Flappy Valve Diagnostics

Flappy Valve Diagnostics

I'll try to explain how the intake flap valve works, but I am not an expert, so this will be very basic. 1987 and later cars have a variable intake. The intake flap simply controls airflow into the engine. It is vacuum and electrically controlled. Evidently, if it's not working, you will notice a lack of low end torque and a surge at 4000 RPM. It is located forward of the air filter box, in the plenum on the very top of the engine. If you look on top of the intake plenum, you'll see what looks like a black rubber knob. This is actually a cap. If you pry (twist...pull...it's rubber) that off, you will be looking at the top of the shaft that the flap rotates on and a C-clip that retains it. In order to see the entire assembly, you must remove the plenum. I haven't done this, but I may have to at some point. Anyway, to test the operation of your flap, take a piece of masking tape, and stick it on top of the shaft with some of it sticking straight up, like a flag (this is so you can see it from the driver's seat while you crank the engine). Next, while starting the engine, look at the masking tape. If your flap is operating correctly, you will notice it move CCW 90 degrees (this is true whether the engine is cold or warm) as the engine fires, then return to it's starting position. The next test is to quickly punch the throttle hard and accelerate the engine to 4000 RPM (be careful when you do this, for obvious reasons). You should see the tape move CCW again. I did this test twice and mine did not work while it was cold, but worked okay when warm. I have the dreaded sticky flapper. I just squirted the top of the shaft with liquid wrench, hoping to free it up for a cold start, which I'll try tomorrow. So now you know what all the flap is about the flap. Give it a try and let me know how it turns out.

ps- you can do a search of the archives on rennlist by typing in 'flap' and et some more background info and problems that a number of people had.

Adam Cultraro wrote:
>Order of business #1: My flap doesn't flap. I know this has been talked
>about lots but does anyone have a procedure for re-introducing flappyness?
>Does the whole plenum have to come off?

Adam -

Before you dig too deep, check to see if the flap is just sticking:

1. remove the rubber cap from the top of the plenum
2. remove the circlip
3. grip the spindle with lineman's pliers and turn to counter clockwise

did the spindle turn? did you feel it unstick?

If you can't turn it, you probably got some major work ahead which I'm not qualified to talk about (as if that ever stopped me?). If it does turn, spray some WD40 on to the spindle and work it by turning a few times, then...

4. Clean off the spindle
5. Fold a 1" piece of electrical tape over itself in to "Y" and
press it to the top of the spindle

You should now be able to sit in the driver's
seat and see the tape sticking up

6. Keep your eyes on the tape and start the engine...
you SHOULD see the tape turn counter clockwise and return to closed

7. Now slowly rev the engine up to 4500 rpm or so...
you should see the tape turn counter clockwise and REMAIN open
When you let off the accelerator the flap should close again.

You may need to spray the spindle head with WD40
and work it in and let it sit overnight and re-apply.

If the above procedure does not work then you have to check to see if you have vacuum... and I'll have to leave that to someone else to describe.

Good luck,
Michael Gerard

I agree with the above, however, no need to remove the circlip. You can rotate the spindle even with
the circlip on.

Michael ran through the basic diagnostic. The next step is to check the vacuum system.

Locate the electronic vacuum valve that switches the flappy's vacuum. The valve is about the size of a roll of 35mm film. You should find it bolted the driver's side (LHD) timing belt cover. You should also find two vacuum lines connected to it. One runs from the valve over the cam cover via the fuel rails to the back of the engine where it plugs into the four-way splitter at the check valve near the brake booster.

The other vacuum line runs from the valve into the dark innards of the throttle body / intake assembly . Carefully, detach this line from the valve and, using a vacuum hand pump, apply a little vacuum. With the rubber boot removed from and your tape on the spindle, you should see and hear the flappy move when you apply vacuum.

If the flappy moves then the problem is between the electronic valve and the brake booster inclusive and is probably vacuum related. If you do not hear or see the flappy move then the problem is the vacuum line under the intake, the flappy vacuum actuator itself, or a stuck flappy. If the vacuum line holds vacuum, the flappy isn't stuck (per Michael's test above) then the actuator is toast.

-- David Chamberland
91 928GT (Amazonagrunmetallic: Green. NO! Blue! Ahhhhhhhhhhh!)

If your flap does not work and you know the flap is free to move then there are three main possibilities:
1. The solenoid actuated valve is naff.
2. The actuator on the underside of the manifold is naff.
3. Your vacuum system is naff.

To diagnose:

1. Solenoid valve [just in front and below the inlet manifold]: Disconnect the plug. Apply 12 volts over the two terminals on the valve. You should hear the solenoid engage and disengage. You can also disconnect the vacuum lines and when the solenoid is open you should be able to "blow through" the valve. If the tests are positive move on. When the car starts the system should transiently put 12 volts on the solenoid terminals for a second or so. You can try to measure this on the connector terminals. If the solenoid valve is functioning O.K. move on.

2. Disconnect the vacuum line that runs from the solenoid valve and disappears into the front of the inlet manifold. If you apply very light pressure to pull the line towards you, you should feel resistance from where it connects to the flap actuator. If there is no resistance the vacuum line has come away and it will pull straight out [ I stress very light resistance!!]. If you suck on the free end of this line you should be able to actuate the flap. If you cannot move the flap this way then the problem is with the actuator under the inlet manifold or the vacuum line has disconnected.

Move on.

3. If your "passive vacuum system" has a problem you can diagnose this by sticking a vacuum gauge on the vacuum line that runs into the solenoid valve from the 4-way connector. When you disconnect the vacuum line from the solenoid valve for this test, if things are O.K. you should hear the vacuum system sucking in air.

Run up the engine for a couple of minutes to build up the vacuum in the system. The vacuum gauge should now indicate about 500mm Hg [Hg = mercury]. The flap needs about 250mm Hg vacuum before it thinks about moving.

When the vacuum has maximised switch off the engine. The system should hold vacuum for ages. If the vacuum decays rapidly [i.e. you can see any change in the gauge reading within an hour] you have a problem with the vacuum system.

If the vacuum is insufficient it will be because of one of the following:
a] Vacuum reservoir collapsed [inspect in the driver side wheel arch].
b] leaky 4-way crossover,
c]leaky check valve [the litle device mounted on the inlet to the brake booster]
d]Your inlet system in general has bad leaks and you cannot pull a decent vacuum anywhere [possible but most unlikely]
e] You have a leak [probably an actuator] somewhere in the hv/ac system.
My S4 suffered a] d]and e] simulataneously!!!

Next step:

Disconnect the single tube leading to the hv/ac system at the 4 way crossover and plug the empty connection- if you can hold vacuum now the problem is in the hv/ac system. If you cannot hold vacuum at this stage the problem is in the vacuum reservoir or the suction system from the inlet manifold.

If the problem is in the hv/ac system re-connect the vacuum line and proceed as follows:

The hv/ac system is more complicated to diagnose and a pain to fix properly. You need to enter the side of the centre console by pulling back the panel to the right of the drivers leg.

You will see the nylon vacuum tubes running from the row of vacuum valves out to the actuators [the tubes are colur coded]. I isolated the tubes one at a time and stuck on the vacuum gauge. I found that I needed to snip some of the vacuum tubes to get good access and used vacuum connectors to re-instate them.

One by one you test the lines until you find the faulty one. When the main system holds vacuum the actuator you have disconnected is the one that is faulty [unless of course you have more than one faulty unit at the same time!].

There are 5 actuators in the hv/ac system:
Heater valve [acessed from the engine bay]
Footwell Flap
By-pass flap [used for a/c duty]
Main closing flap
Defroster flap

If you are really unlucky the problem could be elsewhere under the dash regrettably you will have to fathom this out for yourself. The heater valve is easy to diagnose as you can put the vacuum gauge on in
the engine bay. I have two leaking actuators in my system. I disconnected both of them hence I do not get air into the footwell and the main flap on the blower discharge is permanently open [I do not know why it was there in the first place]. To help diagnose which actuator is leaking you need a copy of the vacuum

One little modification I have considered is to stick one of those little check valves on the line to the vacuum system. By doing this you can protect the flap from failing if you have a problem in the hv/ac sub system. It surprises me that Porsche never fitted this in the first place.

Hope the above is useful. I am sure it has all been said before but I worked all this out the hard way.


Fred R
1990 S4 auto -gradually plugging the leaks!

928 Tips Home     Greg's Home