Popping noise from airbox and CAI during idle

[Le31504]

I have one more issue i had for i while... nothing serious i hope but anyway...

When the car stands on idle after driving its an popping sound from the airbox and cai...
It sounds almost like an 2 stroker MX on idle (but far from that loud, but I can hear it clear with the hood closed)

Anyone has the same or know why it does that.. it started sometime this summer

I heard of one more guy here in sweden with the same thing, but he hasnt found the problem either...
when he cutoff (pressed with his fingers) the line to the EGR valve it stopped... and as soon as he let go of it the noise started again...

I dont know if i have the same thing... havnt tryed it yet, but i will later...

Anyone?

Thanks!
Best Regards
Lars

[alpinestar180]

maybe a bad check valve?

[juttin3]

I have same issue with stock intake. Sounds like crackling popping sound from inside intake box. It might be the secondary air pump cause its very loud on my car on cold starts. Idk.

[Le31504]

I have same issue with stock intake. Sounds like crackling popping sound from inside intake box. It might be the secondary air pump cause its very loud on my car on cold starts. Idk.

mm my inst only on cold start... might the secondary airpump or an checkvalve... but.. there is like 16 check valves in the vacuumsystem... hard to fint the ringt one (suspect vacuume system cus its only on idle... cant here it with a little gas)

[old guy]

A slight gurgling sound from the air box at idle is normal. If you have a CAI it will probably be more pronounced since the air box is lined with sound absorbing baffles to damp the intake noises.

[diagnosticator]

The N112 solenoid valve is probably leaking a little, causing the Combination valve to open slightly. That is why pinching the hose stops the noise. BTW, our A4s with 1.8T does not have an EGR valve.

[Le31504]

Combination valve?

The sound is not like slight gurgling... its a more or less popping (like exhaust osund on en 2 stroker motocross on idle, but more silent then that)

[old guy]

The N112 solenoid valve is probably leaking a little, causing the Combination valve to open slightly. That is why pinching the hose stops the noise. BTW, our A4s with 1.8T does not have an EGR valve.

I believe this is what you meant to say ;-)

[old guy]

hmm ok... isnt the N249 and EGR the same thing?
Combination valve?

The N249 controls the diverter valve. The combi valve adds extra air to the exhaust stream during a cold start. It is controlled by the N112 valve. As diagnosticator stated the 1.8T doesn't have an EGR system. The SAI pump along with the variable cam timing perform a similar function.

[Le31504]

confused... The N249 valve shouldnt have anyting to do with this... right?
the N112 maybe... but you are talking about an combination valve... and ist the combination valve the EGR (its in the same spot on the engine anyway... back of engine... metal mushroom thing

[old guy]

confused... The N249 valve shouldnt have anyting to do with this... right?
the N112 maybe... but you are talking about an combination valve... and ist the combination valve the EGR (its in the same spot on the engine anyway... back of engine... metal mushroom thing

The combi valve is the metal valve on the back of the motor. It is opened by a vacuum signal from the N112 valve located beside the N249 underneath the intake manifold. The combi valve allows extra air pumped by the SAI pump to enter the exhaust stream during a cold start. The extra O2 combined with the exaggerated valve overlap created by the variable cam timing heats the catalytic converter up quickly. An EGR valve (exhaust recirculating valve) pumps exhaust gasses back into the intake stream to help reduce exhaust emissions. It does this by displacing some of the O2 thus reducing the combustion temperatures to below 2,500 degrees which is the the threshold where NOx is created.

Edit: To correct misinformation. That was my brain fart for the thread. Thanks BBC!!

[diagnosticator]

I believe this is what you meant to say ;-)

Yeah, thanks 'old guy' the N112 solenoid is the one I had in mind. Fixed.

[Le31504]

ok...

Lets se... the Combi valve is the same as the secondary air pump? are athlest its called secondary airpump?
The N112 manage the signals to the combivalve by vacuume...

The N249 can i forget cus it has nothing to do with popping airbox... its an controll valve for the DV or something?!

So the BEX engine does not have an EGR? I thought most cars had an EGR... so i can understand why its an mixup with the name and function cus thay do almost the same thing

How can i check if the N112 is leaking?
Can it be an faulty Combi valve rather then the N112, or any checkvalve thats goen bad? (one of about 16)

Trying to learn so please have patience with me :-)

[diagnosticator]

confused... The N249 valve shouldnt have anyting to do with this... right?
the N112 maybe... but you are talking about an combination valve... and ist the combination valve the EGR (its in the same spot on the engine anyway... back of engine... metal mushroom thing

Sorry for my error. Brain fart. The mix up should not be causing any misunderstanding now.

If the N112 is leaking, that will apply a low amount of vacuum signal to the Combination valve opening the Combi valve slightly. If this is the case the exhaust pressure pulses are being heard through the Combi valve, through the SAI pump into the air cleaner via the SAI air supply pipe from the air cleaner. The fact that pinching the Combi valve signal line closed stops the noise is consistent with the N112 leaking slightly and the symptoms you describe.

To check the N112, use a vacuum pump with a vac gage attached, connect to the vac supply port on the N112 solenoid valve, disconnect the vac signal hose from the N112 to the Combi valve, and apply vacuum with the pump. If the N112 is leaking vacuum to the Combi valve, the vacuum applied to the N112 supply port will decrease/leak off, as shown on the pump vac gage, and will not hold steady at the test vacuum value applied by the pump. Don't apply to much vacuum to the N112 for the test. ~4 inHG or equivalent vacuum is enough test vacuum.

BTW, if the Combi valve is open even a small amount with a warmed up engine, and without the SAI pump running, the Combi valve can be damaged by overheating the valve seat inside from the exhaust gas flowing backwards into the Combi valve. To check the sealing of the closed Combi valve, remove the valve from the head and try and blow air through the Combi valve. It should not be possible to blow through the closed Combi valve. If you can blow air through the closed Combi valve, that will cause the popping noise in the air cleaner also, and is a result of the leaking N112 solenoid valve. If this is the case replace the Combi valve.

The 1.8T uses INTERNAL EGR, and is a function of the VVT, there is no separate external EGR valve.

[old guy]

Yes you can forget the N249. It controls the diverter valve. The secondary air pump is located in the lower front of the motor. When it is running during a cold start it pumps air to the combi valve on the back of the motor. The variable cam timing increases the overlap between the intake and exhaust valves which pumps a percentage of unburned fuel into the exhaust stream. The extra air from the SAI helps burn this fuel and heat up the catalytic converter. Read this excellent article on how the 1.8T managed to get a ULEV (ultra low emissions vehicle) rating. Clicky click Pay particular attention to section 2.1.3 and 3.1.1.

Edit: Sorry! I mostly just repeated myself. Anyway, read the article. Also, try clamping off the line from the N112 to the combi valve as diagnosticator suggested and see if the sound goes away.

[Le31504]

Ok now i understand...
So if its something with this that make my popping noise it should be the N112 Valve that leaks a little, and not the SAI pump or the combivalve that are cracking up... or maybe any leak on the hoses from or to any of these components?
I know it could be something else that makes the noise, but its an good start!

Thanks!

[BBC]

The combi valve is the metal valve on the back of the motor. It is opened by a vacuum signal from the N112 valve located beside the N249 underneath the intake manifold. The combi valve allows extra air pumped by the SAI pump to enter the exhaust stream during a cold start. The extra O2 combined with the exaggerated valve overlap created by the variable cam timing heats the catalytic converter up quickly. An EGR valve (exhaust recirculating valve) pumps exhaust gasses back into the intake stream to heat up the catalytic converter.

EGR is to recirculate Exhaust gas to Dilute mixture and reduce combustion temperature, thereby reducing NOX emissions, has nothing to do with heating up CAT. That is the SAIs job.

[Le31504]

But if the N112 was leaking and causing the combivalve to open wouldent that show up on an exhaust reading, like bad CO ans HC readings? Now im taking of an warm engine
This was my readings during an test av exhaust on idle and 3000rpm
CO: 0,0
CO 2005: 0.0
HC: 000
Lambda: 1.01

[BBC]

If the combi valve is leaking, then you will know when the intake pipe to the SAP is disconnected from the air cleaner. you will hear and feel the exhaust comming out. Have seen this issue with improper cam timing on cars that have had a major head repair/ rebuild, or valves that are bent causing them to not seal. Also check for Carbon on valve stems causing lack of valve sealing.

[diagnosticator]

But if the N112 was leaking and causing the com bivalve to open wouldent that show up on an exhaust reading, like bad CO ans HC readings? Now im taking of an warm engine
This was my readings during an test av exhaust on idle and 3000rpm
CO: 0,0
CO 2005: 0.0
HC: 000
Lambda: 1.01

No, because the exhaust gas is flowing backwards through the Combi valve if the N112 or the Combi valve itself is leaking. Since the normal Combi valve function only effects the exhaust gas after the combustion process, by allowing fresh air to flow into the exhaust ports, from the SAI pump, it can't effect the raw emissions from the cylinders.

[Le31504]

Can this in some strange way be connected to an faulty (not broken) G62 temp sensor?
Lets say that the G62 tells the ECU that the car is cold, and sends signal to N112 to open the combi valve a little??? i dont know... just thinking out loud...

[diagnosticator]

Can this in some strange way be connected to an faulty (not broken) G62 temp sensor?
Lets say that the G62 tells the ECU that the car is cold, and sends signal to N112 to open the combi valve a little??? i dont know... just thinking out loud...

I don't think so. There is more involved with the ECU running the SAI pump and opening the Combi valve for normal cold starts.

[Le31504]

If the combi valve is leaking, then you will know when the intake pipe to the SAP is disconnected from the air cleaner. you will hear and feel the exhaust comming out. Have seen this issue with improper cam timing on cars that have had a major head repair/ rebuild, or valves that are bent causing them to not seal. Also check for Carbon on valve stems causing lack of valve sealing.

But if there was a problem with valves not closing then it wouldent stopp popping when i pinch the vacuume hose from the N112, right?

It only seams to be an problem during idle and warm engine... It starts like 10 seconds after i stop and let it be on idle.. so i dont know if its leaking while driving... What i mean is that he popping starts after like 10 seconds on idle after a ride, not immediately when stopping

[diagnosticator]

But if there was a problem with valves not closing then it wouldent stopp popping when i pinch the vacuume hose from the N112, right?

It only seams to be an problem during idle and warm engine... It starts like 10 seconds after i stop and let it be on idle.. so i dont know if its leaking while driving... What i mean is that he popping starts after like 10 seconds on idle after a ride, not immediately when stopping

Yes, that is correct. IMO, the 10 second delay is consistant with a leaking N112. The leaking is probably occuring while driving too, but because the engine is under load at higher RPMs, you wonl't hear the noise like you do at idle.

[Le31504]

Thats true...
I will try to see if i can test the N112... Should it generate any codes anyway? (vag-com)

[diagnosticator]

The ECU cannot directly detect any problems with the N112 solenoid mechanical valve parts. Only by inferred methods measuring other engine variables is that possible, and I don't think all of the necessary variables for inferential detection of mechanical faults are monitored by the ECU for the various solenoid valves. I don't remember ever seeing DTCs relevant with solenoid mechanical function listed in the DTCs for the 1.8T. Therefore, no, there won't be any DTCs associated with a leaking N112.

[Le31504]

mm ok...
Would the leak be detected if i spray startinggas on it.. (dont know if its called that in english) an gas on can that you can use to spray directly in airintake to start old cranky engines with...
An high flamebal gas anyway

If its an vacuumeleak it should suck in the gas and the engine rpm would rise for a while

do you think thats possible?

[Le31504]

Still pops...

Tested to pinsch the vacuumeline from the N112 to the Combi valve, but it was still popping in the airbox...
I disconnecte the connection to the secondary airpump on the airbox... and it was still popping in the airbox... no popping from the hose i disconnected... no exhaust smell och pressue... so i belive its not the seconday airsystem (the combivalve or the N112)

So whats next?
What can it be?

It there anyting under the airbox that can make popping sound? thinking it can be someting else that pops thats close to the airbox and not from inside the airbox itself? (the sound is transported by the airbox or what to say)

[old guy]

Try clamping the line from the n249 to the DV and see if the noise stops. Next try clamping the line from the N80 to the TIP and see if that makes a difference. you obviously won't be able to drive that way but it may help determine what's making the noise.

[Le31504]

mm i can try that to...
more likly the N80 rather then the n249 if thats the case (if you look at position and funktion) The 249 controlls the DV and there is no sound from the turbo or at the front of the car... its more or less from or close to the airbox... so it shouldnt be the n249 or even the n75...
The N80 is clicking normaly during idle so it seems to work...
But I will absolutly try them both!
Thanks!

[Le31504]

yea i edited my post but you were too fast :-)
mm it could be, but my DV is low down in the front... I dont say it cant be that one, but i dont really know how that sound could be transfared from that distanse to the airbox.... and i tryed listning al around the engine and cant hear anyting from the front of the car...

If the DV was pulsing wouldent that show up on the Boostgauge? like pulsing vacuume on the gauge? mine is at -0.7bar with some +/- depending on how cold the engine is, if the AC are on or off and so on...

As i sad will absolutly try it! any ideas you have are good ideas :-)

[awdtfsi]

I have the same issue with 2.0T. The airbox sound at idle (irregular bass sounds). Only at idle. Im am new in this engine. I`m affraid of what "BBC" said abouth intake valve sealing.

EDIT:
No more noise. It was a broken PCV rubber membrane.

[al_oakos]

I have the same issue.
Popping sound (not n80 clicking) stops when squeezing the pipe immediately after MAF that goes to ventury line of Suction Jet Pump


I also have hissing sound from Suction Jet Pump that stops when squeezing the vacuum line that goes to intake manifold:


I will try to replace Jet Pump and I will report