoscope to find the source of the whistling. One end in your ear, the other end you wave around the engine bay and poke into the rats nest to find the open (loud) location.
Hmm. So I have 3/8" PVC tubing, would that work? I also have 1/4". I sterilize and stick one end in my ear? Like an ear bud headphone?
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Old 05-18-09, 05:06 PM   #10
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1. bad intake gasket
2. bad injector seal(s)
3. brake booster hose/check valve

what kind of car?
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Old 05-18-09, 11:07 PM   #11
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without knowing more...I concur with jhota.
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Old 05-18-09, 11:12 PM   #12
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Hmm. So I have 3/8" PVC tubing, would that work? I also have 1/4". I sterilize and stick one end in my ear? Like an ear bud headphone?
You need something flexible so you can move it around easily.

Put one end into your ear and the other end at various places near where you think it's coming from. It centers the sound.

Or another thing that I did was use a loooooooong flat head screw driver with a plastic handle and do the same, with the plastic to your ear. The sound will travel up the metal. You can't get pinpointed as well as with a flexible hose but it will give you an idea.
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Old 05-19-09, 12:07 AM   #13
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So, you want to hook up a vacuum hose to your ear? What, you don't like having ears?

Spray bottle with water, spray mist around until the engine stumbles. Or spray a flammable liquid like carb cleaner, pepetrating oil, brake clean, etc. A propane torch with a rubber hose hooked to it turned on low can get you there also.
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Old 05-19-09, 02:53 AM   #14
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Yeah, use a propane torche (unlit) and spray propane around all the gaskets and interfaces. The idle will change sound when it sucks in the propane.
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Old 05-19-09, 08:40 AM   #15
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The car is a '93 accord. I swapped the F22 intake manifold with an H23's. I am going to try the PVC tubing first, then the flame. My ultrasonic humidifier produces a ton of water vapor, I've thought about using that in lieu of smoke.
But first I'm going to retorque everything
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Old 05-19-09, 08:53 AM   #16
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If you drive the car into a pool, you can probably find where the water is getting sucked in.
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Old 05-19-09, 09:03 AM   #17
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which fuel rail did you use - H or F? since the F is in the way of the EGR valve placement on the H manifold... if you moved or removed the EGR valve, you could have a vac leak there.

what gasket did you use, and did you do any porting?
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Old 05-19-09, 10:01 AM   #18
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Yeah, use a propane torche (unlit) and spray propane around all the gaskets and interfaces. The idle will change sound when it sucks in the propane.
This is better than what I do which is to use starter fluid.

Listen to Danno!
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Old 05-19-09, 10:20 AM   #19
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I used the F fuel rail. The folks on cb7tuner.com, a forum dedicated to my car, said that this was best because the H rail is backwards. I encountered no difficulty here.

I used a Felpro gasket set from Autozone for the intake manifold, both IAB gaskets, and the throttle body gasket. No porting.

So here is what I did:
Removed air filter tubing, so air is sucked in directly from throttle body. I blocked this, and the engine stopped. Good sign!

I reattached the air filter assembly and started the engine. Idle goes to just under 3krpm. For fun, I disconnect my IACV electrical connection. The idle drops to about 1300rpm but fluctuates rhythmically +/- 150rpm. Of course the lack of an IACV makes a check engine light, which I hate to see.
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Old 05-19-09, 04:27 PM   #20
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so the IACV may have come apart inside. that's a pretty easy fix. all you have to do is take it apart and reassemble the piston thingy.
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Old 05-19-09, 05:24 PM   #21
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Would the IACV coming part explain the higher idle? As soon as I disconnect it, the idle drops to a fluctuating 1500rpm.
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Old 05-19-09, 06:19 PM   #22
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yep. the IACV functions as a bypass for the throttle body.

since Honda PGMFI is a speed/density system rather than a mass-airflow system, idle is controlled by bypassing the throttle body. how much air bypasses it is controlled by the IACV. if the IACV is stuck, dirty or otherwise faulty, a (very) fast idle can result.

also, without knowing what year the new manifold is, another possibility is the FITV is stuck open.

but given the change in idle when disconnecting the IACV, i'd check it first.

and did you change/check the plenum-to-runner gasket?

i'm a big fan of Hylomar for intake manifold gasket installation, too.
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Old 05-19-09, 06:36 PM   #23
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Alright, I'll investigate the IACV tomorrow. About that FITV....I plugged up the holes; no FITV is present in this configuration. I would like to get an FITV, but I need to get this car drivable first so I can pull one from a junk yard. I kept the coolant hoses as is and simply connected the two with a coupling.

The gasket kit I purchased contained that of the intake manifold, for between the plenum and IAB plate, and between the IAB plate and the runners. All were replaced. I didn't use Hylomar or any other gasket making material.

Something else I noticed is that if I block off the end of the air intake tube, NOT the end of the throttle body itself, the car will slow it's idle down to about 800 rpm. If I block off just the TB entirely, the engine dies.

The manifold is from an H23A engine, found on Preludes from '92-'96
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Old 05-19-09, 09:29 PM   #24
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well, i think the IACV draws from the inlet side of the throttle body - so if that's the issue, blocking the TB would cut off all airflow to the engine.

as for blocking the intake tube, i'd bet the car's running off of the air in the crankcase (drawing through the breather hose) in that case. assuming a "perfect" seal on the end of the intake tube.
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Old 05-19-09, 10:02 PM   #25
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well, i think the IACV draws from the inlet side of the throttle body - so if that's the issue, blocking the TB would cut off all airflow to the engine.

as for blocking the intake tube, i'd bet the car's running off of the air in the crankcase (drawing through the breather hose) in that case. assuming a "perfect" seal on the end of the intake tube.
Wait a moment. So it could be normal for the breather hose to be providing enough air for the engine to run? Where is the air coming from then? I'm confused
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