sprie

My setup seems to be fairly similar to the described in this link:
https://rennlist.com/forums/928-foru...-source-2.html

I have good vacuum coming in from line 1.
Line 3 I think does go to vacuum reservoir (as it needs lots of pumping on the Mityvac to get the vacuum up, but it holds fine)
Line 4 also holds good vacuum.


Line 2 is the one i have the issue with. It goes to I believe part number 911 618 123 01 on the front of the engine.
When the engine is running, Ii i put the Mityvac on the input to the valve actuator, it holds no vacuum, and if i put the Mityvac on the line from the valve actuator that disappears under the intake, it holds no vacuum.

I presume this is not what should happen? What is the issue and the resolution?

FredR

Sounds like the diaphragm in the actuator has gone **** up. Flappy system tubing is routed via a solenoid valve that is actuated as and when needed so as to apply vacuum to the actuator. The diaphragm in the actuator invariably fails- I rate them good for 10 years but after that can go at any time. Changing it out requires an inlet manifold off job and thise actuators are about US$80 a pop- bumer! I actually fitted an actuator from a new heater valve just because I do not use the actuator on my heater valve that is generally wired closed as I do not flow coolant through the heater matrix- it is by-passed.

Trying to evacuate the passive vac system reservoir with a Mityvac is no fun- you probably end up with carpel tunnel syndrome!

worf928

I looked at the thread in your link. I can’t make heads or tails of what you mean by line 1,2,3,4.

That thread does provide a 404 URL to my Intake Removal Guide. Here’s a non-404 link to my guide:

http://www.928intl.com/repair/intake1b.pdf

Short description of flappy vac system:
- line from change-over valve (COV) on left-side belt cover that runs into the guts of the intake should hold vacuum. Applying vacuum to that line should result in you hearing the flap ‘click’ shut. Releasing vacuum should result in a ‘click’ as the flap flips to open. That line should be connected to the COV via the fitting that points ‘up.’
- other line from the COV (which should be gray and connected to the COV via the fitting that points to the water pump) runs to a 4-way splitter near the brake booster. The splitter should be connected to a blue/black check valve that’s connected to a fitting on the brake booster check valve. The ‘black side’ of the check valve should be connected to the booster fitting and the ‘blue side’ to the 4-way.
- disconnect the blue/black check valve from the booster and pump down the vacuum reservoir through the check valve.
- remove the rubber cap over the flap spindle in the middle of the intake and put a piece of colored tape on the spindle.
- start the engine: if everything is working, you should see your tape rotate 90° upon start-up and then rotate back 90°

sprie

To my (pleasant) surprise, when i tested the line with the MityVac to the flappy valve with the coloured tape on the spindle, it turned and held the vacuum.
The leak appears to be in the Change Over Vacuum Valve (911 618 123 01). These are £110 in the UK, so could be worse/could be better - and at least i don't have to take the intake off.
I don't suppose these are repairable?

FredR

Test the solenoid valve separately.

As I recall the inlet port from the vac reservoir should hold vacuum with power off and the outlet port to the flappy should vent to atmosphere. With power on the vacuum is then applied through the valve and should hold vacuum through the valve.

worf928

Test the COV on a bench. In 20+ years and countless 87+ 928s I have replaced that bit exactly once… because I broke it. Unlike the modern Porsche COVs that are designed, it seems, to fail the instant the factory warranty expires, the 928 COVs are very robust.

The COV has three ports: two connected to the 928’s guts and one that is capped. If I were to guess, I’d guess that the unit is leaking because the cap is loose/non-original/somehow-borked.

Mrmerlin

IIRC on that valve the one thats capped is the vent port,
(thus letting the flappy have a way to return to atmosphere when not activated)
NOTE the cap prevents ostriches from making their way into the vacuum system.

worf928

It’s been 20 years since I tested one; your memory may be better than mine. My recollection was that that port behaved in opposition to the port hooked to the vacuum actuator: When inactive it ‘passes’ vacuum and when active is at atmosphere. I will not however wager a farthing on my memory at this point.

On the gripping hand, I think it is possible that we are both correct. But, I’d need more coffee.

Mrmerlin

My understanding is the Flappy valve port is open to atmosphere when the electricity is off .( to the capped ported line)
When the valve is energized ,
the port is closed then the vacuum side of the valve is then directed to the flappy line thus activating the flappy.
NOTE to test the system,
remove the rubber cap and put a piece of blue tape on the flappy shaft,
start the engine,
once the engine starts you will see the flappy shaft close for a few seconds then go back to open .

sprie

I started the engine and tested the socket on my multimeter that connects to the solenoid valve - and there was no voltage (which i presume is not what is to be expected?)
I can test the solenoid valve in isolation to see if that works - is it 12v or something lower that it is expecting?

worf928

Yes.

IIRC the valve’s operation is very quiet. I added my vacuum hand pump with gauge to the test apparatus.

FredR

The solenoid valve is at either zero volts or 12 volts depending on the load demand. The first switchover point happens around 3700 RPMs as I recall, the system then holds with 12 volts applied until around 5200 RPMs and above that operating point the resonance flap reverts to the zero position again As the revs fall the reverse happens or so I understand

Mrmerlin

please test the system as I have described first

sprie

To my surprise (not the first time), the flappy valve solenoid seemed to work ok when i tested it
i.e.
1) i plugged the exit from the valve solenoid that leads to the flappy valve
2) I connected mityVac to the input to the solenoid
3) i applied 12v to the connections from a battery,
I heard a click, and my MityVac was able to pull and hold a vacuum.

So i went back and tested the electrical supply to the solenoid, and there was no voltage. This was a idle revs, so i didn't try it at 3700 revs.

One point that confuses me a bit: i have vacuum going to the 4 way divider by the brake servo - 1 is input, 3 output. One of these outputs goes to the solenoid above. If it is expected to not operate until 3700 revs, that means there is an one "open" tube from the 4 way divider, which goes to the solenoid and then to the flappy valve which would hold no vacuum. I would have thought this would compromise the vacuum from the 4 way divider (which includes HVAC heater and the auto gearbox).

Questions:
1) do i need to do the electrical test, but this time with 4000 revs?
2) am i missing something about the "open" vacuum?

FredR

Ian,

The solenoid valve has a very simple logic- it is closed when deenergised and open when energised. In the closed position it has to vent the vacuum actuator to atmosphere.

The 4 way manifold has vacuum in, vacuum out [to the reservoir], vacuum to the flappy [via the solenoid] and vacuum to the HVAC system.