928autobahndreamer

I am currently doing some work on my 928 and am getting ready to order parts. I am pretty sure my flappy is non functional due to the solenoid.

With the hood up and the cap off of the top of the intake over the flappy bearing, there is no movement on start up or with increased revs (I did the tape trick).

Last winter I did a full intake refresh, including replacing the flappy actuator and fixing all vacuum issues. If I apply vacuum directly to the flappy, it works as it should, but when plugged into the solenoid, no action.

Is there a method to check the solenoid function directly, or should I just buy a new one and replace?

I guess my question is if there could be another area which could cause the flappy to be non functional. I am 99.9% sure that the problem is the solenoid, but I want to make sure.

Thanks.

fraggle

Just pop the vacuum line frm the solenoid to the flappy.

Then you can get a vacuum tester on the line to the flappy and test its actuation manually.

Nexy you can attach the tester to the flappy side of the solenoid. It should show zero (atm) pressure until you actuate it. When actuated you should get a vacuum signal.

If you check the input to the flappy solenoid, there should always be a vacuum signal when the motor is running, and should remain for some time after the motor is shut down is your vacuum system is in good condition.

Good luck!

Tom in Austin

I think flappy doesn't open until 4,000 rpm or something. There may other criteria as well, like engine temp, etc. so it may be tricky to invoke it with the car sitting in place.

The fact that you can see it working when you apply vacuum to the line certainly eliminates a gross vacuum leak or a mechanical failure, so I would agree that either vacuum to the solenoid or the solenoid itself are the next logical sources of trouble.

Tails

Simple tests to check resonance valve diaphragm by vacuum hand vacuum pump or "suck" test.

Remove the rubber connector to electrical solenoid valve in right front top of engine just to the left of distributor cap and located on top of the TB cover and "suck' on the tube. If the "flappy" resonance valve opens and remains open then diaphragm is OK. Vacuum pump is better as you can watch for leakage on vacuum gauge if diaphragm has a hole.

Check on the suction side of the electrical soleniod valve with vacuum pump and if it maintains approximately 0.6 bar then suction side from vacuum tank is OK.

Check power supply to solenoid valve.

Bridge terminal 17 and terminal 21 on LH control unit plug with the help of an auxiliary lead with small spade terminal ends. Apply ground on terminal 34 of control unit plug with another auxiliary lead and the resonance flap should open. (battery need to be connected for this test).

If the valve does not open, connect terminal 17 and terminal 21 on LH control unit plug. Disconnect plug on solenoid valve and connect voltmeter on plug receptacle (2) and ground and this should display battery voltage.

If no voltage display, check power supply to solenoid valve via LH relay XXV to wiring diagram and test power flow from LH control unit terminal 34 to selonide valve plug connection 1. (amphere draw via DMM).

If you have power flow and solenoid valve still does not operate and solenoid valve will function with 12 volt externa supply then the next step is to check the control signal from the LH unit via an oscilloscope.

Connect LH control plug. Disconnect plug on solenoid valve. Connect tester and check to see if you get a rectangular wave form while stating as well as with engine running and abrupt opening of throttle around 3,500rpm. If you get this far and the resonance valve is still not operating then you have a problem with you LH and you should try and get a swap in LH to check operation to confirm whether you LH is faulty.

A full description of this test procedure can be located in Porsche's service booklet titled "928S4 Test Plan EZK-Ignitiopn and LH-Jetronic '87" WKD 493 921 test step 17 on pages 2 - 35 & 2 - 36.

This booklet is included in Jim Moorehouse's technical CDs which can be obtained from Roger at 928Rus.

If you don't have a set then purchase them as it will be the best investment you will make. If you have the CDs then I would suggest that you read up on your model car if you are going to be a serious DIYer owner operator of your 928 or otherwise ensure you have deep pocket with money.

Tails 1990 928S4 auto

mark kibort

flappy is tough to test. you cant get load with RPM, you need to get it to work usually, as it is just not rpm based. the fact that it doesnt initiate itself, dose tell you something.

928autobahndreamer

Thanks for the replys.

The flappy will be tough to test right now as the front is disassembled for a timing belt job.

I feel pretty confident that the flappy actuator and the vacuum system are in good shape though. I did an intake refresh last winter and made sure that was all in good order. The actuator is new and it works perfectly with minimal application of vacuum with a mity vac. I actually have a thread somewhere about what I did with the flappy from when I was doing my intake. (There was some question regarding whether or not the spring on the actuator arm should be wrapped around once or twice. Also, my flappy was missing the the Stop, and I made one using a drilled out hex head of a bolt.) So, know the flappy resonance valve opens when I apply vacuum, and I know my system holds vacuum well.

It is possible that my flappy is working. I was under the impression that the flappy would activate on start up and then again if rev'd past 4K. I had put a piece of tape on the flappy resonator valve bearing that you can see rotate when you apply vac directly. I watched this on startup and with reving past 4K and saw no movement. I assumed this ment that it was not functioning.

I may just wait until I have the car back together and do the process as outlined above to check power and signal from the LH.

Thanks again for the responses.

Tom. M

If you are not holding vacuum after shutdown, it won't flap on startup.. Also to get it to flap at >4000 rpm you really need to rev fast through 3000 to 4000 rpm....basically stab the gas pedal hard and rev to 6k..it should flap then.

WallyP

Make sure that you have full manifold vacuum on the supply line to the solenoid valve - it is not unknown for the supply line to fail or be disconnected.
Make sure that you have the vacuum lines hooked to the correct port on the solenoid. Test with the mityvac - you want the supply line on the port that holds vacuum. The other port (the one that hooks to the flap actuator) will not hold vacuum.

If all is well, the flap will move when you start the engine - just a quick rotation, then back to rest.My guess is that this is intended to exercise the flap, so that doesn't stick due to non-use.

kmascotto

I was checking to make sure the solenoid that operates the flappy valve was working correctly. So what I did was remove the rubber cap on the top of the intake and I marked the shaft with a marker pen (a line from 12 to 6 O'clock). I then started the car to see if the solenoid would actuate the flappy ( on start up the computer checks and actuates the valve, which it did )
So it looks like its working....but the strangest thing would occur. Every time I did this, the mark that I made would rotate 1/4 turn from its last position. So why would the line that I made that was at 12 & 6 o'clock...the next time i started the car be at 9 & 3 o'clock?... When I checked the shaft, to see if the valve was maybe stuck open or it closed...it was closed! the spring is good and moves freely.

Why would the shaft not go back to the 12 & 6 O'clock position in the closed or rest position...

SteveG

Kent,

I don't know if your question is answered in the thread below, but a quick "search" came up with this primer on the intake. I believe there have been threads on lube and replacing the flappy bearing = mechanical, but it could be vacuum (not likely if the vacuum is working to activate it initially) or electrical (solenoid). I think.

Back to the mechanical: what you describe sounds like something is interfering with the spring that closes it. The shaft the flap is mounted on could be bent or inhibited in some way.

https://rennlist.com/forums/928-foru...and-ideas.html

SeanR

Quick and easy test is to take the cap off. Place a piece of tape on the flappy rod that sticks up. Get in car and get in the seat. Took through the space between the cowl and the hood and look at the tape. Start the car, slam throttle to the floor so the RPM's get above 4k. If the tape moves your flappy is working. If it doesn't you have a problem.

SteveG

Sean: If you are answering Kent's question, he wrote He seems to be observing something odd. If he has marked the shaft correctly, something is obstructing it or causing it to advance from the closed position -- weird.

kmascotto

Hi Steve and Sean,

Steve the flappy bearing is function perfectly it has great movement and is closing shut after being actuated.

Sean, I did not use tape, but I marked it (the top of the shaft) with a marker pen. I then had someone else start the car so I could watch for movement...which did occur, which is great!

But as I indicated, every time I did this the mark that I made would rotate 1/4 turn from its last position, yet appeared to be seated closed when I checked it manually. So why would the line that I made that was at 12 & 6 o'clock...the next time i started the car would be at 9 & 3 o'clock... When I checked the shaft, to see if the valve was maybe stuck open or it closed...it was closed! the spring is good and moves freely.

Why would the shaft not go back to the 12 & 6 O'clock position in the closed or rest position...

Mrmerlin

use a piece of tape to mark the shaft then perform the test as Sean has described,
maybe you marked one of the washers and the washer is turning.

from your description there is not any possibility to have the flappy shaft moving into any other positions than open or closed

kmascotto

I still need to check this, but here is what i think is happening.

When I start the car and the computer does its check of the flappy valve by actuating its movement. However, for some reason its staying open when it should be closing after this actuation. The solenoid must be holding the vacuum to the flappy and keeping the valve open instead of releasing the vacuum. So when I was testing the flappys movement using pliers on the flappys shaft, I was confusing the closed position for the open position....because of the vacuum that was still in the system