Air Box Rear Flap

[DeTomato]

I was interested to see a flap at the rear of the AirBox on my 1.8 Corolla 2003 VVTLi. It appears to open and close with ease when the engine is cold, but when the engine is hot is remains closed.

Is there some reason for this flap on the AirBox? What is it for and what does it do? Would my flap on the airbox be sticking closed, or is it operated with some heat sensitive operating mechanism?

[Gerg]

I think if you have a picture of the part it might help give a better idea of what this mechanism is doing.

Perhaps someone knows without needing a picture...

[DeTomato]

Hi Gerg, so I have managed to get a photo of this rear flap on the Airbox where the top part in the photo swings (or levers) up and down on a spring internal to this unit mechanism. It goes into the base of the back in the Airbox and inside the airbox it pivots to open and close the flap also. It looks very dirty outside on this unit but inside it is really clean.

So question remains, what is this flap for and when is it actuated on a 1.8 VVTL-I 2003 model, and what activates it? Some sort of air vacuum perhaps? What does it do and what is it for? Thanks.

[Gerg]

I have no knowledge of your model, but the flap is likely to be operated by a diaphragm and a connecting arm  The diaphragm is usually in a circular enclosure, usually plastic.  It will powered by normal engine vacuum (like the brake servo) and controlled remotely by a solenoid valve somewhere along the vacuum feed.  The solenoid valve will be controlled by the engine ECU.

The valve will be designed either to allow more air into the air box (less restriction but at the expense of more noise), or simply to make the engine sound more energetic, or both.

Either way, Toyota will have managed to get the system to meet any legal minimum noise requirements (if there are any) by carefully chosen closure of the flap (making the engine less noisy when the car is running at a speed or rpm that the noise test is mandated to read at), so getting past the noise regulations.

If this is what I have said above, it follows on from many similar mechanisms used to allow noisier car and motorcycle exhausts and air boxes to be quiet at certain speeds, but still meet the drive-by noise tests for new car or bike testing.  Yamaha used this as far back as 1989 on their EXUP-equipped motorbikes (Exhaust Ultimate Power Valve, marketed as a performance device but also used to control noise), and more recently in the exhausts of certain car models e.g. Jaguar F-Type.  Although these are a bit more sophisticated and can use electric servo motors to give infinitely variable adjustment.

Yamaha used a device that is very similar to your one (if it is what I've guessed it is) on the airbox of their 2002 model TDM 900 motorcycle (now out of production), and probably others as well.  In that application the flap reduced noise when closed at lower revs, but also probably allowed a different resonant frequency of the airbox at low speeds for slightly improved pick-up, so it is often said. 

Vaguely related, but maybe not, Yamaha have a long association with Toyota and designed the engine in the (James Bond featured!) Toyota 2000GT of 1967 (You Only Live Twice), for example.  This car is worth googling, if only to check out its looks.

I could have got it completely wrong about this part.  It could have been designed by the office cleaner after everyone else had gone home for the weekend, and just slipped through the net.

Perhaps an owner of one might know more about this.

[DeTomato]

Thanks Gerg. That makes sense. It does appear to have a plastic diaphragm above to the right just out of the photo if looking at it. There is definitely a different engine note at certain Revs, like a throaty burbling before acceleration above 3000-4000 Revs.

When flooring it, there is a sustained whistle sound (like a Turbo) up to 6000 Revs where the Lifters then kick in which I suspected was air being sucked in from the blocking of this flap and the small drain holes in the AirBox. I can't see any air leaks in any hoses to cause this whistle otherwise.

The engine appears to run ok. But does have what I can only describe as a throttle "Balance" check or slight falter at 1400 Revs when decelerating, as though it was doing some feedback loop check prior to dropping down to idle at 875 Revs. This is where I thought this Airbox flap may be "sticking" or causing this, but checking it again there seems to be no problem with it as far as I can tell, though it does seem to need a clean I can see now! 

The Throttle Position Sensor is new because the old one just wore out about 6 months ago and the ohms were all over the place, so I wouldn't expect this to be the cause. Struggling to see if this is a REAL problem, or if it's actually running as it should before coming to idle at 875 Revs from the "check" at 1400 Revs - could it be the de-pressurisation from the PCV valve which I know is working well (I cleaned this not long ago so it's not oiled up now). There are no fault codes so I was suspecting this "check" slight falter at 1400 Revs is normal, unless anyone can advise me otherwise. Thanks again all. 

[Gerg]

After a very quick internet search, I now see that your Corolla has the 2ZZ-GE engine (I think), which a quick further search shows as being a Yamaha-designed engine, as you probably know. 

Maybe the Celica forum members (who have this engine fitted too?) have some insight on the design and idiosyncrasies of this motor?  I guess you've already looked there.