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Air intake manifold swollen with air

Guys, I got a strange one.
Whenever I push the gas quickly I hear a hissing sound and it looks like the air intake fills with too much with air (don’t think that’s normal). This hissing sound also stalls the engine for a second. It’s really hard to describe in words but a video tells the whole story :slight_smile:

The car is European (Seat Toledo - 2002) but the engine is the same one as you have on the VW Golf 2.3 V5 170 bhp

LOL… That manifold isn’t swelling its relieving a heavy vacuum that was present before opening the throttle. When the throttle is closed its pulling heavy vacuum, when you crack the throttle that vacuum is reduced so the pressure heads back toward atmospheric and thus relieves the walls of the manifold of the heavy vac that was present a moment ago.

You need to check or replace your air filter… In fact remove the air filter all together and do the same test in the video. If the manifold movement is reduced or goes away…you just found out your air filter was clogged. The other item it could be is the AIC or Air Idle control circuit… How does the engine idle? Do you have idle problems? We can move into that AFTER you remove your air filter and run the same test you just did… Need the results from this test.

Time to go outside and remove your air filter and start the engine.


Hey. thanks for taking the time!

What you’re saying makes sense, about the vacuum. However, i must say it does look like it is swelling exactly when i press the throttle, but could also be an optical illusion :slight_smile:
Regarding the filter - i actually changed it a short while ago - didn’t solve the issue. I also cleaned the MAF sensor, still didn’t sort it.
doing the same test with the filter box open does sound like an interesting idea. Is it safe to do that? Won’t i get too much air in the engine?


The manifold does indeed look as though it is swelling…but you need to consider it is returning to its non vacume state when you hit the throttle.

Imagine sucking the air out of a soda bottle…the bottle collapses…when you remove your mouth from the bottle it snaps back to its normal resting position. This is the best analogy I can come up with for this issue.

Yes it is safe to remove your filter or leave the airbox in half for this test…just dont go driving around all week without your filter or with the airbox apart. For this test it is no problemo…

Its a quick test really. Manifolds have to handle vacume… if its not a clogged filter or anything restricting airflow after the throttle body…such as a rag sucked into the intake boot or something… It IS possible your intake manifold is weak or fatigued…it is plastic afterall, certainly possible. Do the test and report as its rather important before trying to help further.

Also be sure there is nothing inside that rubber intake hose going to the airbox lid… ** It wouldn’t hurt to clean your throttle body of any oil or gunk and carbon deposits that always build up at the butterfly valve in the throttle body. Excess deposits restrict airflow past the butterfly valve…but if you had big deposits you would have idle issues most likely… Its just always good to have a clean throttle body around the throttle butterfly valve…perhaps it is gunky and relying on the Air idle circuit too heavily… Again…what issues are you having other than watching your engine?

This may just be the plastic intake manifold flexing because its either defective or weak…

You never really complained about any sort of problem associated with all this. Are you just curious as to why it could be moving or is this an effort to solve a problem?? If you are just looking at stuff under the hood…I can tell you that many things move and flex under there under normal conditions… SO you need to be clear if you are having some kind of issue…or if you are just an under hood Peeping Tom. LOL

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That actually makes sense, with the soda bottle example. thanks :slight_smile:

I will definitely do the test tomorrow when i go by the garage where i keep my car.

And to answer your query - i wish it was just a case of Peeping Tom, but it’s not. This issue actually makes it a bit difficult to get moving from a stop light especially if it’s uphill. The car has a manual transmission and instead of gas it feels like I’m only getting air in the engine while i’m releasing the clutch and slightly applying acceleration. As a result, i have to accelerate harder in order to get moving.
Another symptom is the this one: I quickly accelerator, RPM goes up, i hear the hissing sound, then upon release of the pedal the RPM goes back but really low (400-500 RPM) and the engine starts shaking like it’s stalling, before going back to normal idle.
So clearly something is not right in there.

I will do the test with the air box open to see what happens, i will clean the butterfly valve with a special spray and i will try to do a smoke test and see it smoke comes out from anywhere. I’m not quite sure yet where i’m going to insert the smoke from, but i will look for some hoses on the air box, if i have any and try to blow it from there.

And again - thank you for your time :slight_smile:

when motor is off, there is no vacuum. car is at rest. what does airbox look like than? have a 2nd person start car while you stare at box. what does it do at idle?

OK, now that we know what issue you are actually experiencing it is easier to try and guide you. The fact that your intake manifold is actually deflecting to the degree in the video makes me suspect the integrity of your plastic intake manifold. HOWEVER… Since it can show its under vacume by sucking in upon itself…that may prove its holding air pressure or vacume… The other idea would be a clogged exhaust… how does exhaust flow feel to you?

Either of these conditions will produce symptoms you describe, but in different ways. Vac leaks usually produce a high idle speed… An exhaust clog has a certain feel and sound to it as well. I apologize, I’ve been doing this crap for so long I can hear and even smell problems with engines to be honest so when I am onsite with the machine my senses help me as much if not more than diagnostic equipment. So its hard for me to write all the hundreds of theories and thought processes my mind quickly rifles through in minutes when diagnosing something. For me to write them all for you gets a little silly especially since I cannot hear or see the vehicle.

Let us know the result when you remove the air filter and check for any intake obstructions…this is important.

While you are at it…have someone or yourself and an assistant…verify exhaust flow…

The direction of the solution changes dramatically once we know these vital stats…

Keep us posted.

I checked :slight_smile: it looks deflated

Hi. That is a good theory and you do make a very good point. I’m sure my problem is around there somewhere. It all makes sense.
And I didn’t have time to go to the garage and check i’m afraid … the wife prefers to have me home for Christmas :slight_smile: :slight_smile: I will go very soon though

Hello :slight_smile:
So, I went by the garage the other day and i’m starting to think this will not be an easy fix.

I checked the manifold before turning the engine on - it seems deflated, same as it is when the car idles. so it only “inflates” when i accelerate.

Also, I opened the airbox and i got the same result as if the box would be closed (apart form the fact that it was even louder). so this didn’t solve the problem.

In addition, i noticed the engine is quite hot even after a short drive and as far as i know this can indicate too much air in the mixture. The temp indicator in the dash was still within limits thought.

i did look down the main hose to see the butterfly and all seems fine, nothing stuck in there.

At this point, after what i saw and after you guys told me, i think i will have to take the car to a mechanic. it might be that the plastic manifold is indeed faulty and that’s not something i can change by myself.

so i guess that’s that… thanks for all your help!

You skipped checking how the exhaust flow feels to you… Does it seem free flowing? Also remember the “inflating” you are describing is the manifold going back to atmospheric pressure… instead of being sucked in on itself under high vacume at idle.

I didnt ask the obvious… is the engine light illuminated? Any codes to go off of here ? The manifold may have always done this dance you describe and you are just noticing it. The other symptoms you have could be something as simple as the MAF sensor being dirty or failed. SO any engine codes will get you to a solution much quicker.

Sorry, forgot to tell you. Yes, all seems fine. It even sounds really well. Doesn’t have any hiccups.

Well, i just cleaned the MAF sensor recently (i thought that was the issue) - but it didn’t.
and no, the engine light is not on. However, i order an OBD II and it should arrive soon. Do you think it will give me any indications on my issue, even without a code? I saw it can show thing like air flow/air pressure/air temp

Or maybe i do have a code and somebody erased it before i bought the car…hope that’s not the case.


When you turn your key on…engine off… Do you see the check engine symbol illuminate. This is when your dash tests all the lights it can illuminate, if the CEL light is not among them… You may indeed have codes because your engine light bulb cannot illuminate. Just an idea…

yep, the engine light is there when i turn the engine on. so it does work :slight_smile:

OK Good… Did you try to drive the vehicle with the airbox open or the filter out? Just for fun? You wont hurt anything just by performing this test. Im curious…does the condition improve when running with no filter or box open.

Do that test real quick and see what happens… If it still the same…and with no codes it would seem this is something physically wrong that the computer cannot see. Like a Banana in the tailpipe sort of thing…or a ruptured intake manifold.

You can also spray some type of solvent or WD40 on and around that intake manifold to see if idle quality suddenly changes and thus exposing a breached intake manifold. It is plastic and its deflecting enough for you to see it…maybe it also lost ability to seal itself completely. If it is sucking unmetered air, you would get symptoms like this… you should also get a higher than normal idle but…i dont know… Im running out of ideas from this distance.

I am suspicious of ANY intake manifold that moves… That is not supposed to be occuring…and could signal a breech of the manifold.

Is a clogged cat a possibility? It almost looks like it’s getting more air in the engine than it can get out. You’d probably notice some driveability issues if the exhaust was clogged that bad, though. Might try removing the pre cat oxygen sensor and see if the intake still swells…?

Its not swelling… it is just going from a vacume…back to close to atmospheric. I also had concern about exhaust flow and he stated it seems ok. If the manifold is flexing like this it is highly suspect to be leaking unmetered air. Who knows…may be impossible to diagnose from this distance…there are clues onsite that just arent available here.

I thought he said it looked “deflated” with the engine off? If that’s true, there’s no vacuum with the engine off, right?

Although, now that a think about it more, I guess a plugged exhaust wouldn’t cause the intake to inflate anyway, since air isn’t forced in to the intake (assuming this isn’t a turbo engine). Any swelling with a plugged exhaust would probably be past the intake…I think…

Not an expert or anything but I think if the exhaust was obstructed, not all of the gasses would escape the cylinders. That means there would be less air to enter into cylinders on intake stroke creating loss of vacuum. i would think the only way enough pressure exiting the engine into the intake with enough force to fluctuate a healthy intake would be leaking intake valves.