Foo - 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?

can you make something to release smoke and see where it is being sucked in?

That would be optimal. I don't have a smoke machine though.

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.

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.

:lol: 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.

How about a sniffer bottle, maybe from eye drops, nasal spray etc, and some talcum poweder? Make a small puffer.

Or contact the HVAC repair guy for the campus. He may well have an air current tester, smoke puffer.

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.

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?

1. bad intake gasket
2. bad injector seal(s)
3. brake booster hose/check valve

what kind of car?

without knowing more...I concur with jhota.

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.

So, you want to hook up a vacuum hose to your ear? What, you don't like having ears? :p

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.

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.

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 :)

If you drive the car into a pool, you can probably find where the water is getting sucked in.

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?

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!

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.

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.

Would the IACV coming part explain the higher idle? As soon as I disconnect it, the idle drops to a fluctuating 1500rpm.

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.

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

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.

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 :o

no, it wouldn't be "normal," but we're not dealing with an engine in normal condition. and the crankcase isn't really "sealed" on these cars - you could be drawing in air (in this case) around the spark plug holes, through the dipstick tube, really just about anywhere there's a gasket on the block.

you could have a leaking intake pipe (are you still using the factory rubber one, or an aftermarket replacement?), as they tend to crack over time.

there's any number of possibilities.

just one more set of reasons i don't work on Japanese cars (anymore).

Would the IACV coming part explain the higher idle? As soon as I disconnect it, the idle drops to a fluctuating 1500rpm.Imagine running a hose from the IACV port in front of the throttle-body and connecting it to port after the TB. You're essentially allowing more air into the engine, similar to opening the throttle more.

A lot of times, the IACV can get contaminated with dirt and grit when it's removed. This can cause the moving parts to stick and not work correctly. So... you've already confirmed that IT is the part that's sending extra air into the engine and causing a high idle. What you need to do now is figure out why the IACV isn't working correctly. Cleaning it out and spraying some carb-cleaner through often works. Otherwise, you'll need to replace it. Here's some instructions: Cleaning IACV, FTIV, TB (http://www.honda-tech.com/showthread.php?t=1844067).


On a footnote, I can't believe the engineers that designed TWO parts into an engine that controls idle-air. It's so much simpler to have just one idle-valve and use the computer along with air & coolant-sensors to come up with an idle-algorithm for both cold and warm starts. In each case, it only needs to control a single idle-valve. The fewer mechanical moving parts you have, the more reliable it is and easier to troubleshoot.

Problem solved :)

My process: I noted that blocking the TB completely makes the engine die. Obviously this means I have no major vacuum leaks. I did detect, using the PVC tubing as suggested here, the tiniest leak from the manifold gasket. This was easily remedied with a few ft lbs of torque.

Next I used my finger to block off the innerhole of the TB which allows air to be drawn in during idle, bypassing the throttle body's butterfly valve. The car continued to run, though very slowly. So something in the idling system is leaking...

I used compressed air to figure out the path that inbound air travels on during idle. Obviously one thing along it's journey is the IACV. While not powered up, the IACV completely blocked off air passage between the two holes. I know the IACV is working then because the idle rises tremendously when I plug it in, suggesting that the motor opens the passage. This leaves one other piece...

The starting air valve. This discrete and seemingly meaningless valve was attached, but the plug was simply capped off as I was told the valve was unnecessary for operation. But I found out that unless a vacuum is applied to it, it leaves the passage completely open! I applied a vacuum to this from my intake tube and viola, normal idle :D

On the FITV and IACV: Danno, I agree with you, it is ridiculous to have two valves for the purpose of controlling idle speed. The FITV is mechanical and factors in only coolant temperature. Supposedly this valve is responsible for ensuring higher idling speed with a cold engine, it does so by regulating air flow from a second port which was engineered into the throttle body. Because I don't have a FITV, I covered the holes with duct tape. The IACV is electronically controlled, and the ECU factors in many parameters when controlling it. I am certain that one of these factors is coolant temperature; there exists a temperature sensor on the car anyways, and it relays to the ECU! With the FITV not even present, and a cold engine, the IACV happily made the car idle at 2000rpm. After a few minutes the idle began to drop until it reached my set 700rpm. Conclusion: FITV is a useless piece. Unless one falls into my lap for under 15 dollars, I'll be machining a nice plate to elegantly cap off the holes.

On the FITV and IACV: Danno, I agree with you, it is ridiculous to have two valves for the purpose of controlling idle speed. The FITV is mechanical and factors in only coolant temperature. Supposedly this valve is responsible for ensuring higher idling speed with a cold engine, it does so by regulating air flow from a second port which was engineered into the throttle body. Because I don't have a FITV, I covered the holes with duct tape. The IACV is electronically controlled, and the ECU factors in many parameters when controlling it. I am certain that one of these factors is coolant temperature; there exists a temperature sensor on the car anyways, and it relays to the ECU! With the FITV not even present, and a cold engine, the IACV happily made the car idle at 2000rpm. After a few minutes the idle began to drop until it reached my set 700rpm. Conclusion: FITV is a useless piece. Unless one falls into my lap for under 15 dollars, I'll be machining a nice plate to elegantly cap off the holes.

the FITV was introduced with PGMFI for warm-up idle purposes. idle speed was controlled with a bypass screw and a vacuum servo for the A/C. this pretty much sucked, so the IACV was added to the system in 1988 with Honda's adoption of what is usually called OBD-0, allowing the ECU to control idle.

this was intended to control idle speed for varying load issues - such as power steering load, A/C compressor cycling, or what gear position the AT lever is in. warm up was still left to the FITV.

but the ECU still had no way to monitor operation of the valve until the adoption of OBD-II. thus, in 1996, since the ECU was now "smart" enough to sense and control how much the valve opens (instead of a general "open" or "shut" case). then they (reasonably) dumped the FITV.

if the car is OBD-I (which i assume it is, being a '93), i'd still run a FITV. the system is designed to operate correctly with it in place, and i've got a lot more faith in Honda's engineers than my own redesign of their system.

Starting fluid around the intake is a good thing...the gasket could have slipped, and that might be a problem. Also, knowing the vintage of the car, are you sure that you got EVERY vacuum line reconnected and in good working order?

Uh, I think he solved the problem. Good job PC2! I guess you found yet a 3rd component that controlls idle-speed. I suppose the starting air-valve is only open on the 1st couple of cranks before the engine develops vaccum. This is kinda like holding the throttle partly open when cranking. Then when it starts, it cuts off the extra air, like letting up on the throttle.

From the evolution of the system that johta described, it appears Honda was just extending the lifespan of its earlier systems by piggybacking on additional components. Makes sense to save engineering effort and costs, but it does make the overall system more complex. On all my racecars, I always rip out the entire factory ECU and harness and install a MoTec or Electromotive system. So much simpler and infinitely more programmable and adjustable. And only one idle-valve needed. Starting enrichment, supplemental air, ignition, etc. are all adjustable and can be weighted with varying factors based upon ambient air-temp, coolant-temp, elevation, etc.